Jump to content
Holidays
Local
Radar
Windy?
  • entries
    17
  • comments
    68
  • views
    51,445

About this blog

Musings on the current, previous and forecast weather of the Dundee/Edinburgh area and more!

Entries in this blog

 

Winter Forecast 2015/16

I usually start these off with a recap of last winter, but this year I covered that in a separate blog, so here's one I made earlier:
Moving onto this winter, there are a number of factors that make this a really fascinating one to watch. I'll go through each of these in turn, explaining what they're likely to do and how that's likely to affect us, before going on to look briefly at the methodology of the forecast, and, finally, getting to the fun bit, where I pull all of this together to make a (still wildly speculative) detailed winter forecast.
Factors to Consider
El Nino
The most influential factor globally at the moment has to be El Nino - temperature anomalies in the central ENSO region are almost 3C above average http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf putting this easily in the top 3 strongest El Ninos on record. The impact in many regions is acute, with well above average Sea Surface Temperatures in the Indian Ocean contributing to an unprecedented two cyclones hitting Yemen in the space of a week.
However, for Northwest Europe the impact is far more subtle and indirect, with its main contribution being that Sudden Stratospheric Warmings (more on that later) increase in frequency in El Nino years http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00207.1 . Therefore, in isolation, a strong El Nino marginally favours a colder than average winter, although nothing compared to the hype of the tabloids (and even the Sunday Post) who seem to think it guarantees an oncoming ice age. Of course, it's more complex than that, particularly when you consider that there are essentially two types of El Nino - a 'traditional' east based El Nino e.g. the giant red pincer that was the last huge El Nino event back in 1997, the precursor for a mild winter in 1997/98:

and an El Nino Modoki, which is a more central-west Pacific based event e.g. in the exceptionally cold winter of 2009/10:
This winter's El Nino doesn't quite match either of those, with much weaker warming in the eastern Pacific compared to 1997 and in the central/Western Pacific compared to 2010:


This suggests that, while there will be an increased chance of a Stratospheric warming as a result of El Nino, the idea of a winter similar to 2009/10 isn't a firm favourite on the basis of this, at least. However, the sample size with El Ninos of this magnitude is relatively small,
My model of analogue years weights El Nino in a number of ways: firstly, by weighing the Multivariate ENSO Index to the current value, secondly by weighing towards neutral El Nino Modoki years, and thirdly by considering the joint impact of El Nino and the QBO which I'll get onto in the next part.
  QBO
The Quasi Biennial Oscillation is a measure of the stratospheric wind near the equator. It runs in an approximately 2 year (hence quasi-biennial) cycle, where it reverses from easterly to westerly and back. This winter, we're looking at a very strong westerly QBO bottom graph:

which in isolation would suggest a more westerly dominated winter.
Last winter saw the strongest easterly QBO in the recorded dataset, which was one of the reasons many people were leaning towards a colder than average winter - an easterly QBO in isolation slows the westerlies at higher latitudes and hence weakens the vortex, leading to increased high latitude blocking and generally colder winters. However, as last winter's failure made clear, taking the QBO in isolation is deeply flawed - in particular, its interaction with both ENSO and the solar cycle complicate matters. In the former case, both El Nino and and East QBO increase the wave activity acting on the vortex, with the East QBO acting to advance the onset of the El Nino forcing on the stratosphere earlier in winter while the West QBO delays this signal, though the difference between the two QBO states in the mid-lower strat. seems to disappear almost entirely by late winter: Calvo paper This would suggest that we're likely to see a strong vortex developing early, but with the potential for more blocking as winter progresses. Thus far, in spite of this weekend's early cold snap, we've seen the stratospheric vortex behave in a manner consistent with this, with stronger than average westerly winds and near record low heights at the pole:



As (hopefully) an improvement to last winter's forecasting method, the QBO itself is not weighted but rather than ENSO/QBO and Solar/QBO interactions (more on these in the next passage) are, and in addition years with similarly strong early stratospheric vortices are factored in too. These can work both ways - on one hand in the short to medium term they tend to be associated with more zonal, westerly conditions prevailing, but can also increase the likelihood of an SSW further down the line,

Solar Activity
This is certainly one of the 'hotter' topics in medium/long range forecasting discussions, and has been since the out of the blue very cold winter of 2009/10 coincided with the deepest solar minimum in nearly a century. The current solar cycle was unusual in two ways: firstly, it was one of the shallowest in a long record, and secondly it had a secondary peak in activity which was stronger than the initial (and what was assumed to be at the time the main) peak. As you can see if you read the recap of last winter's forecast, this secondary peak's emergence during last winter appeared to be one of the main reasons the vortex didn't behave as expected. Anyway, what of this winter? Well, it looks as though we're finally heading back towards solar minimum values (approximately <100 sfu) and if the next cycle plays out as expected it may not get much above that mark for the next 15-20 years:
The issue we now have with the solar/QBO relationship this year is that, while last winter solar max conditions acted along with the negative QBO to enhance westerlies, this winter the QBO flip means we're once again at a relative disadvantage when looking for vortex disruption, compared to if only one of these variables had flipped. Nonetheless, the lack of moderate/low solar, WQBO strong ENSO winters in the dataset means we don't have that much to go on, so confidence is pretty low, and low solar activity might at least inhibit subtropical heights from developing too strongly.



Other SSTs (Atlantic and the PDO)
The most notable factor this summer for many of us was the Atlantic cold pool, which contributed to a rather wet and cold winter overall, moreso the further north you went:

This cold pool remains in place, albeit somewhat less anomalously cold and flanked by record warm tropical waters, and is expected to remain for the rest of winter, again prompting some less scrupulous sources to start ramping about exceptionally cold winters. But is that really the case? Certainly, you would expect Arctic sourced westerlies that are usually heavily modified to be less so, which could potentially mean more cold zonality type outbreaks like we saw last winter. But looking at the area where the cold anomaly was strongest over the winter (15-45 degrees west and 40-55 degrees north) the correlation between SSTs and Scotland's winter temperatures is weak, and in fact weakly negative, although with the coldest SST winters seeing generally below average temperatures, suggesting that it does perhaps have some effect. The main reason for the weak (or perhaps negative) correlation for this region is that it overlaps very strongly with the northernmost of the NAO tripoles - this is the signature SST pattern which indicates the prevailing pressure pattern over the Atlantic.For a -ve NAO signature you'd be looking for colder waters further south, with anomalously warm waters north of 50 degrees e.g. the opposite of this:

Therefore, this is potentially a signal for a +ve NAO/ Low pressure dominating at around our latitude (for early winter at least), but also for those westerlies to be colder than they otherwise would be.
I've also weighted for the PDO, which has a more significant effect over the US but is another indirect factor, particularly since one of the key features of the SSW loading pattern appears to be the Aleutian Low, which tends to be associated with a +ve PDO:

Coming into this winter we have a positive PDO pattern, although not quite a classical one - the warm anomalies in the Central Pacific are atypical for a +ve PDO set up. While most studies have shown that El Nino, +PDO winters tend to be more associated with displacement SSWs i.e. ones where the stratospheric vortex is displaced off the pole rather than being split in two, the Cohen precursors for each type suggest that this warmer pool, which is likely to encourage higher pressure in the mid latitudes, would be more consistent with a split type SSW later on than a displacement.

Snow Advance Index + Arctic Sea Ice
These aren't linked as such but for the purposes of this can be thought of in similar ways. The Snow Advance Index, developed a few years ago by Judah Cohen ( https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiY6Y_aqKTJAhWFgQ8KHTGMC4YQFgggMAA&url=http%3A%2F%2Fwww.aer.com%2Fnews-events%2Fblog%2Fsnow-advance-index-new-tool-predicting-winter%25E2%2580%2599s-severity&usg=AFQjCNF2cjqp4wFUVmmZdXpWEsShHJTBsA&sig2=BvbNGHy0fC49jfGPhAAGoA  is a measure of the advance of snow cover during October above 60 degrees north, which has been shown to correlate inversely with the Arctic Oscillation, primarily because strong Eurasian snow cover advance in autumn encourages the growth of the Siberian High which is another key feature in the precursor for SSWs, and hence one of the key factors driving vortex disruption. This winter, as in almost all winters I've been on the forum for, this index is strongly positive, hence indicative of a negative Arctic Oscillation. However, it's worth remembering that last winter's SAI was also strongly positive and yet we ended up with an above average Arctic Oscillation.
Arctic Sea Ice is important because it has been shown that low ice extent, as again has been common in the last few winters for obvious reasons, is correlated with a weaker polar vortex and hence negative Arctic Oscillation. Therefore, weighting for these increases the likelihood of a colder, more blocked winter.
These are also factors which makes it a bit more difficult to find decent analogue years - both 1998 and 1983 had negative SAI values, and neither had sea ice extent as low as this one either.
Long Range Models
[A quick look at what the long range models are showing, given they did pretty well last winter.
The Met Office's GloSEA model had looked pretty promising in its October update for late winter at least,but November's update is a bit more sobering - low heights to the north, higher heights to the south, indicative of a generally milder than average, and probably wetter than average, winter:

though again there is just a hint of a possibility of a bit of Atlantic/Greenland blocking developing later on:

The CFS has, for a while, been going for a significantly milder than average winter for almost all of Europe, but seems to have toned it down a bit, particularly for December, in its most recent update:
http://origin.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/imagesInd3/euT2mSeaInd1.gif
Hints also of a potentially more blocked February after an unsettled December:
[http://origin.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/imagesInd3/glbz700MonInd3.gif
http://origin.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/imagesInd3/glbz700MonInd1.gif
The Japanese JAMSTEC model, on the other hand, is going for a colder than average, drier than average winter for the British Isles, which mostly looks to be driven by the SSTs:

The above factors were weighted and a index of the deviation from this winter was calculated (accounting for the lack of data for SAI and Sea Ice during the 50s and 60s), and used to weight the composites years, with La Nina winters disregarded. 
The composite years generated are 1995 (x2), 1964 (x2), 2007, 2005, 2003, 1978, 1988, 1993, 1970, 1983, 1987, 1958 and 1998.

While not perfect (and understandably weaker due to the fact that we're averaging years), the analogue years do pick up some of the key features we're looking for from this November, both at the stratospheric and tropospheric levels:
   

The main exceptions to this are tropospherically upstream, where it has heights much lower over the US than they actually were, and around the Kara Sea in Northern Russia, where there's again a positive anomaly instead of a negative one. The latter is actually quite significant, as autumn blocking in the nearby Taymyr region is shown to correlate with a negative Arctic Oscillation( http://onlinelibrary.wiley.com/doi/10.1002/joc.3968/abstract ). Given this is linked to a high SAI and low Arctic Sea Ice, the lack of very recent analogues is probably the reason for this failing, and I will be taking that into account.


As for December, my analogues set up a somewhat drier month than might be expected given the current modelling. The low heights over Greenland remain, but with a positive height anomaly over Scandinavia there's at least some scope for drier, frostier interludes between more Atlantic driven spells.

Taking the Temperature and Precipitation data would point towards a month of around average rainfall, with temperatures around 0.5C-1C above average for Scotland, and similar for the CET zone.
Overall, I'd perhaps lean on it being a bit more Atlantic driven, with a bit more of a north south split temperature-wise, but certainly a signal there for a quieter period of weather at some point, which will come as a contrast to the last two Decembers.
As for the stratosphere, the general picture is of a still strong vortex being squeezed from East Asia:
For January, a more interesting month if you're interested in cold weather - signs of somewhat above average heights towards Greenland, with low heights to our east, hinting at the prospect of a more northerly driven regime. However, there's definitely a lot going on there - firstly, the block has a chance of setting up too far west for us to benefit greatly. There's also some danger from the anomalous heights to the south - while the Labitzke QBO-solar linkage would suggest that the best chance of blocking heights would come from increased solar activity in the next month or so the flipside of this might be a rise in heights to the south, as we saw occur during 2012/13, somewhat hampering what could've been a very cold winter here. The main factor affecting temperature here though is SSWs - when these occured in January the mean Scottish temperature was 0.8C below average, but above average without.
Rainfall from these analogue years is again around average, but with some huge variance. The common factor in most of the wet months is a cold Atlantic, which suggests that perhaps we're more likely to be above average on rainfall this winter, particularly further south.
The composites suggest that any Stratospheric Warming is most likely to result in at least a chunk of the stratospheric Polar Vortex ending up on our side of the NH (and most likely from the analogues a displacement rather than a split) : However, given some of the specific factors at play this winter I feel a split is on the table more than usual. My own feeling is that we'll come close to a proper split quite early, not quite achieve it but eventually end up with something close to a technical SSW but more likely a displacement, and likely to be a bit of a mess all round until we eventually get the vortex properly cleared out by late February/March.
For what it's worth, here are the February composites:    This comes out as the coldest month for Scotland with a mean of 2.3C, and as you might expect the driest month too, slightly drier than the average. These are the kinds of composites we were looking at for last winter, albeit a bit more toned down, but it does give the impression that blocking to the northwest may become a bit more central Greenland based the deeper into winter we go. On the other hand, the threats of both a west-based NAO and height rises from the south remain. Given all of that, there's definitely enough there to cheer a 'coldie', but it doesn't quite look like one of the classic deep freeze months either.
March could be interesting too though:







2015/16 Winter Forecast
December
Now we get to the fun bit - the actual forecast. Again, it's worth a health warning that the predictions in this section are pretty speculative.
December is likely to start off in familiar fashion - unsettled, with a familiar train of low pressure systems making their way past. Expect high winds and very heavy rainfall to be a bit less of a factor than the last two, but still quite a wet and windy opening week, with the potential for at least one transient snowfall for Scotland from a backedge cold front. High pressure will make multiple attempts to build in from the south, resulting in generally mild and less cold conditions for the south and southeast, and eventually I'd expect mid latitude heights to develop by around the 10th. The most settled conditions, and potentially the most frosty, will again be in the southeast of England, with low pressure not too far from the north of Scotland, but in between a mix of mild drizzly days and colder, clearer days, but overall temperatures are likely to be generally above average. However, this will see our first attempt at 'proper blocking', as heights build towards Scandinavia and a continental flow develops. I expect this to fail, and for a more cyclonic regime to assert itself by the 20th, but there's a small possibility things click into place by this point and we end up with a more substantial cold spell to end the month. Even without this, though, I'd still expect there to be a reasonable shot at a white Christmas Manchester northwards, with deep low pressure to the northwest combined with a bit more amplification developing upstream giving the potential for some cold zonal setups like we saw for much of last winter, with this type of pattern holding until the end of the year.
Temperature wise, I'd expect a CET of around 4C, with a Scottish temperature mean of 3-3.5C - a cold mid month for England being offset by a milder start and slightly above average finish, with temperatures for Scotland generally highest mid month, particularly further northwest. Rainfall will mostly be average but with big regional variations - quite a bit above for northwest Scotland and Northern Ireland but quite a dry month for the east coast and the southeast in particular. Snowfall is unlikely to feature for southern England (unless the easterly mid month can develop into something more substantial), will fall a few times but unlikely to lie in northern England and could be relatively frequent other than mid-month in Scotland but away 2from altitude it'll be pretty transient, though maybe an outside chance of a more prolonged spell of snow on the ground between Christmas and New Year.
Similar months you might remember: early December a bit like any of the last 3-4 Decembers but with less wind, mid month similar to Decembers 2002 and 2006, late December similar to Christmas 2004, last January.

January
A tough month to forecast, as ever, but even moreso this year. In the analogues we have some of the mildest Januaries on record, one of the coldest spells in the entire 350 year CET dataset, and a few average to moderately cold months thrown in for good measure too.
The month may well start in similar fashion to December - with a cool unsettled regime giving way to pressure rising from the south. This could be a particularly mild spell, with a Euro High and low pressure to the northwest giving a mild southwesterly regime, with significant rainfall for northern and western parts. Hemispherically, however, the signs of a big change should materialise around the same time, with the vortex being shunted off the pole, and by the latter 3rd of the month a genuinely cold north/northeasterly regime is likely to have set up. This pattern change is likely to dominate the rest of winter, however where the UK lies in relation to the main blocks is hard to pin down, but the prospect of at least one prolonged cold spell is pretty reasonable.
Overall, January is likely to sit around average temperature-wise, though perhaps a little above depending on just how long and mild the mild spell is (if it does occur at all of course) with the Scottish temperature likely to come in around 2.5C, the CET maybe more like 4C. Similar to December, a northwest/southeast precipitation split could be the main feature, with the first snow of winter potentially not until mid-late January for central/southern England. However, when it does arrive it does at least give the potential for some more significant falls, with slow moving troughs in the vicinity once cold air gets embedded.
Similar months: first half of month similar to very mild mid January 2005, second half a slightly toned down second half of December 2009.
February
As the forecast goes on it gets a bit sketchier, however hemispherically again there are decent reasons to believe that February gives the greatest potential for a more blocked pattern to become established.
Starting off from cold late January, we are likely to be very much on the edge of the cold for most of the month. Troughing is likely to cycle between Scandinavia and the northwest Atlantic, giving the potential for some big snowfalls inland, but also for quite a wet month, particularly for Wales and southwest England, with lows stuck out to the west for days at a time. By month's end high pressure is likely to made a reappearance in some form, which depending on the positioning could either herald an early spring or the first proper easterly of the winter, but with the potential for a cold March the former would likely be premature and short lived...
On balance, this is likely to be the coldest month of the winter, with Scottish temperatures of around 2.2C and a CET around 3.1C (though potentially significantly lower if the early spring doesn't materialise). Rainfall, above average in the south but below further north, with snow totals generally largest down the spine of the country from frontal events, but with the threat potentially shifting more towards northeastern areas at times. 
Similar months: February 1978 (not that I'd remember that), February 2010, if we're particularly unlucky then February 2014.

I hope you've enjoyed the forecast, let me know if there's anything that needs clarifying, or if any of the images that seem like they should be there aren't.

LomondSnowstorm

LomondSnowstorm

 

Recap of 2014/15 forecast (2015/16 forecast to follow...)

Before I start on this year's forecast, it's worth recapping last year's one for a look at what went right, and (moreso) what went wrong, and why. If my 2013/14 forecast was wrong but for the 'right' reasons i.e. the synoptic pattern for the NH that materialised was similar to the one that was predicted even if it was milder than I'd predicted for here, last winter's was pretty much the opposite, for Scotland at least.
A look at the predicted/analogue (left) vs the actual height anomalies (on the right) show a rather poor miss:
[attachment=263251:blogentry-9298-0-92446000-1414943296.png][attachment=263250:2015 winter.png]
There are a few similarities, including a weakish negative height anomaly over southern Europe and mean ridging from Alaska to central Russia, but the main point of interest from the analogue, a stonking Greenland high with a negative anomaly stretching across from the US to southeastern Europe, is absent, replaced by a fairly typical +ve NAO pattern. Why was this, when so many indicators (-(very -ve QBO coupled with weak El Nino, positive Snow Advance Index, negative OPI [the latter index clearly needs a bit of work to say the least]) pointed towards a very blocked pattern? The clue, as is often the case, is in the stratosphere:
[attachment=263259:strat.png]
Much as it tried, and there was some pretty hefty wave 1 activity through the first half of winter, the SSW that we probably needed to fulfill the predictions simply never came through, and we ended up with a fairly strong Greenland based vortex. Part of that may be linked to solar activity - after a bit of a drop off in summer 2014 we again saw a peak during the winter [attachment=263263:solar-cycle-10-cm-radio-flux.gif]
This appears to have a particularly strong impact during the -ve phase of the QBO, where papers suggest the -ve QBO actually acts to enhance the solar signal and propagate westerlies northwards, strengthening the polar vortex ( [url="http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50424/full"]http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50424/full[/url] )
As Tony (Lorenzo as he's known on here) pointed out a month or so ago, taking an 'additive' approach to these variables(as I did last winter) may not be the most effective method, and I'll certainly take that into account this winter.
The other measure which seemed to buck the trend were the long range forecast models. I've long been sceptical of their performance, particularly of the CFS, but actually both it and the Met Office's GLOSEA5 model were pretty close to the mark. Given these were very much contradicting my own analogues last winter I will put a significantly greater weighting on their outlook for this winter's forecast. There will obviously be misses, but compared with the models' accuracy a mere 5 years ago when the Met Office's seasonal forecast had to be pulled after the 'barbeque summer' fiasco (and the CFS seemed to fare even worse overall) it looks as though recent upgrades have paid dividends.


The bit that, if you weren't paying that much attention to the overall NH picture and lived in Scotland, seemed like it might have made the forecast vaguely ok was the temperature - in spite of a lack of 'proper' high latitude blocking we managed to end up with temperatures around average overall (although a bit above in the south) and a number of snowfalls. These were mostly pretty marginal and were obviously nothing on the scale of 2009/10 or 2010/11 for longevity or depth, but given how long I'd personally been waiting for 'cold zonality' to deliver seeing charts like these was really quite fun:
[img]http://modeles.meteociel.fr/modeles/gfs/archives/gfs-2015011012-0-6.png[/img] [img]http://modeles.meteociel.fr/modeles/gfs/archives/gfs-2015011312-0-6.png[/img]
The second one gave me my first sighting of proper lying snow in central Edinburgh since March 2013.

Anyway, there's a good reason charts like those, which on the face of it don't look especially snowy, delivered to a number of low lying areas - this:
[img]http://www.ospo.noaa.gov/data/sst/anomaly/2015/anomnight.1.12.2015.gif[/img]
The cold Atlantic, while potentially acting to enhance those storms, inhibited temperatures sufficiently to turn what might otherwise have been cold rain into snow (though, for most of that period for most of central/southern England, simply turned mild and wet into cold and wet).
In comparison to 2013/14 the angle of attack of the storm systems was more favourable for cold, with the lack of blocking to our north allowing some proper Arctic air into the mix, and overall storms were less frequent with the jet tracking further north (though it hardly eased up much - see the horrific storm that hit Lewis on January 9th with 2 hours of sustained hurricane force winds), but there were definitely some similarities between the two for NW Europe.
So are we heading for a third straight winter where 'stormy' headlines dominate 'snowy' ones? Will we finally get the 'coldest (actually we really meant [s]wettest[/s] [s]windiest[/s] [s]sunniest?[/s]) winter in 1000 years' as the Express has been promising since just after the last genuinely cold one? Am I massively overhyping a forecast of a pretty average winter? All will be revealed in the next few days...

LomondSnowstorm

LomondSnowstorm

 

(Preliminary) Winter Forecast 2014/15 - Return to Cold?

[size=3]Before I get started on this I must at least explain last winter's forecasting failure on my part – I did indeed get the mild, wet start to winter right but the second half was so epically wrong that it more than offsets the relative successes of my previous few attempts. In my defence, the analogue charts weren’t that far off for late winter and in fact the cold trough over the US was correctly placed, but on this side of the Atlantic heights were never quite high enough west of Scandinavia to divert an incredibly strong jet far enough south for cold air to back west, though it was cold enough for over 9m of snow to accumulate on the top of Cairngorm at one point. Anyway, a few lessons from last winter:[/size]
[size=3]1) [/size][size=3]Sometimes strong wave 1 activity (as predicted by many before winter kicked off) can’t stop the Atlantic winning out, particularly when all the wave 1 activity did was to compress the vortex into NE Canada creating a turbo charged jet[/size]
[size=3]2) [/size][size=3]The OPI (read on to find out more about what this is) is good, and its creators did well at pointing to the overall hemispheric pattern pretty well, but the jury is still out on its ability to predict the AO – last winter came in neutral in spite of having the second highest OPI value in the series. It may well be that this was a one off, and given its remarkable hindcast correlation I am willing to give them the benefit of the doubt that this was a particular exception, but this winter will be a key tester for the OPI[/size]
[size=3]3) [/size][size=3]Analogue charts can be deceptive – trying to work out how our little corner of NW Europe benefits from low heights generally to the south and higher heights to the NE is a difficult task. January’s height anomaly was superficially similar to that of January 2013 but the outcome was completely different for most of western Europe.[/size]
[size=3]So, with those things in mind, what does 2014/15 have in store for us? I’m going to go through the factors I’ve taken into consideration (this may end up being even more lengthy than last year’s) and finally arrive at a forecast for the winter ahead broken down by month, so if you don’t fancy reading through the technical bits then scroll down to the bottom and get the ‘will it snow in my backyard on Christmas Day’ rundown (top tip – I don’t know the answer to that question and nor does anyone else, but this should at least give you a rough idea of what to expect).[/size]
[b][size=5]ENSO/QBO[/size][/b]
While last two winters have seen generally neutral conditions prevailing over the ENSO region, the expectation for this winter based on current conditions and the modelling consensus is that we’re likely to see a weak El Nino i.e. 0.5-1, developing over the course of the winter:
[attachment=228579:El Nino forecast.gif]
In addition, the Quasi Biennial Oscillation, which fluctuates roughly every two years between positive(westerly) and negative(easterly) is strongly negative:
[url="http://www.esrl.noaa.gov/psd/data/correlation/qbo.data"]http://www.esrl.noaa.gov/psd/data/correlation/qbo.data[/url]
What’s the significance of this to our winter? Well the current research suggests that the stratospheric vortex over the winter months is generally warmer during El Nino phases than La Nina, in particular when combined with a negative/Easterly QBO, and that Sudden Stratospheric Warmings (I’ll get into that a bit more later) are more likely during higher amplitude ENSO events than when ENSO is neutral, which means that the likelihood of cold air being displaced from the Arctic to mid latitudes is greater. To illustrate, and this will directly feed into my overall forecast, I’ve put together a list of analogue years for the expected –QBO and weak to moderate (0.5-1.5 MEI) El Nino winters, along with 1976/77, which was very marginally under the El Nino threshold for the DJF period. This is the height anomaly:
[attachment=228581:ENSO QBO anomaly.png]
As you can see, without even taking into account the more ‘experimental’ or recently developed indices, the signal for this winter is for strong blocking over Greenland and the Arctic with low heights across Europe, a classic –AO/-NAO pattern. Of the 7 winters, all 7 had overall negative Arctic Oscillations and 5 had negative NAOs, with the mean temperature anomaly for Scotland 1C below average and 16 of the 21 months were below the 61-90 average.
[size=5][b]October Pattern Index[/b][/size]
Last winter the October Pattern Index had just been developed and, while I used it in my forecast, it was still very much an experimental index. It remains in an experimental stage, with a paper on it due to be published after this winter, but at least we have a reasonable understanding of how it’s calculated. Last winter’s results were mixed – its creators nailed the specifics across the pond of an Alaskan Ridge and the vortex dropping into Eastern Canada but as a result of this the value of the AO was significantly lower than the OPI, although still positive overall. However, as a general measure of the difficulty of shifting the vortex it worked out well – the jet just wouldn’t let up, even when heights started to build to the northeast in January as anticipated, and given the significance of the correlation between the OPI and the AO in hindcast it would seem foolish to ignore this completely. Fortunately, it doesn’t particularly disagree with the prevailing consensus. If you want to read more on the OPI I’ve attached links to both the thread on it, with the original explanation along with some fine analysis on it, but essentially it views October as the critical month in vortex development and extrapolates that a strong, compact vortex in October leads to a strong, compact vortex, and +AO, over winter, and the same for a weak vortex. The OPI for this winter is [b]-2.08[/b], the second lowest value in a series going back to 1976/77. The lowest value ever? -3.4 in October 2009, just before the coldest winter since 78/79 in England, and the coldest since 1962/63 in Scotland, so as you’d expect very negative values correlate pretty well with the most brutal winters. Taking winters with OPI values below -1.5, and to extend the timeseries a bit I’ve also taken into account winters with an AO below -1.5, which essentially assumes that the AO/OPI correlation holds going back further than 76/77, gives this result:
[attachment=228580:OPI analogues.png]
Given the winters we’re looking at this isn’t a surprise but it is excitingly, or if you hate cold worryingly, similar to the ENSO/QBO analogue chart. The average AO for these winters is -1.8, with the highest AO value at -0.9 in 86/87, while 9 of the 10 featured negative NAOs, with 5 of the 6 lowest NAO values since 1951 being found in the list of winters.
It should also be noted, however, that [font=calibri][size=2]the Italians’ even more experimental new toy, the Indice de Zonalita Emisferico, disagrees somewhat with the 'consensus' so far, going for a very negative AO for December but positive January into February and coming out neutral overall. [/size][/font]
[size=5][b]Snow Advance Index[/b][/size]
Another tool, devised by Judah Cohen, is the Snow Advance Index (SAI) which is a measure of the increase in snow extent from the 1[sup]st[/sup] to the 31[sup]st[/sup] October. This has also been shown to correlate strongly with the AO, with positive SAIs generally leading to –AO winters and vice versa. This October, it appears we’ve had a very substantial gain in snow cover over the last month - October finished with a snow extent similar to 2009/10 in spite of a big fall in the last few days, with the SAI value up there with the largest values of Octobers 1976 and 2009.

[size=5][b]SSTs[/b][/size]
A brief overview of these – in spite of being in what appears to be a long term –ve Pacific Decadal Oscillation phase, the PDO this winter will be positive, which for the US means high pressure around the Pacific NW with colder conditions for the east and southeast with the jet tracking further south than normal. This, combined with solar activity being still reasonably high (although far lower than at this stage in most cycles), leads to the likelihood of a very strong jet once again this winter. However, a +PDO is not all bad – the last consistently +PDO winter was 2009/10, where the jet was displaced far to the south of the UK by persistent height anomalies over Greenland:
[attachment=228589:09-10 pattern.gif]
As for the Atlantic, we’re once again looking at a pattern of anomalous warmth for high and low latitudes with colder temperatures in the mid Atlantic, which may aid a -ve NAO, although it doesn't look like much of a factor either way.
SSTs are taken into consideration, with analogues (using an incredibly sophisticated system of looking at others' analogues for SSTs and having a look to see if it's similar to our current pattern) used to weight the sample to generate our composite charts.

[size=5][b]Solar Activity[/b][/size]
After a monster sun spot flared up last month, causing a spike which went against the general downward trend as we head towards the next solar minimum, we're back down to a more normal solar flux of 120 and sunspot number of 71 at the time of writing:
[attachment=228591:solar-cycle-10-cm-radio-flux.gif].
This means we're straddling the borderline between high and low sunspot activity. Generally speaking, colder winters are associated with solar minima, and in particular the chances of a Sudden Stratospheric Warming are greater with low sunspot activity combined with a -QBO (or high activity combined with a +QBO), so if you're looking for cold you'll be looking for solar activity to continue to fall over the next few months. This is also factored into my analogues, with analogue years with sunspot numbers between 110 and 40 being weighted more strongly.
[size=5][b]Stratospheric[/b][/size]
For those looking for a detailed explanation of why the stratosphere is so important to winter weather, look no further than the strat thread:
[url="https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/"]https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/[/url] I certainly don't claim to have as much expertise in this area as the likes of Ed, so much of this is based on rather crude statistical analyses using the FU Berlin strat site (and probably doesn't take enough account of early strat warmings earlier than 2009/10 which aren't listed as Canadian Warmings).
From my list of ‘strong’ December composites i.e. with 2 or more variables matching the expected pattern, 7 out of 12 winters, or 14 out of 22 winters when taking into account those that were double or triple weighted, featured either a Canadian Warming or bottom up wave 2 wavebreaking over Greenland (2009/10). This is the key to early winter – winters featuring this warming saw an average December temperature for Scotland of 1.5C (1.4C below average) and a CET of 3.7C(0.9C below average), but without it saw mean temperatures for Scotland of 3.1C and a CET of 5.2C. While in either case the chances of a below average January and February remain fairly high, it does has a major impact – analogue winters without a CW saw mean temperatures of 2.4C for Scotland for the winter as a whole, as opposed to 1.1C with. Basically, the November/early December strat, despite the favourable early signals, really does need to play ball and help us out with some lower-mid strat vortex disruption early doors. Without it, there still exists the potential for a very cold January and cold February (the sample size of winters with such favourable conditions are very low, and these tend to manifest later on in the winter), but our chances of getting a below average winter, particularly in Scotland, are vastly better if winter gets off to a decent start.
So how does this winter look? Well, rather interesting to say the least. The vortex has struggled to get particularly organised at any level, and it has generally been found towards the Eurasian rather than the Canadian side of the pole (very much unlike what we saw last winter). Moreover, there are already signs of vortex disruption, both from wave 1 activity propagating downwards stretching and compacting the vortex and also, potentially, from wave 2 activity from the troposphere. Today's ECM ensemble mean for D10 highlights this well [attachment=228593:EDH101-240.gif] This, combined with the other factors (which to be fair the analogue years already factor in, suggests that, to date, we're more likely to be in the first category of winters, one with a disrupted November vortex, than the second, where the vortex strengthens through early winter.
As for later on in the winter, the analogues, and most of the research papers on the strat forcings I've read, suggest an SSW mid winter is more likely than last winter, and if not formally an SSW at least something similar to it.

[size=5][b]Long range models[/b][/size]
I will mention these as this is yet another interesting test of how ‘good’ they are. However, I would very much advise anyone who takes these too literally to have a read of this: [url="http://www.esrl.noaa.gov/psd/events/2014/arctic-predictions-science/presentations/tue/arctic-wkshp-051314-cohen.pdf"]http://www.esrl.noaa.gov/psd/events/2014/arctic-predictions-science/presentations/tue/arctic-wkshp-051314-cohen.pdf[/url]
A compelling case for why the long range models are particularly bad at forecasting winters. Essentially, the (correct) modelling of warmer Arctic temperatures, particularly around the Taymyr Peninsula (expect to read a lot more about that in the coming months), is modelled as bringing milder weather to mid latitudes, when in reality there’s no mechanism for this and in fact what happens is colder air spills out into the Eurasian continent and across into Europe, with the same thing happening over the other side of the pole. They also appear to struggle with the stratospheric modelling, with some of the models not even extending as far up as the top of the stratosphere. These factors appear to have been responsible for the general misses by the long range models of 2009/10 and December 2010, so it's not clear that, if we are heading towards a cold winter this time, the models would be able to pick up on it.
Anyway, the signal from this is somewhat mixed, although more models are going for a milder winter than a cold one, with a generally +ve NAO and AO signalled by most, going against most of the factors listed above. I'll link to the two most commonly cited long range model based forecasts, the Met Office long range outlook the CFS [url="http://www.cpc.ncep.noaa.gov/products/CFSv2/CFSv2_body.html"]http://www.cpc.ncep.noaa.gov/products/CFSv2/CFSv2_body.html[/url] for reference.

[b][size=5]Background Factors Summary[/size][/b]
[size=5][size=4]Before we get onto the actual forecast I'm just going to sum up the signals for the coming winter:[/size][/size][list]
[*]-ve QBO and El Nino points towards a more blocked winter than average for the Northern Hemisphere and hence greater chance of a colder than average winter
[*]very -ve OPI points towards strongly negative AO and also a more blocked winter than normal, though this is still in its infancy
[*]very +ve SAI again points towards a negative AO and more blocked winter than usual
[*]SSTs generally point to a more active jet than normal, but also somewhat towards increased high latitude blocking
[*]So far the stratospheric, and tropospheric, vortex has struggled to become established and this looks likely to continue through November, potentially leading to a cold end of November/start of December
[*]Long range models still don't really buy any of the above, going for a milder than average winter with low pressure to the northwest, similar to last winter
[/list]
[b]Composite chart methodology[/b]
Using the criteria listed above I've compiled a list of composite winters. However, given the apparent divergence between December and the rest of winter the factors assessed for the December composites are somewhat different. As with last winter I've decided to use a broader composite matching than most, meaning that some months are double or even triple counted if they are very similar. For December, the composite months are as follows:
1958(x2), 1965, 1968 (x2), 1969, 1976(x3), 1979, 1984, 1986, 1993(x3), 2002(x2),2009 (x3), 2012 (x2)
For these months, the mean AO is -1, the mean OPI (for those months where an OPI value is available) is -1.6, the mean winter Nino3.4 value is 0.6 with the mean QBO at -10.1.
For January/February, the composite months are as follows:
2013, 2010 (x3), 1994, 1987(x2), 1985(x2), 1977(x3), 1970, 1969 (x3), 1966, 1959, 1952
For these winters, the mean OPI is -1.9, the mean winter AO is -1.7, the mean Nino3.4 value is -0.5 and the mean QBO is -10.1.


[b][size=5]A quick look at November[/size][/b]
I've already given a brief overview of how November is shaping up for the vortex in the strat section, but a slightly more detailed look is perhaps called for given that, in Scotland at least, the second half of November has the potential to behave very much like a winter month, and also because, if the OPI/SAI is right about this winter, November should look similar to other -ve SAI/-ve OPI Novembers.
So what are we looking for? The Cohen high SAI SLP anomaly for November doesn't seem to be publicly available, but the characteristics of it, by the account of those who have access to it, is very similar to this, the height chart for the six highest SAI values that I crudely calculated using the Rutgers monthly data: [attachment=228619:sai.png] Troughing to the northwest stretching to the UK with mean heights over Scandinavia/Western Russia (driving Wave 1 activity in the strat). The height anomaly for the 7 sub -1.5 OPI Novembers looks similar[attachment=228621:opi nov.png] What about this year? Well, this is the current 8-14 day height anomaly from the NOAA [img]http://www.cpc.ncep.noaa.gov/products/predictions/814day/814day.03.gif[/img]

Once again, reassuringly similar to the analogues above. The pattern itself isn't, immediately at least, a particularly interesting one for the British Isles - generally wet with alternating cooler and somewhat milder periods when heights manage to extend a bit further west or cool and dry when low pressure slackens off. But there is major potential down the line - the omnipresence of the scandi high combined with a vortex which, unlike last year, seems determined to stay as far from Greenland as it possibly can, leads to the possibility that by month's end we're likely to see an attempt at an easterly. While November 2010 was a ridiculously extreme example, I still think that there's a chance for us to see some widespread snowfall before the start of winter proper, and at the very least before Christmas. Overall though, I'd expect temperatures to be close to average given the mild start, with a CET around 6-7C and a Scotland temperature average around 5C, with precipitation well above average.
[size=6][b]Winter Forecast 2014/15[/b][/size]
[size=3][size=4]Caveats: as usual, the more specific I am the lower confidence it has. [/size][/size]
[size=6][b][size=5]December[/size][/b][/size]
[size=4]My analogue charts for the month are as follows. Heights:[/size]
[attachment=228638:dec comp.png]
Higher heights centred around Novaya Zemlya/Svalbard stretching across to Greenland and to the pole, giving at least a fighting chance for colder air to come at us from the east. Low heights around the UK suggests a very unsettled period, although whether that results in a snowy month or a very wet and windy one, or one where both feature in roughly equal measures, is hard to say.
Temperature: [attachment=228639:Dec temp.png]
You'll notice that, I think largely as a result of such a large sample size, the values aren't particularly large, but that the signal is generally for cold around our latitude, with the height of the cold sitting around Scandinavia/western Russia stretching across to Scotland, but with slightly above average temperatures for southern Europe.This is consistent with a cool, unsettled theme, with troughing too far north to deliver to southern Europe but potentially far enough south to deliver for our neck of the woods.
Precipitation: [attachment=228640:dec precip.png]
Again, consistent with higher heights over Scandi, troughing further south, lots of precipitation for the south/southwest of England.
[size=4]I'm expecting a month which is largely unsettled but, potentially, very cold, if we continue to see major vortex disruption in the stratosphere over the course of November. The month may well start with a cold spell from the east but I'm expecting most of early-mid month to be dominated by low pressure systems bringing a lot of wind and rain. Depending on how they're orientated and how far west high pressure can make it, we could well see some transient snowfalls, particularly further north and east, but the general pattern is for above average precipitation and average to somewhat below average temperatures, with occasional southerly winds bringing in the odd milder spell (and drier for the south of England). However, from mid month onwards pressure across the Greenland-Iceland-Scandinavia corridor is likely to rise, bringing in colder conditions. This transition is likely to take the form of a brief settled spell, with high pressure ridging up from the south to the Scandi high and eventually retrogressing to Greenland with a scandi truogh, to bring in a significant cold spell from the east and eventually north-northeast to end the month. During this period, snowfall would be expected to be above average, with any drier periods bringing some deep frosts, and if the timing is right we could well end up with a widespread white christmas. HOWEVER, and this is a big caveat, this outcome is dependent on things going the right way over the course of November - if the vortex does manage to recover by the end of this month a more likely outcome is an unsettled month without any prolonged deep cold, although with the potential for snowfalls from the northwest to Scotland or more widely further south when low pressure dives into the continent and with temperatures generally on the cool side. I'd put it at about 60/40 in favour of the colder variant at the moment(although of course it isn't in reality a binary choice), but I'll update towards the end of the month to let you know which way it's heading.[/size]
[size=4]In the colder variant we're looking at Scottish mean temperatures around 1.5C and a CET around 3C, with precipitation above average, moreso the further south you are, and perhaps below average for the northwest of Scotland, and a good amount of snow about. If the vortex isn't significantly weakened we're more likely to see a Scottish mean temperature of 2.5C, much closer to the average, with a CET 4-4.5C, with precipitation well above average everywhere and snowfall near average for most, but with the wind a major feature. Either way, not a very mild month with the prospect of at least some snow for most.[/size]
[size=4][b][size=5]January[/size][/b][/size]

Strangely, I think January is likely to pan out colder than average no matter how the early part of winter goes. If it's the colder setup in December, we'll simply see a continuation, perhaps even a deepening, of the cold, with the jet undercutting to give snowfall potential. If not, the analogue years would hint at a fairly abrupt switch to cold via a Sudden Stratospheric Warming.
Composite heights: [attachment=228641:January 15 height anom.png] Stonking positive height anomaly over southern Greenland, modest low height anomaly stretching from the eastern seaboard, strongest in NE France, -ve AO, with vortex remnants over central Russia - for snow lovers this is about as perfect a chart as you could wish for for January, pointing to a month dominated by winds somewhere from North to southeast.
Temperatures: [attachment=228642:January 2015 brrrrr.png] Bloody cold - a much more robust negative anomaly, once again centred around the Baltic extending right across Europe, and a clear >1C below average.
Precipitation: [attachment=228643:Jan precip.png] Below average across the Northwest Highlands and more generally for western Britain, closer to average elsewhere.
Stratosphere:[attachment=228644:strat temp jan 15.png] temperatures much above average, any sign of a stratospheric vortex is over towards the Pacific.
Basically a colder than average month, with an increased risk of snowfall. Biggest potential probably around mid month, but frankly difficult to pick a less cold period, other than there is obviously likely to be one at some point. 1/2 of the analogue months give sub 1C Scotland mean temperatures, and only 2 are above average - 1969 and 2013, and for the CET zone 60% came in sub 3C (a modern benchmark for a particularly cold month), with this time Januaries 1994 and 1969 coming in as the only above average months. In 1969 we saw for long periods what I think the main danger for January/February not delivering major cold is - a west based -ve NAO, where the pattern is too far west, resulting in the UK getting stuck in a mild, wet oasis among a sea of mid latitude cold. Of course in both winters this frustrating period was made up for with severe cold a bit further down the line, with February 1969 an exceptionally cold month, moreso in Scotland where proper Arctic northerlies swept the country, and March 2013 being colder than any of a not too mild winter 2012/13 and seeing all precipitation in Edinburgh from about the 5th March through to the start of April falling as snow. My CET punt would be 2C, although with the potential for it to end up quite a bit lower, particularly if we are starting from a cold point in late December, with the Scotland temperature average a spectacular 0.5-1C.
[b]February[/b]
In many ways February has a very similar look about it to January - only 1, February, 1959, in the analogue list comes in above the 81-10 Scotland average and only 3 do for the CET zone. However, a look at the height anomaly gives a subtle clue as to why February may not be shaping up to be quite as cold as January:[attachment=228664:feb 2015 height anom.png]
The low height anomaly over the Atlantic has strengthened and moved further north, pointing to a more active jetstream, with the potential for mild incursions from the southwest along with possible battleground snowfalls. The temperature anomaly still suggests that below average temperatures are likely to persist but the 0C anomaly line is significantly further north than in January [attachment=228665:Feb 2015 temp.png] Precipitation for southern and central England looks to be above average once again but potentially below average again for the NW Highlands: [attachment=228666:feb precip.png]
The stratospheric temperature perhaps gives an indication of what might be causing the jet to power up - a suggestion of the vortex drifting towards Western Greenland for the first time [attachment=228667:feb strat.png]
Generally, this month looks to be characterised by a north/south split - Scotland seeing temperatures remaining well below average with snowfall inland from a variety of setups, with a mix of northerlies and battleground setups, while south of the border, and moreso for the Midlands southwards, it will feel cold but likely end up with a wet and windy month dominated by storm systems. The boundary line is fluid, and means that we could either end up with a month which trends towards milder weather away from the Highlands, or in fact the boundary could shift a bit further south and leave more of us with the cold, but this looks the most likely solution to me. Predicted temperatures will reflect this shift, with a CET around 3.5 but a Scottish mean temperature of 1-1.5C.

I won't do a forecast for March in full but all I'll say is that it looks pretty cold as well at this stage, and if you're looking for your snow fix and it hasn't panned out by February there's still potential for March to deliver something interesting.

So, in summary, a return to cold winters is expected after last winters mild 'blip'. Strong signs for a negative AO which will at the very least push the jet far enough south to make things interesting and with the potential for some really classic spells of cold and snow, particularly but not exclusively for January, and moreso for Scotland. Expect another good ski season, although one more likely to see useable snow (and probably not 9m this time) and the potential for big snowfalls almost anywhere, but at the same time there's always likely to be a fair chance of a breakdown, and the further south you go the more likely you are to end up with a lot of rain. The mean winter CET on my projections would be somewhere around 2.7-3.5C, so cold but perhaps not extremely so(somewhere between 1985 and 2008/09, most likely on a par with 95/96 or 10/11), with the Scottish mean temperature around 0.8-1.8C, which would put it somewhere between 1976/77 and 95/96, potentially in the top 10, or even the top 5, coldest winters since 1950.
I'm planning to update this by the end of the month when the early winter prospects are, hopefully, a bit clearer, maybe even a video update if I can work out how to do this adequately, but for now I hope you've enjoyed reading this.

LomondSnowstorm

LomondSnowstorm

 

Winter Forecast Recap/Update

With December nearly over (and with me having some spare time on my hands for once) I thought it would be a good time to assess my winter forecast so far and give my thoughts on where we're likely to head weatherwise in the New Year.
This was my NH composite forecast for the general height pattern for both the troposphere and stratosphere for December, along with my own adjustments to the composite maps:[quote]
[size=5][b]December[/b][/size]
[url="http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-31045200-1383526045.jpg"][img]http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-31045200-1383526045_thumb.jpg[/img][/url]
For December, the Aleutian Ridge and negative height anomalies over the Western side of the Arctic are the most notable features, but otherwise the anomalies are fairly muted, with a positive anomaly out in the mid Atlantic and actually a slight mean trough over Europe.Half of these winters featured either a Canadian Warming or SSW in the first half of winter, with 8/10 featuring a notably cold stratosphere initially, so this may well be the key to our winter once again. The Stratosphere composite looks like this for December:
with a very strong West Greenland centred vortex, but already there are signs of weakness creeping in from the Eurasian side.
[url="http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-21243900-1383526044.jpg"][img]http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-21243900-1383526044_thumb.jpg[/img][/url]
[b]Adjustments[/b]
Given the SST pattern I’d suggest that the mean height anomaly for February is centred somewhat further west over Greenland, while low heights south of 60 degrees north are likely to be confined to southeast Europe through December.[/quote]
Although the month isn't yet over, the anomaly to the 23rd is likely to be broadly indicative of the final anomaly, albeit with a less emphatically strong negative anomaly for north of the GIN corridor, a less strongly positive anomaly across continental Europe (perhaps neutral to negative for the British Isles into Northern France) and a more neutral anomaly for western and central Russia:
[attachment=200213:compday.jopyVF2xYl.gif]
Bearing in mind the likely movements listed above and also the correction suggested in the original forecast, the December anomaly was actually quite a decent match, picking up on the mean ridge across the mid Atlantic to the southeastern US, a strong Aleutian Ridge, low heights into the central portion of the US and generally low heights directly to our north. As for the stratosphere, bearing in mind positive height anomalies around SIberia over the next few days, the vortex positioning anomaly was also pretty reasonable all things considered:
[attachment=200259:strat anomaly Dec.gif]
So how about the actual text forecast?[quote]
A very zonal period to start the month following a short, sharp Arctic blast at the end of November, with west-northwesterly winds and very limited blocking. Near constant low pressure systems will bring wind and rain throughout, with only brief drier interludes. Temperatures above average in England and Wales but near or even slightly below average for Scotland, where colder upper air temperatures will bring the odd smattering of snow even to lower levels in spite of a lack of frost, with precipitation above average initially everywhere.
From around the 12[sup]th[/sup] onwards things will quieten down, with high pressure building in from the south, bringing a brief spell of very mild southerlies followed by a dry and eventually frosty spell in the run up to Christmas. Temperatures above average everywhere up to the 20[sup]th[/sup] but cooling down towards average from the south as heights transfer northwards, precipitation generally below average away from the far northwest Highlands where they’ll be around average. By Christmas, heights will transfer westwards with an initial spell of rain followed by a genuinely cold northerly blast as low heights dive temporary southeastwards, bringing more widespread snowfall and low temperatures, although with accumulation generally confined to the usual spots (which of course vary depending on the exact wind direction) before the dam breaks and the heights sink once more by month’s end. Temperatures will be generally above average for most of England and Wales, with an initial CET punt of 5.6C, the Scottish mean will sit around average at 3.4C. Precipitation will be marginally above average for all of Scotland and much of northwest England but for southern England and Ireland it will be around or below average. In other words, a fairly typical December.[/quote]
I think, all in all, a decent enough forecast, somewhat underestimating the storminess around the 13th-27th and maybe overestimating the cold potential for Scotland but generally a reasonable account of the month. The CET and Scottish mean temperature punts both look a bit on the low side but moreso for the Scottish Mean, which looks like it'll come in around 5C, while the CET is currently sitting around 6.7C.
So how's the forecast looking for the rest of winter?
Well, after having had a positive Arctic Oscillation for the entire month of December we may be looking at the first hints of a more blocked NH profile with the AO forecast to go negative for the first time this winter:
[img]http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.sprd2.gif[/img]
We can see how this materialises from the 8-14 day height anomaly forecast chart - positive anomalies across the pole centred most strongly over Alaska with a mean UK trough flanked by positive anomalies over Western Russia and the Azores:
[img]http://www.cpc.ncep.noaa.gov/products/predictions/814day/814day.03.gif[/img]

What does this mean for us? Well, it's a slightly messy setup but ultimately one where a broadly westerly pattern will prevail into the first week of January. However, that doesn't quite tell the whole story, as, if the height rises over the Arctic verify, we will be looking at a generally cool regime with transient blocking both in the Mid Atlantic and over Russia providing temporary cold snaps and battleground snowfalls as the month progresses (albeit the details of this is very sketchy and it's fine line between these, cold zonality and regular zonality). Looking even further out, the ECM 32 dayer along with MOGREPS appear to back up this generally unsettled but with a hint of something more seasonal at times into mid month, and (apparently) even as far as the end of January. In addition, both the MJO and AAM look to be giving a slight helping hand to more amplified solutions and this will also give some encouragement to those looking for snow (the favourable phases at this time of year are the 7-8-1 wheel).
[img]http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/UKME_phase_23m_small.gif[/img]


My original LRF for January broadly backs this outlook up for the first 20 days but indicates the possibility of major stratospheric disruption occuring around mid January which could potentially usher in a much colder pattern.At the moment, however, while there is some reasonable warming in the 'surf zone' forecast, it doesn't look nearly enough to actually force an SSW - for the first half of January at least. I still feel that we are looking at a winter of two halves (although if the OPI wave forecast is anything to go by we may have to wait a while before we see anything meaningful) and there have been some increasingly encouraging teleconnection forecasts as we head into the New Year, so I'm broadly sticking to the original for now, with the caveat that an SSW, if it occurs at all, is unlikely until the very end of January into February at the earliest.

LomondSnowstorm

LomondSnowstorm

 

Lomond Snowstorm's (epic novel) Winter Forecast 2013/14

Well here we are again folks, that special time of year when the anticipation of Christmas is matched only by the joy of the now annual tradition of the LomondSnowstorm winter forecast. Last winter promised much and delivered some, although on the ground it was generally more of a slushy hinterland than a winter wonderland. So can we continue the trend of generally below average winters which have developed in tandem with a more southerly tracking jet, or will the glorious summer of 2013 usher in another trend bucking warmer than average season? In this forecast I’m going to look at a number of factors, including the El Nino Southern Oscillation, Global Angular Momentum, the Quasi Biennial Oscillation, Arctic Sea Ice, Eurasian snow extent/autumnal gain and Solar Activity to put together a forecast for what the winter of 2013/14 has in store for us. Once again, the usual disclaimer goes out about the low confidence nature of the more detailed parts of the forecast, but it’s good fun and paints a picture more readily for those who don’t have a particularly strong meteorological background.
[size=6][b]ENSO[/b][/size]
The El Nino/La Nina state, an index of the Sea Surface Temperature anomaly in the Eastern Pacific, has been shown to be critical to the global circulation pattern, so I’ll address if first. At present, as it has been for about a year now, we’re in a neutral phase (between 0.5 and -0.5 amplitude), and the model outlook suggests that it will stay that way throughout the winter:
[attachment=191357:enso.jpg]


Neutral ENSO winters have been shown to have SSWs less frequently than in those with either a clear La Nina or El Nino state (Butler and Polvani, 2011) which would indicate a lower chance of Polar vortex disruption and therefore a higher chance of more ‘normal’ winter conditions than we saw last year. However, neutral/weak ENSO values are also associated with a greater frequency of –ve NAO conditions, which is strongly correlated with below average temperatures.
[size=6][b]QBO[/b][/size]
The Quasi Biennial Oscillation is another key factor to consider, with easterly (negative)QBOs encouraging a weakening of the Polar Vortex and westerly QBOs a strengthening. However, it is more complex than that, and a combination of a Westerly QBO and a peak in Solar activity has been shown to increase the probability of a Sudden Stratospheric Warming Event. Currently, the QBO is in a Westerly state and will remain so (given that it’s a two year cycle) for the rest of the winter:
[attachment=191369:QBO.jpg]
[size=6][b]Solar Activity[/b][/size]
This is one of the hottest (if you’ll pardon the pun) areas of meteorological research at the moment, and along with the Stratospheric developments this has contributed a huge amount to the advancement of medium range forecasting in recent years. The interest began when the historically cold winter of 2009/10 occurred during a deeper and more prolonged sunspot minimum than we’ve seen in 100 years:
[attachment=191377:sunspot.jpg]
The inevitable upturn in the 11 year cycle has taken place since then which takes us up to near the peak of the cycle this winter, although it’s a peak which is far more akin to those of the freezing Dalton minimum years than those of more recent years, . Nonetheless, remember the linkage between Solar Activity, West QBO and an increased prevalence in SSWs, because it may be critical to the coming winter.
[size=6][b]Arctic Sea Ice Extent[/b][/size]
For the unitiated this one might seem a bit strange, but in fact there is a reasonably strong basis for [i]lower [/i]Sea Ice extent being associated with an increase in the prevalence of blocking highs and a subsequent decrease in average temperatures across western Europe. As anyone who pays much attention to environmental news knows, Sea Ice in the Northern Hemisphere has been at historical (at least during the post 1979 satellite age) lows in extent. However, this year has seen something of a comeback for the ice, with the extent so far for the year being the highest since 2006:
[attachment=191371:sea ice.jpg]
Still, it remains below the long term average so this can be tallied up as being a slight mark in favour of increased blocking. October [size=6][b]Eurasian Snow Cover/Snow Gain[/b][/size]
This one makes perhaps more sense, although again perhaps not for the reasons you’d imagine – increased snow cover in October and November across the Eurasian continent causes low ground temperatures which leads to stagnant rising air and hence high pressure systems to form across high latitudes, which leads to a weakening of the vortex from below and the displacement of cold air southwards to our latitudes. After an exceptional end to September for snow gain, the uptake since then has been less spectacular, with the ‘Snow Advance Index’ for October being somewhat below the long term average, although we’re still sitting hemispherically at above average once again thanks to recent gains in the Eastern US.
[attachment=191373:Snow cover.jpg][attachment=191372:snow cover 2.jpg]
Overall then this index is neutral, with the gain being far less spectacular than it could have been but partly because of a record high starting point.
[size=6][b]North Atlantic SSTs[/b][/size]
The feedback between Sea Surface Temperatures in the North Atlantic and the North Atlantic Oscillation, or in other words blocking over our side of the Northern Hemisphere, was shown to have been largely responsible for the record cold December of 2010 (Met Office, 2013). The key signature of a negative NAO is a tripole in the North Atlantic, with a cold section around 30-50 degrees latitude just off the coast of Eastern Canada, with warmer sections to the north and south:
[attachment=191376:SST correlation.jpg]
The signal is rather muted but a tripole of sorts has been showing in the North Atlantic in recent weeks, which is a fairly promising sign, although we need to watch to see if this sustains and develops further.
[attachment=191375:SST 2014.jpg]
[size=6][b]Arctic Oscillation tendency[/b][/size]
Two recently developed indices of snow cover and height anomalies respectively have an exceptional record of predicting the Arctic Oscillation tendency - Cohen’s Snow Advance Index, which as previously discussed is indicative of a moderately positive Arctic Oscillation, and the very recently developed October Pattern Index, which can be thought of as the atmospheric effect of the SAI, which is actually at its highest value since 1991/92. Anyway, given the agreement between the two and the high predictability both have, I’d put our chances of seeing a +ve Arctic Oscillation for the winter at 90% plus. This doesn’t necessarily preclude a below average winter, and is less reflective of temperatures at our side of the pond than the North Atlantic Oscillation, but hemispherically it points towards high latitude blocking being generally scarce and does reduce our likelihood of a cold winter. However, this does not imply a permanently positive Arctic Oscillation, and we should bear this in mind.
[size=6][b]Long Range Forecast models[/b][/size]
These have been fairly mixed, with their output mid-October on the back of large initial snow gains being very much favourable to cold and snow at mid latitudes, but almost all of them, including the GloSea and last week’s CFS average, call for a milder than average winter. However, these are generally pretty volatile, don’t handle the Stratosphere particularly well (with the exception of the EC long range forecasts which was impressive last winter) and some also include a positive height anomaly across Greenland, which is of course very much linked to below average temperatures across the British Isles. While their inputs are valuable, I’d be impressed if any of them were able to pick up on a major pattern change more than a month out given the current and near future strength of the vortex.
[size=6][b]Global Angular Momentum[/b][/size]
Finally, we have GLAAM, effectively the ‘turning force’ due to the Earth’s rotation, frictional torque, Mountain Torque etc. This broadly follows the pattern of the ENSO, although there is a bit of a chicken-egg debate going on regarding which causes which. Anyway, with the slightly negative ENSO values this index has also been tracking negative but it does look likely to increase towards a positive state as the winter goes on, which could be key in disrupting the vortex
[size=6][b]Composites[/b][/size]
Having looked at years from 1958 onwards (prior to that the datasets become sketchier and it’s tougher to pattern match) and basing on the criteria outlined above I’ve come up with 11 composite winters which broadly match the likely winter ahead. These were all either ENSO neutral or weakly positive/negative (value less than 1), featured positive QBOs and are ‘weighted’ towards those which are more similar. While some years with negative Arctic Oscillations were counted, years were double weighted based on the Arctic Oscillation value given the likelihood of a positive one this year. The most similar winter was 1990/1991, which is counted four times in the composite charts, with 1971/72, 1980/81, 2001/02 and 2008/09 counted twice and the rest just once. The full list of years are:
2008/09, 2001/02, 1990/91, 1985/86, 1980/81, 1978/79, 1971/72 ,1966/67, 1961/62 and 1959/60.
[size=5][b]December[/b][/size]
[attachment=191365:December height.jpg]
For December, the Aleutian Ridge and negative height anomalies over the Western side of the Arctic are the most notable features, but otherwise the anomalies are fairly muted, with a positive anomaly out in the mid Atlantic and actually a slight mean trough over Europe.Half of these winters featured either a Canadian Warming or SSW in the first half of winter, with 8/10 featuring a notably cold stratosphere initially, so this may well be the key to our winter once again. The Stratosphere composite looks like this for December:
with a very strong West Greenland centred vortex, but already there are signs of weakness creeping in from the Eurasian side.
[attachment=191364:Dec the strat.jpg]
[size=5][b]January[/b][/size]
Onto January and we have a very different looking anomaly – Greenlandic height rises come into play but with a very strong negative height anomaly stretching right across from NE Canada into southern Europe. Again, this is very far from clearcut, with evidence of some vortex disruption particularly in our neck of the woods but still with negative or neutral height anomalies across the pole:
[attachment=191368:Jan the strat.png]
A look at the stratospheric picture once again may help to enlighten us further – the stratospheric vortex looks to end up split over the North American and Siberian sectors with a mean strat. ‘ridge’ over Europe and the Atlantic:
[attachment=191367:jan strat.jpg]
Given the predicted height anomalies it does look more like a split than a displacement SSW (and in fact there are a few ‘hints’ of the Cohen tropospheric response model in the anomaly) if it comes off, with the timescale being somewhere around mid January. Last winter, bizarrely, may end up being a reasonable analogue for January, although given an almost polar reversal of tropospheric conditions it is likely to play out somewhat differently, with the tropospheric vortex more likely to recover.
[size=5][b]February[/b][/size]
With the recovery of the tropospheric vortex over the Arctic, it then remains a question of how long and whereabouts the blocking can hold on for. Perhaps unsurprisingly given the initial split, it appears that the Eurasian-Atlantic sector may well continue to see the effects of the split vortex well into February:
[attachment=191366:feb height.jpg]
Once again, a mean Euro trough is evident with the main area of blocking from Iceland to Scandinavia while globally we do appear to return towards positive Arctic Oscillation values. At the stratospheric level, things are somewhat different though – the stratospheric vortex does not appear to recover fully even while the tropospheric one rebuilds, which again suggests potential difficulties for snow lovers afoot with propagation. Still, if it turns out similarly to the composites shown we should have a decent crack of the whip before we see a breakdown of the pattern.
[attachment=191370:r2Udu38dtx.png]
[b]Adjustments[/b]
Given the SST pattern I’d suggest that the mean height anomaly for February is centred somewhat further west over Greenland, while low heights south of 60 degrees north are likely to be confined to southeast Europe through December.

[size=7][b]2013/14 Winter Forecast[/b][/size]
[b][size=5]December[/size][/b]
A very zonal period to start the month following a short, sharp Arctic blast at the end of November, with west-northwesterly winds and very limited blocking. Near constant low pressure systems will bring wind and rain throughout, with only brief drier interludes. Temperatures above average in England and Wales but near or even slightly below average for Scotland, where colder upper air temperatures will bring the odd smattering of snow even to lower levels in spite of a lack of frost, with precipitation above average initially everywhere. Perhaps something along the lines of this:

From around the 12[sup]th[/sup] onwards things will quieten down, with high pressure building in from the south, bringing a brief spell of very mild southerlies followed by a dry and eventually frosty spell in the run up to Christmas. Temperatures above average everywhere up to the 20[sup]th[/sup] but cooling down towards average from the south as heights transfer northwards, precipitation generally below average away from the far northwest Highlands where they’ll be around average. By Christmas, heights will transfer westwards with an initial spell of rain followed by a genuinely cold northerly blast as low heights dive temporary southeastwards, bringing more widespread snowfall and low temperatures, although with accumulation generally confined to the usual spots (which of course vary depending on the exact wind direction) before the dam breaks and the heights sink once more by month’s end. Temperatures will be generally above average for most of England and Wales, with an initial CET punt of 5.6C, the Scottish mean will sit around average at 3.4C. Precipitation will be marginally above average for all of Scotland and much of northwest England but for southern England and Ireland it will be around or below average. In other words, a fairly typical December.
[size=5][b]January[/b][/size]
I’m anticipating a switch around in January, where transient colder snaps brought about by an excessively strong vortex are replaced by a more wintry pattern. Nonetheless, it won’t start off that way – more bog standard Atlantic frontal systems will dominate the first half, with temperatures and precipitation widely above average. However, as the month progresses, mid latitude height rises will ridge northwards, with the jet finally being diverted southwards with colder air encroaching from the east. With still a large chunk of the vortex situated over Canada the height rises will take a week or two to become properly established, with a number of transitional snow-rain-snow events interspersed with more settled milder days, and temperatures will generally be around or slightly below average, but eventually, by around the 25[sup]th[/sup], a cold easterly flow will be established, ushering in one of the main cold spells of the winter. With the centre of the high between Iceland and Scandinavia rather than further east and with still a fairly impressive cold pool over the Arctic the UK could tap into some severely cold uppers if the setup works out. This would bring a period of very low maxima, perhaps sub zero, with the potential for significant snowfall right across the British Isles, but particularly for areas exposed to the easterly wind (including the Forth-Clyde streamer area) where showers would merge into longer periods of snow, although with low heights anywhere in the south of England could see some impressive snowfall totals, perhaps upwards of 8 inches quite widely IF we tap into the cold pool before the flow is cut off. (Note: the timing and severity of this event are very much low confidence, so I’d wait until at least early January before stocking up on tinned goods). Temperatures for the month as a whole will be below average but not massively so – a CET of 3.1C and a Scottish mean of 1.8C are my current bets. Precipitation totals generally around average, with the exception of the Western Isles and Northwest Highlands where it will be somewhat drier than average and southern England where it will be above.
[size=5][b]February[/b][/size]
With the easterly flow cut off by the start of the month as heights lower from the north a brief spell of anticyclonic weather will prevail through the first part of February, away from the far south of England where the odd snow shower may remain. Temperatures at the surface would be well below average even without the upper cold pool as the snow fields caused minima to drop like a stone under clear skies. With the southerly arm of the jet still dominant, and the vortex still not really managing to get a foothold east of Canada, a reload of the cold looks likely, with a weak Scandi trough/ weakfish southeast Greenland high providing a possible route, propped up by the strong southerly jet. This would give more snow to eastern parts, with Aberdeenshire in particular taking a bad hit, and temperatures way below average once again. By mid-month the pattern will look to shift westwards, with height rises over Western Greenland, leaving us in a west based –NAO state (think February 2010) with a rather messy cold trough, bringing a rather dour mix of snow, sleet and cold rain from a variety of wind vectors before eventually height rises over southeastern Europe build northwestwards and introduce a milder flow to end the month.
Precipitation once again above average, although moreso in eastern parts, with temperatures very much suppressed until the very end. My CET punt is a very cold 0.8C, the coldest February since the sub zero 1986 (one of the ‘lesser’ composite years incidentally) with the Scottish mean a positively balmy 1C.
I hope I haven’t bored/scared you too much, and I’ll be looking to update it throughout the winter and give an honest assessment of if/when it goes completely bust and we end up with a heatwave early February. I’ll be updating this in the next few days too with one or two synoptic charts which maybe give a better representation of my thoughts than the description does.

LomondSnowstorm

LomondSnowstorm

 

Factors affecting a lack of snowfall in Embra city (blogged from the thread, for future reference perhaps)

[quote name='LomondSnowstorm' date='13 February 2013 - 16:14 ' timestamp='1360772050' post='2607974']
[quote name='by-tor' timestamp='1360771157' post='2607928']
No entirely sure I understand all the factors in the Edinburgh snowshield but it definitely exists. We came back down the A9 from Boat of Garten earlier, heavy snow in poor visibility almost all the way until we got close to the north side of the Forth Road bridge. On the Embra side, visibility vastly improved, rain, and even on the higher parts of the bypass, in some places snow was completely absent.

Some kind of recurring geographical, topographical, micro-climatological thing going on there but whatever it is, its bloody effective at turning snaw into naw.
[/quote]
I think it's a mix of factors, including:
1) Urban Heat Island Effect
2) Coastal modification off the Forth and the North Sea during an easterly flow
3) topographical issues - being in the lee of the Pentlands and the Lammermuirs means precipitation totals are significantly lower than the surroundings with southerly wind vectors.
4) The same issues with northerlies that the rest of the central belt and mid Scotland has aka the Grampians.
5) proximity to sea level, certainly for the middle of the town and up towards Leith and Western parts too to an extent.
Its site may have done a good job of keeping the English out during the middle ages but it also does a very good job of keeping the snow out. Somewhere between an east and northeasterly wind is probably the ideal setup for here, which is actually the same for Freuchie, but without some of the added hang ups listed above.
[/quote]


Source: [url="http://forum.netweather.tv/topic/75835-scotland-regional-discussion-050213-08/page__view__findpost__p__2607974"]Scotland Regional Discussion 05/02/13 08 -------->[/url]

LomondSnowstorm

LomondSnowstorm

 

Lomond Snowstorm's Winter Forecast 2012/13

Well it’s that time of year again when I, and what seems like half the world, publish their thoughts on the coming winter. Many of us have already had a taste of winter, with fairly significant snow down the east coast, away from the now famous snow shadow here in central eastern Scotland. The northern hemispheric pressure patterns that delivered that snowfall and which have been showing up for some time now consistently on the models has borne a striking resemblance to those of 09/10 and early 10/11, with the PV significantly disrupted and blocking prevalent towards Greenland. This has, justifiably, prompted many to ask if this is a harbinger for the coming winter, and whether this is likely to become the defining theme of the winter of 2012/13. This forecast will look holistically at most of the major known drivers of Northwestern European winters, both longer term global dynamics such as ENSO, the QBO and GLAAM along with more localised leading indicators such as Snow Cover, Stratospheric Temperatures and the current Arctic and North Atlantic Oscillations to give a detailed forecast for the winter ahead.
[b] El Nino Southern Oscillation[/b]

Most people should be relatively familiar with the concept of El Nino/La Nina, the temperature anomaly of an area of ocean in the Southern Pacific that has been found to be a major driver of seasonal and annual climatic patterns around the world. We are currently at the very weakest possible El Nino state, with forecasts indicating that ENSO is likely to become neutral, perhaps more likely positive than negative, but essentially such a weak signal indicates that this will not be a major driver this winter.
[img]http://iri.columbia.edu/climate/ENSO/currentinfo/SST_table.gif[/img]

[b] Northern Hemisphere Snow cover[/b]

The relationship between the extent of Autumn snow cover across the Northern Hemisphere, in particular Eurasia, and Northern Hemispheric pressure patterns has been well documented.
[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image004.jpg[/img] [i]Correlation between October snow cover and Sea Level Pressure as a spatial anomaly map. The bright oranges over the poles indicate that greater extent of NH snow cover is correlated with higher Sea level pressure over the poles and therefore greater blocking[/i]
[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image006.jpg[/img]
[i]Spatial correlation between October NH snow cover and 30mb Stratospheric Temperature (more on stratospheric temperatures later). Again, There is a significant positive correlation for the poles and negative one for southern latitudes, which again shows an increased tendency for upper latitude blocking in winter following Octobers with greater snow cover extent.[/i]

So what does that mean for this winter? Well this October’s snowcover extent has been the 8[sup]th[/sup] snowiest in the Northern Hemisphere in the last 45 years, being above average both across Siberia, Scandinavia and North America, after a September that was 36[sup]th[/sup] out of 44 years. This rapid gain of snowcover is a very strong indication of a continuation of the predominance of high latitude blocking over the last 3 or 4 winters, which of course is a strong indication of a colder than average winter.
[b] Global Angular Momentum[/b]

This is probably the most complex index covered here and one which has only relatively recently been widely used in longer range forecasting (an explanation of GLAAM can be found here [url="http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281971%29028%3C1329%3AGAMBET%3E2.0.CO%3B2"]Newton, C 1971[/url]). Currently, Global Angular Momentum is in a negative state and, in part due to the weak or slightly negative ENSO state, is expected to remain negative.
[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image008.jpg[/img]




[b] QBO[/b]

The Quasi Biennial Oscillation is another key tool in our armoury, as its state fluctuates between easterly and westerly over a roughly 2 year period. We are currently in a negative (easterly) phase, which peaked around August and is starting to decline.
[img]http://www.cpc.ncep.noaa.gov/products/CDB/Tropics/figt3.gif[/img]

A negative QBO tends to lead to a weakened Jetstream, and is perhaps another indicator of a more blocked pattern this winter.
[b] Current Weather Patterns[/b]

As most of us are probably aware of, this Autumn has featured some particularly potent northerly blocking, with the NAO and AO both being persistently negative throughout October:

[img]http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.sprd2.gif[/img]
[img]http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif[/img]

In particular, there is a strong correlation between the Autumn Arctic Oscillation and winter pressure patterns, which suggests that the current prevalent blocking over the northern hemisphere is likely to persist into the winter months:




[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image014.gif[/img]
[b] Stratospheric Temperatures:[/b]

Developments in this field have probably contributed the most to improvements in seasonal forecasting over the last few years. Currently, upper latitude stratospheric temperatures are below average and cooling quickly:
[img]http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/30mb9065.gif[/img]
Given that generally cooler stratospheric temperatures at upper latitude are associated with a stronger Polar Vortex and therefore a more zonal westerly flow, this does not seem especially encouraging news given the high weight placed on the stratosphere in recent years. However, given the background of an easterly QBO and low Angular Momentum I would expect the stratospheric situation to change as the winter progresses, especially given that highly vaunted Sudden Stratospheric Warmings tend to occur when temperatures have effectively ‘bottomed out’.
[b] Model Forecasts[/b]

There is no strong signal either way in terms of temperature anomalies across northwestern Europe according to the CFS forecasting model.
[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image018.jpg[/img]More interestingly, and something which I haven’t as yet touched upon, is the rainfall anomalies, as there is a reasonable signal for above average rainfall, which would suggest a more Greenland or mid Atlantic based blocking rather than further east. [img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image020.jpg[/img]

[b] Forecast:[/b]

This is based on years which are similar to the consensus view of the key drivers outlined above:

[i]Temperatures are expected to be near or below average for northwestern Europe, though there is a strong signal for a very cold January:[/i]
[attachment=144301:winter 2013 temperature.png]
[b]Precipitation:[/b]
[attachment=144302:2013 precip.png]

[img]http://www.esrl.noaa.gov/psd/tmp/composites/188.74.99.104.315.18.51.28.png[/img][b][i]This is suggestive of lower than average precipitation for northern parts and greater than average for the far southwest and into southern Europe, which is consistent with a continuation of a southerly tracking Jetstream.[/i][/b]
[b][i]Pressure patterns[/i][/b]
[attachment=144303:heights.png]
[img]http://www.esrl.noaa.gov/psd/tmp/composites/188.74.99.104.315.18.52.26.png[/img]
[b][i]A very strong signal for a negative AO/NAO, with low pressure centred over southwestern Europe and blocking across the arctic.[/i][/b]
[b]Textual Forecast (warning: excessive precision is largely for descriptive purposes and I won’t be held to account for snowfall being a day or so late)[/b]
[b]December[/b]
[b]I’m expecting another early start to the cold, with strong blocking to set up sometime around the 2[sup]nd[/sup] week of December. The first week is likely to be fairly unsettled, with temperatures around or slightly below average, and precipitation largely above average. Initially, blocking is more likely to be centred towards the west, with a fairly potent three day northerly around the 8[sup]th[/sup]. A period of a slacker north-northwesterly flow will follow, bringing fairly cold but mostly dry conditions, before a shortwave drops out of the Arctic. Temperatures in this period will be well below average and there will likely be widespread snowfall across northern and eastern parts of the British Isles and another more transient one further south as the jet tries to undercuts the block. The snow is likely to relent somewhat as the high drifts southwards and a drier, milder spell is possible as the block sits just to our west, with rainfall and some hill snow towards northern Scotland. However, around Christmas look for the blocking to reform, leading to a more sustained period of northeasterlies as blocking establishes properly over Greenland. This should be a fairly snowy period, with at least one major snowfall affecting the bulk of the country.[/b]
[b]Temperature: With the source of cold generally being northerly rather than easterly the cold will be centred further north and east, though it will still be colder than average across England .Scottish Mean: 1.8C CET 3.7C[/b]
[b]Precipitation: Generally average for the north and east, below average for the south and east.[/b]
[b]January[/b]
[attachment=144304:january 2013 pressure.png]
[b]January will begin with a continuation of the cold, with blocking centred to the northwest and the flow generally being north of east, though largely slack for much for the first week. However, this relative calm will give way to the jetstream, bringing above average precipitation but still generally below average temperatures, leading to some major but transient snowfalls. Blocking will be more peripheral, with the low pressure dominant through mid month and with some milder interludes at times, but by the end of the month the block will re-establish and extend across to Scandinavia to bring a potent snowy easterly and the coldest period of the winter will be the final third of January into the start of February. [/b]
[b]Temperature: cold start, cool middle and a very cold end, with the cold generally centred over the spine of the country rather than the north. Scottish Mean: 1.1C CET 1.3C[/b]
[b]Precipitation: Well above average across the country[/b]
[b]February[/b]
[attachment=144305:february 2013 presssure.png]
[b]February will begin with intense cold, with temperatures struggling to get above 0C anywhere with widespread snow cover. It will be mostly dry, with heights again transferring southwards, but this will gradually lead to a thaw and temperatures will start to recover towards average by mid month. With the jet still on a southerly trajectory, though, the block will not sink and some battleground setups with renewed cold uppers from the still frozen continent giving snowfall to eastern areas before giving way to mild southerlies by the end of the month. [/b]
[b]Temperature: very cold start, cool to average middle and much milder towards the very end: Scottish Mean 2.6C CET 3.7C[/b]
[b]Precipitation: Around or slightly above average for the south, Below average further north.[/b]

[b]Note: this forecast was completed in a bit of a hurry so content is subject to slight tweaks over the next week(also if the images don't work I'll get that fixed too), but basically this will be my definitive winter forecast.[/b]

LomondSnowstorm

LomondSnowstorm

 

Lomond Snowstorm's 2011/12 Winter Forecast

I perhaps left this forecast a bit late, to the point that it coincides with the Netweather one, but I felt it was worth doing one anyway even if it overlaps with much of what others are saying. I'll have a look at the factors to consider, then summarise each individual month, both in terms of the general pattern and for Scotland in particular.
[b][u][size=4]Key Factors[/size][/u][/b]
[size=4]ENSO - We're most likely looking at a weak-moderate La Nina, perhaps similar in strength to last year, though the forecast is for a significantly weaker La Nina than was thought a month or so ago [url="http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/images2/nino34SSTMon.gif"]http://www.cpc.ncep....ino34SSTMon.gif[/url][/size]
[size=4]Global Angular Momentum - This is currently weakly negative and is likely to stay generally negative or neutral through the winter.[/size]
[size=4]Solar Activity - We are quite a long way from the relatively long sunspot minimum that many believe was largely responsible for the last few winters( [url="http://www.nature.com/ngeo/journal/v4/n11/full/ngeo1282.html"]http://www.nature.co...l/ngeo1282.html[/url] this too links to my next key factor). This increase in sunspot activity could perhaps contribute to lower stratospheric temperatures and hence discourage blocking. [/size]
[size=4]QBO - Generally speaking, a negative or easterly QBO tends to be disruptive to the polar vortex, but moreso in years of low solar activity (see the stratospheric temperature thread for more in depth analysis of these factors). This is currently also weakly negative and will remain negative through the winter, which could aid blocking as the winter progresses.[/size]
[size=4]I decided to have a go at making my own analoguous composite series using years with both a weakly negative La Nina, weak easterly/ neutral QBO and slightly negative/neutral GLAAM to see if it was similar and indeed it is - [attachment=123819:2011-12 composite pressure.png] pressure is anomalously high to the northeast and low to the southeast. [/size]
[size=4]Stratospheric temperatures - currently these are below average, which means high latitude blocking is likely to be hard to come by in the first half of winter, particularly given the lag time for propagation down through the atmosphere. However, the main crumb of comfort for those looking for blocking is that the composite years favour an anomalously warm stratosphere over high latitudes, which hints at a potential SSW (sudden stratospheric warming) and eventual disruption of the polar vortex into January and February. [attachment=123820:2011-12 composite stratospheric temps..png] This is the key to the winter - if the SSW splits the vortex unfavourably or doesn't come off at all then we're unlikely to see a significant cold spell at all. However, if it does, and the analogues give a good guide to where blocking will set up, we could have a classic spell of lamping on our hands yet again.[/size]
[size=4]Autumn Eurasian snow cover - This also affects winter atmospheric circulation through the upward propagation of Rossby Waves, causing a weakening of the polar vortex ([url="http://climexp.knmi.nl/data/g20111126_2140_11308_1.png"]http://climexp.knmi....140_11308_1.png[/url]). The October snowcover for both Eurasia and the NH as a whole was below average ([url="http://climate.rutgers.edu/snowcover/table_rankings.php?ui_set=1#eurasia"]http://climate.rutge...i_set=1#eurasia[/url]) while in both 2009 and 2010 it was significantly above average. However, November currently looks to be running above average for Eurasia [url="http://climate.rutgers.edu/snowcover/chart_daily.php?ui_year=2011&ui_day=329&ui_set=2"]http://climate.rutge...ay=329&ui_set=2[/url] which does add further support to the idea of a winter of two halves.[/size]
NAO - In the past 3 winters this has been negative, weakly so in 08/09 and strongly so in 09/10 and the first half of 10/11. As a pressure index, it is probably best used as an indicator of the synoptic pattern over a season rather than something which is a driver in itself, but it is worth looking at anyway ([url="http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif"]http://www.cpc.ncep....a/nao.sprd2.gif[/url]). The outlook into the first ten days of next month suggests it will remain positive, which is in line with the generally poor outlook for lovers of northerly blocking (as it should be given these are model generated forecasts).
Model Output - The CFS has given us this [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst/images3/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] which runs counter to a lot of what has been said in the forecast. Should we take note of this? Well looking at the last two winters' outputs in November, the CFS has fairly mixed fortunes to say the least ( [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst_history/200911/images/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] and last year [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst_history/201011/images3/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] )

[b][u]December[/u][/b]
The first third of the month looks likely to be very unsettled, with very low heights to the north and Polar Maritime incursions if and when the trough tilts NW to SE (but of course, the usual health warnings about believing fantasy charts apply). Precipitation widely above average, temperatures generally close to average, though much colder in northern areas. Even here, though, any snowfall will be largely transient even on high ground with frosts certainly coming at a premium. Into mid-month, I'd bank on a return to more settled conditions, with high pressure fairly close to the southeast of the UK, and temperatures generally above average and precipitation below average, though perhaps around average in the northwest. Christmas may well coincide with a return to less settled conditions and perhaps a transient cold spell from the north before milder air returns at the end of the month. Overall, then, a milder than average month for most of the UK, though for Scotland it may well end up being close to average. Precipitation somewhat above average, probably moreso in the north than south, but there may be a slight easing of drought conditions in the southeast. There's a very high chance of a NW/SE split in snowfall prospects, with most of central and southern England seeing no lying snow in December and snowfall being above average in Northwest Scotland. East-central Scotland's snowlovers will suffer badly from lack of northerly exposure, though depending on 'snowshield' formation I wouldn't rule out one significant but short lived snowfall accumulation.
CET punt - 5.1C Scotland mean 3.2C
[b][u]January[/u][/b]
A very mild southerly flow will gradually give way to drier conditions as high pressure builds in from the southeast. This quieter spell of weather will usher in the change to colder conditions as the high retrogrades to northern Scandinavia. Snowfall will initially be confined to southeast England with dull, drizzly conditions further up the east coast and sunshine for western areas.However, as the high moves northwards and colder uppers move westwards across the continent showers will become more widespread and intense to end the month. Precipitation will be around average in southern and eastern England but well below, for once, across Ireland and Western Scotland. Snowfall amounts will depend on the timing of the SSW but again I see a NW/SE split, this time with the southeast most likely to see above average snowfall. Eastern Scotland, particularly coastal parts, could well endure a spell of intensely dull weather mid-month with a chilly southeasterly flow uppers not quite low enough for a 'sunshine and showers' regime before the real cold arrives.
CET punt - 3.6C Scotland mean - 2.6C
[b][u]February[/u][/b]
February will begin with a period of intense cold and snowfall with the south bearing the brunt of the snowfall from 'channel lows'. At times, low pressure will become more dominant, temporarily shifting the focus further north and melting accumulations in the south and near the east coast. Showers will again become confined to the southeast as the high builds southwards, bringing the coldest minima of the winter in deep snowcover. This will then be followed by a reload midmonth from the northeast bringing more convective snowfall right down the east coast.. The month ends with a rather messy breakdown taking place and neither airmass winning out until well into March, ensuring a gradual thaw. Precipitation will again be well below average in the northwest with central, southern and eastern parts seeing the most precipitation. For Scotland, the southeast will receive the brunt of the snowfall, with frontal systems providing most of the snowfall away from the east coast.
CET punt - 1.4C Scotland mean - 1.3C
[b][u]Summary[/u][/b]
The degree of certainty in the forecast is of course exaggerated. Confidence up to around the end of December is fairly high but the remainder of the forecast is dependent on how a fairly probable stratospheric warming plays out. The building blocks of a dramatic switcharound in midwinter are most definitely there, but whether it comes off or ends up, like February 2009, as something of a disappointment remains to be seen. Here's the composite for temperature [attachment=123821:Winter Temp Anomaly composite.png] and precipitation [attachment=123822:2011-12 composite precip.png] .

LomondSnowstorm

LomondSnowstorm

 

A Christmas Weather Sum Up - Past, Present And Future!(Warning, This Entry Is Entirely Stolen From My Own Post On Regional Thread!)

Firstly, I'd like to have a look at what happened in the west of Scotland today. Basically, a level of upper air had a temperature above freezing , causing rain to fall despite the low ground temperatures [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis06/ukuk/t925/10122509_2506.gif"][color="#284b72"]http://expert.weathe...122509_2506.gif[/color][/url]
This was actually picked up on by the NAE at 12pm yesterday, so perhaps we should've been more on the ball with that one [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/24/basis12/ukuk/t925/10122506_2412.gif"][color="#284b72"]http://expert.weathe...122506_2412.gif[/color][/url] .
However, it was Christmas Eve, so I'll give myself a bit of slack there! However it would be careless if the risk of this occuring again was not examined.
Tomorrow has all the hallmarks of a complete mess forecast wise - models differing in their view of uppers and dewpoints at just the moment when things are most marginal.
The easiest question to answer is probably when - around 5pm for areas west of Glasgow, 7pm for any to get to Edinburgh and about 10pm by the time the heavier stuff makes it east according to the NMM and NAE
[url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/24/basis12/ukuk/t925/10122506_2412.gif"][color="#284b72"]http://expert.weathe...122506_2412.gif[/color][/url]
The where is also explained - heaviest across western Scotland, including Galloway, and the central belt, with Grampian and eastern borders areas seeing very little up to Monday midday [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/rsum/10122712_2512.gif"][color="#284b72"]http://expert.weathe...122712_2512.gif[/color][/url]
What will precipitation fall as is a toughy, so I'll attempt to firstly ascertain what COULD be falling out of the sky tomorrow. I think living things such as frogs and mice can be ruled out straight away, as can dust and sand.
Freezing rain? It must be said that there is indeed a risk of this in East Ayrshire, East Lothian, Kintyre, the Borders and the Northwest Highlands [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/t925/10122621_2_2512.gif"][color="#284b72"]http://expert.weathe...2621_2_2512.gif[/color][/url] with little change in the affected areas by 9am on Monday morning [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/t925/10122709_2512.gif"][color="#284b72"]http://expert.weathe...122709_2512.gif[/color][/url]
How about snow? Any chance of that? Well with the NMM (which I can't post on here) suggests lowland central areas will see almost exclusively rain with dewpoints about 1-2C, though inland and upland areas above about 200m should see snow, at least to start with. The NAE is different - snowfall is for a wider area from central Fife to Stirlingshire to Dunbartonshire (and obviously including M74 corridor and the Highlands) [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/prty/10122700_2512.gif"][color="#284b72"]http://expert.weathe...122700_2512.gif[/color][/url] .
D.Ps look good for most of inland Scotland, though as I already mentioned some areas would see freezing rain rather than snow [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/taup/10122700_2512.gif"][color="#284b72"]http://expert.weathe...122700_2512.gif[/color][/url]
Minimal change at the surface through Monday morning in terms of dewpoints according to the NAE [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/taup/10122712_2512.gif"][color="#284b72"]http://expert.weathe...122712_2512.gif[/color][/url] so if you start with wintry precipitation it's likely that you'll stick with it and if you start with rain you're very unlikely to see it change back barring evaporative cooling with intense precipitation. As for accumulations, who knows really! With temperatures just above freezing, wet snow would still lie reasonably on top of old snow, but of course rainfall would compress snowfall into an icy mess. [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/tmp2/10122700_2512.gif"][color="#284b72"]http://expert.weathe...122700_2512.gif[/color][/url]
I would favour Northern and Western Perthshire and places like Moffat and Lockerbie to see significant accumulations and coastal areas, both east and west of Scotland but especially west, to see a complete thaw. In between that is all to play for - some will come out with more snow on Monday morning than tonight, others will have much the same, many will have an icy pack and a few may see a total thaw, but, predictably, inland and upland is the place to be if you want to see snow from an atlantic front.
By Wednesday at 12pm the models agree that high pressure will have seen off the Atlantic threat [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/96/h500slp.png"][color="#284b72"]http://www.netweathe.../96/h500slp.png[/color][/url] [url="http://www.netweather.tv/ecmimages2/20101225/12/ecm500.096.png"][color="#284b72"]http://www.netweathe.../ecm500.096.png[/color][/url] [url="http://www.wetterzentrale.de/pics/Rukm961.gif"][color="#284b72"]http://www.wetterzen...ics/Rukm961.gif[/color][/url]
Yaaaay! In practice, of course, that doesn't mean we're back to cold and crisp weather. Maxima on Wednesday are hardly much to speak of [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/96/ukmaxtemp.png"][color="#284b72"]http://www.netweathe...6/ukmaxtemp.png[/color][/url]
but with mild uppers and a southerly flow we're likely to fall short of an air frost in most places [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/114/ukmintemp.png"][color="#284b72"]http://www.netweathe...4/ukmintemp.png[/color][/url]
However, it does look a 'cleaner' high than yesterday when a dirty raw southeasterly was being shown. The GFS suggests the flow will be slack by Hogmanay, allowing for a more widespread frost [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/162/ukmintemp.png"][color="#284b72"]http://www.netweathe...2/ukmintemp.png[/color][/url] though a rather unfortunate and purposeless easterly flow would, in this instance prevent eastern Scotland seeing a frost. [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/162/h850t850eu.png"][color="#284b72"]http://www.netweathe.../h850t850eu.png[/color][/url]
However, let's not bother with details as the other outcomes on offer for New Year's Day range from a cold easterly to the beginnings of a northerly to high pressure sitting just south of the UK.
The UKMO is promising, giving a colder easterly to southern England and would almost certainly allow for cold air to move right across the UK as the high retrogresses to Greenland [url="http://www.wetterzentrale.de/pics/Rukm1441.gif"][color="#284b72"]http://www.wetterzen...cs/Rukm1441.gif[/color][/url]
Note that an easterly from the initial low is unlikely to deliver this far north and that getting the high to move northwest is key to getting severe cold back.
The GFS ultimately fails on this front and allows the PV to reform to the north, postponing hopes of a cold spell for a fortnight at least [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/240/h500slp.png"][color="#284b72"]http://www.netweathe...240/h500slp.png[/color][/url]
The ECM is nice, making more of troughing over Scandinavia to allow the high to drift slightly northwest of the UK, and we'd probably be close enough to the centre of it to see a cold and frosty New Year as opposed to the milder, cloudier scenario we ended up with earlier in the month. On the face of it the ECM appears to be headed for a dud, the high drifting south [url="http://www.netweather.tv/ecmimages2/20101225/12/ecm500.192.png"][color="#284b72"]http://www.netweathe.../ecm500.192.png[/color][/url]
However, the troughing to the east and heights building to the north give a clue of the next step in the evolution - [url="http://www.netweather.tv/ecmimages2/20101225/12/ecmt850.240.png"][color="#284b72"]http://www.netweathe...ecmt850.240.png[/color][/url] [url="http://www.netweather.tv/ecmimages2/20101225/12/ecm500.240.png"][color="#284b72"]http://www.netweathe.../ecm500.240.png[/color][/url]
Stunning easterly with a Greenie High.
So, to sum up, anticyclonic weather will kick in midweek onwards, with frost returning to most by the weekend and the possibility of something more significant than mere surface cold into the first week of January.



LomondSnowstorm

LomondSnowstorm

 

The Potential Sleety Breakdown/ Blizzard Of The Decade/ Somewhere In Between Of Boxing Day 2010

After a week where temperatures of -2C have been regarded as a slight warm up for most of us and where snow cover still lies thick on the trees days after its initial fall, the old adage of all good things must come to an end enters the fray. It should have been plainly evident to anyone who has lived in this country for more than two years that spells like this one are not only rare but exceptional, and even the classic winters did not manage to sustain cold like that for over a fortnight. The breakdown of this particular bout of cold has been on the cards for over a week now, initially progged as a Christmas Eve breakdown but eventually changing to a battleground scenario between embedded surface cold, aided by high pressure towards Scandinavia, and the first Atlantic low to get as far northeast as Iceland in a month. The general set up by midday on Sunday seems almost certain to happen http://www.netweather.tv/gfsimages2/gfs.20101223/18/66/h500slp.png http://www.meteociel.fr/ukmo/run/UW72-21.GIF?23-18 http://www.meteociel.fr/modeles/ecmwf/run/ECM1-72.GIF?23-0
The trough to our west is the obvious feature, introducing less cold air from the Azores (initially) and creating a frontal boundary between itself and the cold air to the east. As has been said on the model thread many times, the tilt of the trough is key - southeast-northwest would be ideal as it would allow us to tap into colder air from a frozen continent. A flow from the south would allow lower dewpoints to move northwards, keeping snow loss at a minimum and any precipitation as snow. The worst case scenario, and one which it would appear we may be heading towards, is a southwest to northeast tilt, dragging unmodified Tropical Maritime air up from the Atlantic.
http://www.netweather.tv/gfsimages2/gfs.20101223/18/66/h850t850eu.png
The latest GFS gives us a SSW flow, blowing away the cold air rather quickly http://www.netweather.tv/gfsimages2/gfs.20101223/18/90/h500slp.png
The cold air over the continent is trying hard to get into the UK, but merely clips the tip of Kent http://www.netweather.tv/gfsimages2/gfs.20101223/18/90/ukpaneltemp.png
Needless to say that almost all of the precipitation on low ground from this point onwards would fall as rain http://www.netweather.tv/gfsimages2/gfs.20101223/18/99/ukprec.png
Superficially, this chart looks very similar to the 18Z, but the differences on the ground are stark http://www.netweather.tv/gfsimages2/gfs.20101223/12/96/h500slp.png
The difference is a few degrees in the angle, but it's enough to get that continental air in http://www.netweather.tv/gfsimages2/gfs.20101223/12/102/ukpaneltemp.png
so that when heavier precipitation moves in to all areas it has a fighting chance of being snow http://www.netweather.tv/gfsimages2/gfs.20101223/12/111/ukprec.png
The UKMO is somewhere between the two, probably allowing enough of a continental element to bring snowfall to inland areas as the current BBC forecast suggests http://www.meteociel.fr/ukmo/run/UW96-21.GIF?23-18
The latest Fax chart will be of much interest. As we approach NAE and NMM timeframe I'll update further, but as I've noted slight changes mean huge differences in the outcome.
A quick Christmas update: http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/23/basis18/ukuk/prty/10122512_2318.gif
Looks like central Scotland could see a technical as well as semantically white Christmas.

LomondSnowstorm

LomondSnowstorm

 

Christmas Forecast 2010

This is my first blog of the 2010/2011 winter, and so far we've had over 2 foot of snow falling. But that means nothing if Christmas is a slushy washout, right?
Well perhaps that's an exaggeration but a white Christmas really would top this month off very nicely. Of course there are three types of white christmas. There's the met office definition of a flake falling at one of their weather stations (initially it was on central London but was widened out to major cities in other parts of the UK creating a major betting market). This can be as lovely as an all day blizzard or as awful as a rainfall with a few flakes mixed in. Edinburgh and Glasgow both saw some flakes last year. The second type is what most of Scotland and northern England had last year - mostly dry, with snow cover. Arguably this is better than the first type but having rutted dirty ice on the ground is not particularly brilliant either. The third type is the one that Bing Crosby dreamt of with such mistaken nostalgia. Deep, crisp and even snow cover with some snowfall on the day also.
What we appear to be looking at for Christmas Day 2010 is generally a colder than average day for most areas.
The general pattern appears to be that high pressure will be situated to the north and east with a major low pressure system approaching from the southwest, and the affectlong range pattern looks to be dependent on the development and placement of a shortwave to the northwest of the UK.http://www.wetterzentrale.de/pics/Rukm1441.gif The UKMO would give rainfall to the south of England, and possibly a few showers in Eastern England. Scotland would be mostly cold and dry with a few snow showers to northeastern coasts.
With the GFS we end up in a col between cold northeasterlies and a Tropical airmass, meaning largely dry for most of the UK with surface cold. Again, a risk of rain and high ground snow in the far southwest http://www.netweather.tv/gfsimages2/gfs.20101219/12/153/h500slp.png http://www.netweather.tv/gfsimages2/gfs.20101219/12/138/h500slp.png
The ECM, well it spoils the party somewhat. http://www.wetterzentrale.de/pics/Recm1441.gif Low pressure blasts mild air up the core of the UK, leading to a snow to rain event in the north but a speedy thaw further south and west http://www.wetterzentrale.de/pics/Recm1441.gif .
Another thing to be considered is the ensembles, which at the moment suggest a Christmas Day breakdown for southern and central Britain, though given their volatility I wouldn't trust this either.
There's a lot of uncertainty clearly as to how things will pan out, but generally, out of 4 santas, each regions chances of a white christmas on the ground (by northern Scotland I would include areas north of Angus) :
South West England(away from the Moors) [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
South East England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img](+1/2)
Midlands [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Northwest England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Wales [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Northern Ireland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Eastern Scotland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Western Scotland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img](+1/2)
Northeast England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
East Anglia [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img](+1/2)
Northern Scotland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Yorkshire [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
As for snow falling, a much more volatile beast:
Southwest England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img](+1/2)
Southeast England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Midlands [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img](+1/2)
Northwest England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Wales [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Northern Ireland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Eastern Scotland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img](+1/2)
Western Scotland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Northeast England [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
East Anglia [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Yorkshire [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
Northern Scotland [img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img][img]http://nwstatic.co.uk/forum/public/style_emoticons/default/smiliz19.gif[/img]
These are assuming your average low ground locations in these areas and are mostly conjecture at the moment!
Merry Christmas!

LomondSnowstorm

LomondSnowstorm

 

Mr Data On The Era Of Modernity In Light Of The Winter Of 2009/10

[quote name='Mr_Data' date='01 March 2010 - 23:02 ' timestamp='1267484559' post='1791704'][quote name='North Sea Snow Convection' date='01 March 2010 - 13:17 ' timestamp='1267449454' post='1791378']All this winter has done is confirm my own expectations as to the sort of winter that is still possible to experience in the UK. Indeed colder one's than this are still possible.[/quote]Yes, it has surpassed for coldness at least for the CET the winters of the 1900s, 1930s, 1950s, 1980s, 1990s and 2000sThere were only one colder from the 1910s, 1920s, 1960s and 1970s with three colder from the 1940s.It is probably the 8th coldest winter since 1900 for the CET.There is one person who has been very quiet and that is Ian Brown. For the last few years, he has bombarded the forums with his modern winter, modern "era" theories etc and has been on the record saying that even a 1995-96 may not be achievable anymore. How on earth is he going to explain this? It was obvious to me even if there is a trend in one direction there were too many variables and random elememts to dismiss possibilities. Why Ian could not see this is beyond me.His theory is in tatters, his benchmarks surpassed. This winter has been absolute disaster for him as he says he has written a book ironically called "At least it will be mild".[/quote]Source: [url="http://forum.netweather.tv/topic/61869-what-are-peoples-expectations-for-201011-winter/page__view__findpost__p__1791704"]What Are People'S Expectations For 2010/11 Winter?[/url]

LomondSnowstorm

LomondSnowstorm

 

The Historic Winter Of 2009/2010

Winter 09/10 average for Scotland (provisional up to 24th considering downwards revisions are usually quite likely):0.24 °CWinter 06/07 average: 4.34C 71/00 average:2.7C62/63 average: 0.16CIs a 0.08C revision downwards too much to ask?Still, whatever happens, this has been the most incredible winter for persistance of snow and cold IMO. For this area it was really only about that famous spell which will go down in history as probably the coldest 28 days for a few hundred years - I think it probably eclipsed most of 62/63 in severity, but it was fast becoming a winter remembered for only one major cold spell until the last few days delivered something to rival the highland blizzard of 1978 (I have no idea if it beat it though, perhaps too marginal this time round nearer the coast). It never got mild though, and although the temperature reached 11C on one occasion in late January in bright sunshine after some rain from a southerly, on only one other occasion did it break the 6C threshold. I also cannot recall a winter with as many days with temperatures below freezing - I reckon that if you look at any 24 hour period you will find that at least two stations in Scotland got below freezing, something which seemed 18 months ago like merely a fantasy and not something which could ever occur again. In the context of the last two decades, particularly the last one which failed to see a sub-3C CET month, and the moderate-strong el nino which we were taught by some, though not GP whose main analogue was 65/66 I think, would mean a mild, atlantic dominated winter. In the end no one could have predicted the winter that followed. It will be talked about and built up for years to come - a '63 or '47 for my generation. Truly an amazing time in my life.Time to upload all my winter pictures methinks....Source: [url="http://forum.netweather.tv/topic/61845-scottish-cold-spell-discussion/page__view__findpost__p__1791661"]Scottish Cold Spell Discussion[/url]

LomondSnowstorm

LomondSnowstorm

 

Quick Update On Next Week

Hi all you loyal LS fans. Having already posted this on the model thread I felt I'll keep it safe on here as it isn't too bad a summary for next week.Tomorrow evening looks interesting for anywhere from Fort William to Wick to Comrie to my location, as a band stalls over northeast Scotland for a number of hours before heading southwest. [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/02/15/basis12/ukuk/prty/10021700_1512.gif"][color="#284b72"]http://expert.weathe...021700_1512.gif[/color][/url]The southern extent is somewhere around Dundee, so potentially very interesting for the residents of two of the largest Scottish cities. Temperatures look impressively cold for where precipitation is falling [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/02/15/basis12/ukuk/tmin/10021700_1512.gif"][color="#284b72"]http://expert.weathe...021700_1512.gif[/color][/url] [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/02/15/basis12/ukuk/tmax/10021700_1512.gif"][color="#284b72"]http://expert.weathe...021700_1512.gif[/color][/url] so snow and quickly accumulating on frosty or icy ground is likely for anywhere inland. After this and IMO the UKMO looks impressive for some snowfall in eastern Scotland, with the low fairly close and an easterly wind kicking in [url="http://www.meteociel.fr/ukmo/run/UW120-21.GIF?15-18"][color="#284b72"]http://www.meteociel...20-21.GIF?15-18[/color][/url]The ECM is a lovely run and a better one for most areas. [url="http://www.meteogroup.co.uk/meteo/ecmwf/NSea/2010021512/NSea_2010021512_thgt850_72.png"][color="#284b72"]http://www.meteogrou..._thgt850_72.png[/color][/url][url="http://www.meteogroup.co.uk/meteo/ecmwf/NSea/2010021512/NSea_2010021512_thgt850_96.png"][color="#284b72"]http://www.meteogrou..._thgt850_96.png[/color][/url]The latter is interesting in particular as this is pretty much the same setup that brought 5 inches to central Edinburgh and 10 inches around the M90 from Perth to Edinburgh. [url="http://www.meteogroup.co.uk/meteo/ecmwf/NSea/2010021512/NSea_2010021512_thgt850_120.png"][color="#284b72"]http://www.meteogrou...thgt850_120.png[/color][/url]At +144 a brief northerly bout brings in more geniunely cold uppers across northern Scotland [url="http://www.meteogroup.co.uk/meteo/ecmwf/NSea/2010021512/NSea_2010021512_thgt850_144.png"][color="#284b72"]http://www.meteogrou...thgt850_144.png[/color][/url] , and at +168 everywhere north of Birmingham is under -5 uppers. In fact up to +240 -5 uppers are present across southern Scotland and northwest England [url="http://www.meteogroup.co.uk/meteo/ecmwf/NSea/2010021512/NSea_2010021512_thgt850_240.png"][color="#284b72"]http://www.meteogrou...thgt850_240.png[/color][/url]And even for those further south what's not to love about this chart? [url="http://www.meteogroup.co.uk/meteo/ecmwf/NSea/2010021512/NSea_2010021512_thgt850_192.png"][color="#284b72"]http://www.meteogrou...thgt850_192.png[/color][/url]Not quite what '93 was here but it could be pretty good for northwest England and Ireland if it came off. So in summary, nothing in these charts suggest mild, not even average for many areas, and potentially very snowy further north.

LomondSnowstorm

LomondSnowstorm

 

Weather Anorak

I was having a tidy up the other day in my room when I stumbled across a few printoffs from 2005, when I was eleven. It was, unsurprisingly to most on the forum, a five day weather forecast by the BBC and STV from the 2005 Boxing Day easterly. The date printed? Christmas Eve. It was at that moment that I realised just how obsessed I had become with the weather, moreso in winter than summer of course, from when I was just nine or ten. Another thought also crossed my mind, and that was that the 2005 easterly had largely been erased from my mind somehow, so that when people mentioned it on the model thread I would think 'that sounds like a situation which would deliver here but for the life of my I can't remember it!' I then realised that I was in Edinburgh on the 27th/28th which saw no lying snow while I remember getting home to find two or three inches of the stuff later on the 28th [url="http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120051228.gif"]http://www.wetterzentrale.de/pics/archive/ra/2005/Rrea00120051228.gif[/url]It's a very nice chart though, like a watered down '87, and I guess Edinburgh does less well out of a southeasterly than Fife.But that's not the point of this entry at all. What I wanted to ask was at what age did you become properly interested/addicted in/to weather and why? It just seems such a peculiar interest to many other people my age outside the forum but the appeal to me seems fairly obvious - the drama of the changes in model output, the most innocent and pure pleasure of watching snow fall, the banter with those who also understand these, not to mention the fundamental connection between meteorology and almost every activity partaken on this earth.Will update the blog with NMM/fax charts if and when more interesting weather comes into the reliable timeframe i.e. 72 hours.Keep the Model Thread and hence all the other members and moderators happy by trying to stick on topic and avoid aggression/saying things for the sake of making a noise!LS

LomondSnowstorm

LomondSnowstorm

 

The 8Th February - The Beginning Of Another Major Cold Spell?

Well the date that has been bandied about for weeks now on the model thread is almost upon us - the 8th February. Comments like 'we'll all be buried by the 8th' or 'ignore that horrible ECM 168 chart showing southwesterlies because by the 8th there'll be an easterly' have become commonplace lately. But why the 8th? Is there a reason for the fascination with this date, other than a few implausible FI charts?Well the answer comes from a man born just five miles away, in the Kinross-shire village of Kinneswood, in the catchment area of my dad's secondary school, in 1814. His name was James Buchan, and he was the father of the science of meteorology - he discovered that low pressure systems in the NH have anticlockwise circulation (something some members still seem oblivious to!). In addition to this, he came up with a number of common cold spells and warm spells, the most frequently occurring of which is the 7th to the 14th of February.From memories of the last three winters, I have to say that this still rings true today - last year had the 12th February snowfalls, which brought 4-8 inches in eastern Scotland, but also the breakdown of the cold spell and effectively the winter [url="http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00120090212.gif"][u][color="#0000ff"]http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00120090212.gif[/u][/color][/url] . It would've been a snow day at school but alas, it fell on the five day weekend, as did this southeasterly streamer, which delivered something similar [url="http://www.wetterzentrale.de/pics/archive/ra/2007/Rrea00120070211.gif"]http://www.wetterzentrale.de/pics/archive/ra/2007/Rrea00120070211.gif[/url] .So what about this year?Well tomorrow could well bring about wintry showers for many parts of the area, mostly light in nature and very marginal due to not convicingly cold uppers but perhaps a few flakes nonetheless.[attachment=97237:nmm.png] After this is where a bit more interest lies, with a decaying occluded front bringing what would appear to be snow overnight into Tuesday [url="http://cache.netweather.tv/fax/PPVG89.png"]http://cache.netweather.tv/fax/PPVG89.png[/url] and on Wednesday, as the wind veers back east again, another front may well clip Fife and the Lothians [url="http://cache.netweather.tv/fax/PPVK89.png"]http://cache.netweather.tv/fax/PPVK89.png[/url] . After this, an easterly looks likely, but whether the cold and snowy variety or the chilly and a bit miserable type is yet to be decided based on the cold pool sitting mainly to the south of Scotland. But the real interest for most of Scotland will be the weekend onwards, as high pressure retrogresses, in the classic pattern of this winter, towards Greenland [url="http://www.wetterzentrale.de/pics/Recm1441.gif"]http://www.wetterzentrale.de/pics/Recm1441.gif[/url] . Ominous? [url="http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00120091218.gif"]http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00120091218.gif[/url]What follows is not entirely certain, but it appears that a chunk of the displaced PV will be dragged south with this trough and bring something potentially spectacular to most of the UK [url="http://www.meteociel.fr/modeles/ecmwf/run/ECM1-216.GIF?07-0"]http://www.meteociel.fr/modeles/ecmwf/run/ECM1-216.GIF?07-0[/url].[url="http://91.121.94.83/modeles/gem/run/gem-0-240.png"]http://91.121.94.83/modeles/gem/run/gem-0-240.png[/url]It depends where the trough ends up - too far west and the atlantic may make a return, but if the channel lows undercut perfectly? [url="http://www.wetterzentrale.de/archive/ra/1978/Rrea00119781231.gif"]http://www.wetterzentrale.de/archive/ra/1978/Rrea00119781231.gif[/url]If the troughs from the north end up over us, though, something akin to December is also entirely plausible, perhaps even better if the trough contains colder uppers or sets itself up in the North Sea - but now we're just speculating!20-40% chance of -15 uppers arriving at some point in the next two weeks - not my words, but those of Chionomaniac. It's times like these that make you glad you discovered the forum.LS

LomondSnowstorm

LomondSnowstorm

 

A Rare Dreich, Mild Day In This Extraordinary Winter

Greetings!I finally got round to starting a blog so here goes....The last thirty six hours have reminded us what this part of the world is famed for - rain on and off, though usually not all that heavy, temperatures moving between 4.9C and 5.6C, and that horrible grey murk surrounding the Howe of Fife, not lowering visibillity enough to make it seem like an actual event but just generally making the view seem much bleaker. These are the days when I feel the suicide rate must be at its highest. But oddly enough, these kind of conditions have been seriously lacking this year. For the majority of the winter, it has either been sunny or clear and pretty cold or snow has been falling. This is a real rarity for myself, brought up in the generally bland, less cold winters of the late 90s with only the Christmas period of 2000 being of any great note. [url="http://www.wetterzentrale.de/pics/archive/ra/2000/Rrea00120001229.gif"]http://www.wetterzentrale.de/pics/archive/ra/2000/Rrea00120001229.gif[/url] Then there was the horrific 06/07, which was the year I realised that TheWeatherOutlook's long range forecast was badly inaccurate. Charts like these have been all too common in winters before 08/09 [url="http://www.wetterzentrale.de/pics/archive/ra/2006/Rrea00120061229.gif"]http://www.wetterzentrale.de/pics/archive/ra/2006/Rrea00120061229.gif[/url] But these last two years have suddenly become very interesting for lovers of cold, crisp weather. While last winter was not particularly brilliant for snowfall (perhaps around the average number of days with snow falling and a little above the average for snow lying), there was a real change in the air, with a cold and dry end to December, a spell of cold zonality which brought a number of near misses on the snowfall front and the early February cold spell, which seemed quite decent at the time, though again there was only one memorable snowfall, and this came from the breakdown [url="http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00120090212.gif"]http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00120090212.gif[/url] Some put this down to the much speculated about Hale winter, though like the year when the millenium began, some believed that this winter was in fact the Hale winter. As it transpired, this winter has turned out to be a major surprise to many, who thought perhaps that a month with a CET below 1.5C was pretty much out of reach. And, unless the models perform the biggest turnaround seen in the internet age, it looks likely that some more cold and potentially snowy weather is heading our way in just a couple of days. [url="http://cirrus.netwea...50t850eu.png"]http://cirrus.netwea...50t850eu.png[/url] How much snow is likely is hard to say, nothing hugely significant for most areas of east central initially at least but it does look like some convective showers heading our way on Monday at least [url="http://cirrus.netwea...ukprec.png"]http://cirrus.netwea...ukprec.png[/url] Watch for this to come into NMM timescale (36 hours) as anything further out is unlikely to be all that accurate.LS

LomondSnowstorm

LomondSnowstorm

×