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 Iâ€™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 itâ€™s good fun and paints a picture more readily for those who donâ€™t have a particularly strong meteorological background.
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 Iâ€™ll address if first. At present, as it has been for about a year now, weâ€™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:
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 â€˜normalâ€™ winter conditions than we saw last year. However, neutral/weak ENSO values are also associated with a greater frequency of â€“ve NAO conditions, which is strongly correlated with below average temperatures.
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 itâ€™s a two year cycle) for the rest of the winter:
This is one of the hottest (if youâ€™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 weâ€™ve seen in 100 years:
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 itâ€™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:
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 youâ€™d imagine â€“ 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 â€˜Snow Advance Indexâ€™ for October being somewhat below the long term average, although weâ€™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:
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.
[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 - Cohenâ€™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, Iâ€™d put our chances of seeing a +ve Arctic Oscillation for the winter at 90% plus. This doesnâ€™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 weekâ€™s CFS average, call for a milder than average winter. However, these are generally pretty volatile, donâ€™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, Iâ€™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 â€˜turning forceâ€™ due to the Earthâ€™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
Having looked at years from 1958 onwards (prior to that the datasets become sketchier and itâ€™s tougher to pattern match) and basing on the criteria outlined above Iâ€™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 â€˜weightedâ€™ 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.
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]
Onto January and we have a very different looking anomaly â€“ 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 â€“ the stratospheric vortex looks to end up split over the North American and Siberian sectors with a mean strat. â€˜ridgeâ€™ over Europe and the Atlantic:
Given the predicted height anomalies it does look more like a split than a displacement SSW (and in fact there are a few â€˜hintsâ€™ 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.
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:
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 â€“ 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.
Given the SST pattern Iâ€™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]
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 theyâ€™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 monthâ€™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.
Iâ€™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 wonâ€™t start off that way â€“ 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 Iâ€™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 â€“ 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.
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 â€“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 â€˜lesserâ€™ composite years incidentally) with the Scottish mean a positively balmy 1C.
I hope I havenâ€™t bored/scared you too much, and Iâ€™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. Iâ€™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.
By Angelici Magiovinium
Please use this thread for your regional discussions about the current weather or what is being forecast for the next few days in Scotland
Link to the previous one hereâ€¦
Recently Browsing 0 members
No registered users viewing this page.