Model Output Discussion:

[Cecil Hogmanay Melchett]

Not saying it is going to be exactly the same or will last as long, but there is a suggestion of Feb 1986 about the charts. Very very blocked, the atlantic sent to coventry (with a small c!) quite dry a lot of the time, homogenously cold and last but by no means least some snow showers about.

Very nice indeed!!

I agree Tamara.GFS ens mean brings some very cold uppers (-10) into the SE and around -8 for most of the rest of the UK at 144h.

At the moment it looks cold and dry for the majority but thats fine with me.

Looking at the output this morning another below averge CET month a posibilty?

[Guest North Sea Snow Convection]

I agree Tamara.GFS ens mean brings some very cold uppers (-10) into the SE and around -8 for most of the rest of the UK at 144h.

At the moment it looks cold and dry for the majority but thats fine with me.

Looking at the output this morning another below averge CET month a posibilty?

The implications for the CET are of course excellent here, following on from the sublime return we have under our belts for January. Not that December was half bad either.

In terms of ppn, there is a question mark. In view of the potential for the cold to be embedded for some time, the track of the high after it nestles initially between Iceland and Scotland will need to be watched in this respect. The distribution of any snow might change accordingly

But as long as it stays cold, then snow could pop up just about anywhere yet.

[cooling climate]

If we retain better heights to the north then this will have the effect of sending the troughing and cold

pool further south and west.This will have the knock on effect of bringing in the much colder air earlier

and also unstable air.

Regardless of this it is imo safe to assume that as from Sunday onwards we will start to pick up a

continental flow which will become progressively colder and wintrier as we go through the week with

many areas seeing some snow (this could well be quite substantial in some more favored eastern and

northern areas).

There is also a growing trend in the models to suggest that this could well be a prolonged cold spell of

weather for the UK.

[Glacier Point]

A feature of the current modelling to keep an eye on will be the development of a deep upper low in the western Atlantic (with corresponding significant blocking ridge above it centred eastern Canada)

GFS Ensemble mean 500 hPa height anomaly day 7 picks this out nicely:

http://raleighwx.eas...nomalyNH168.gif

For every trough there must be a downstream ridge.

Longer range guidance, pressumably influenced by a negative zonal wind anomaly signal is programming all of this ridge response towards Greenland and Iceland. Depicted here GFS and GEM day 11-15 ensemble mean height anomalies:

http://raleighwx.eas...htAnomalyNH.gif

http://raleighwx.eas...htAnomalyNH.gif

I'm not sure that the final solution will be anywhere near that clear cut unless we see a significant and very clear negative zonal wind anomaly permeating the mid troposphere. Global Wind Oscillation composites are suggestive of a ridge around 60N, possibly a bit higher or lower.

Therefore, the evolution being shown for the ridge over the UK to retrogress in over a week's time has a potential weakness to it. Now, I'm not saying this will not happen. It potentially could given stratospheric developments over the last 2 weeks. However, I wouldn't be in a rush to support this until we get to shorter range given the difficulties in modelling the polar field and there is a risk that enough energy will be present in the northern arm / heights sufficiently low enough to our north to prevent a full retrogression as GFS and GEM means suggest.

Edited February 5, 2010 by Glacier Point

[Frosty.]

The GFS 06z seems to have a bit more snow potential next week than the 00z for most of eastern britain and not just the se quarter, even the areas which remain dry will be cold next week with widespread frosts and crisp sunny days which is a lot better than mild murk and drizzle. The whole run looks very cold and blocked with a tendancy for the north to see more snow during the second half of the run as hp drifts away NW towards Greenland opening the door for a N'ly flow, albeit fairly slack, plenty more subtle changes to come but the broad evolution is nailed.....it's going to get very cold everywhere.

Edited February 5, 2010 by Frosty039

[TEITS]

Excellent ensembles once again. I have only looked at the control/mean but im very pleased with the trend.

Here is the control.

http://91.121.94.83/modeles/gens/run/gens-0-1-120.png?6

http://91.121.94.83/modeles/gens/run/gens-0-1-168.png?6

http://91.121.94.83/modeles/gens/run/gens-0-1-192.png?6

GEFS mean at +192.

http://91.121.94.83/modeles/gens/run/gens-21-1-192.png?6

Cambs ensembles.

http://cirrus.netweather.tv/ensimages/ens.20100205/06/t850Cambridgeshire.png

Summary of these is at the moment the best chance of snow showers is for E Anglia/SE due to being furthest away from the HP. Beyond +144 and a clear signal continues of the block moving to Greenland as supported by the GFS Ops/ECM.

I will add that between +96/+144 a relatively small shift in the synoptics can mean the difference between scattered light snow showers and heavy, frequent snow showers. So don't worry at this stage about snowfall.

[Cecil Hogmanay Melchett]

Thanks for that update GP, I don't believe upstream signals favour a retrogression of the High and the models will pick up on this over the weekend with the High likely to remain close to the UK before edging away E/SE.

Intresting Ian,can you elaborate why you think the high will sink?I dont see any evidence of that in the outputs? You could of course be correct but id be interested to hear why you think it will sink SE.(The high).

[Glacier Point]

Thanks for that update GP, I don't believe upstream signals favour a retrogression of the High and the models will pick up on this over the weekend with the High likely to remain close to the UK before edging away E/SE.

That evolution would be the expectation in the absence of any stratospheric signal, probably with the high becoming centred directly over the UK rather than further south as per a global wind oscillation orbit through phases 6-7-0.

Edited February 5, 2010 by Glacier Point

[Lewis]

That evolution would be the expectation in the absence of any stratospheric signal, probably with the high becoming centred directly over the UK rather than further south as per a global wind oscillation orbit through phases 6-7-0.

Hi,

But why does the models not show this? I know things can change, but are stratospheric signals and all the other techy stuff only accurate at long range looking for patterns? and not the short range.

Thanks for your great posts as usual, i need to learn more lol.

[Thunder Snow]

From looking at the 06z it looks like just about anywhere could see snow showers at first, however any real snow threat moves to the East as you move towards the middle of next week.

[cooling climate]

With the AO expected to tank again -4to -5SD and with height anomaly charts such as this.

http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/hgt.shtml

plus the latest northern hemisphere charts for the h500 anomaly pattern and predicted

downward propagating easterly winds shown by the ECM stratospheric charts and the

CPC charts, northerly blocking over the Arctic will again dictate.

http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/gif_files/time_pres_UGRD_ANOM_JFM_NH_2010.gif

Northerly blocking with a suppressed southerly tracking jet stream and strong sub tropical jet

will imo give the UK its first negative CET return since February 1986.

[TEITS]

GEFS Parallel mean is very keen on retrogression of the HP.

http://91.121.94.83/modeles/gensp/run/gens-21-1-216.png?6

Without meaning any disrespect to anyone but I prefer to look at +144/+168 and then look to see if what is being shown at +192/+216 is plausible.

The answer is yes because at the moment the models are suggesting much of the energy will be in the STJ. When you have LP systems tracking to the S then retrogression of the HP is perfectly plausible which is why the models are suggesting this. Don't forget the HP doesn't have to fully retrogress to Greenland. Even if this is centred over Iceland our cold spell would be maintained.

Also worth adding that Feb is always the month that blocking is more likely than any other winter month due to a weakening of the jet. If next week pans out as suggested by the models then this will coincide with the Buchan spell!

[nick sussex]

With the AO expected to tank again -4to -5SD and with height anomaly charts such as this.

http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/hgt.shtml

plus the latest northern hemisphere charts for the h500 anomaly pattern and predicted

downward propagating easterly winds shown by the ECM stratospheric charts and the

CPC charts, northerly blocking over the Arctic will again dictate.

http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/gif_files/time_pres_UGRD_ANOM_JFM_NH_2010.gif

Northerly blocking with a suppressed southerly tracking jet stream and strong sub tropical jet

will imo give the UK its first negative CET return since February 1986.

I think it would need everything possible to go right to get anywhere close to a negative CET especially as the first week would be close to average, as we head through February increased solar energy and the chance that even one short further mild snap could easily derail that.

You would need the set up to remain a sunshine and snow showers pattern to keep those frosts going overnight. Although the model output hints at retrogression of the high its hard to say where the pattern could go after that. Realistically you would need the pattern to progress back to easterly then northerly again if that negative CET is to be reached.

Brave on you in going for that.

[karyo]

That evolution would be the expectation in the absence of any stratospheric signal, probably with the high becoming centred directly over the UK rather than further south as per a global wind oscillation orbit through phases 6-7-0.

Thank you for the update! So, if we didn't have the warm stratosphere, the high would be likely to slowly sink southeastwards as IB suggests. However, thanks to the warm stratosphere, the high has a chance to retrogress later in the period?

Karyo

[Backtrack]

Thank you for the update! So, if we didn't have the warm stratosphere, the high would be likely to slowly sink southeastwards as IB suggests. However, thanks to the warm stratosphere, the high has a chance to retrogress later in the period?

Karyo

Yes, because of the positioning of the pressure system, a reload is odds on!

[SMU]

Thank you for the update! So, if we didn't have the warm stratosphere, the high would be likely to slowly sink southeastwards as IB suggests. However, thanks to the warm stratosphere, the high has a chance to retrogress later in the period?

Karyo

I disagree with that- December didnt have a warming event, yet we still had a etrograde high to our North- its not the only influence-

S

[TEITS]

I disagree with that- December didnt have a warming event, yet we still had a etrograde high to our North- its not the only influence-

S

Indeed and im still not 100% sure why the AO dropped to -6 in mid December. I would really appreciate if someone filled me in on this.

[Guest North Sea Snow Convection]

Indeed and im still not 100% sure why the AO dropped to -6 in mid December. I would really appreciate if someone filled me in on this.

There was an upwelling from the warm lower stratosphere/troposphere layers that sustained the cold weather of the first part of the winter. This was initiated by favourable stratospheric condtions during November.

There were other factors as well - best to look at GP's winter forecast as they are mapped out in there

The latest METO update supports the latest model 10 day + outlook with the southerly tracking jet continuing and blocking being sustained throughout. Cold throughout with plenty of opportunity to see snow at some point. As good as you could expect to see given the general situation as it is.

[karyo]

I disagree with that- December didnt have a warming event, yet we still had a etrograde high to our North- its not the only influence-

S

Yes, but it should help retrogression/further northern blocking?

Karyo

[chionomaniac]

I disagree with that- December didnt have a warming event, yet we still had a etrograde high to our North- its not the only influence-

S

It didn't need to Steve. November's anomalously warm stratosphere (almost record conditions) alongside the very cold tropical stratosphere helped create perfect conditions right down to the troposphere for vortex disruption. This continued right until the first third of January and can be seen quite clearly on the NCP mean zonal wind anomalies chart. They negative zonal winds may have not been the only reason for the vortex splitting twice but it must have played a significant part.

Look at the propagation in the last third of the year from this post 244 :

Edited February 5, 2010 by chionomaniac

[Thundery wintry showers]

So, a lot of disagreement on what ultimately happens with the high. One possibility, judging by those composites, is that we could end up with a moderate cold spell like in mid February 1987 delivered by a slow moving high to the NW of Britain:

http://www.wetterzentrale.de/archive/ra/1987/Rrea00119870216.gif

http://www.wetterzentrale.de/archive/ra/1987/Rrea00119870217.gif

http://www.wetterzentrale.de/archive/ra/1987/Rrea00119870218.gif

Looks pretty cold but mainly dry apart from some snow showers possibly in the east.

We should remember, though, that 6-8 January this year was dominated by a weak high pressure cell over northern Britain but the north-easterly flow on its southern flank brought a fair number of snow showers to eastern areas, some of which headed a surprising distance inland especially over northern England. On the other hand if the flow is less unstable it may well be mainly dry. A slight further shift S and E of the high would kill off any shower activity and keep it cold and dry. So cold is looking very likely but precipitation unclear, except in the W and NW where significant precipitation looks unlikely in any event.

The high sinking slowly SE into the continent is the classic "modern winter" evolution but I don't think it is strongly supported in this case by the teleconnections or the models- at least not until well after midmonth.

[Glacier Point]

Indeed and im still not 100% sure why the AO dropped to -6 in mid December. I would really appreciate if someone filled me in on this.

A very weak polar vortex which underwent 2 prior disruptions in November and early December.

http://ds.data.jma.g...lat_u30_nh.html

Add to that a background signal for enhanced Brewer Dobson Circulation, a favourable SSTA, and trigger WAA event induced by wave dispersal from tropical convection and momentum transport in early December.

We are repeating many of these variables now. However, the wavelength changes related to the season are not as conducive for retrogression without strong forcing from a sharp drop in polar westerlies scheduled for days 7+

Latest GFS AO ensembles certainly seem keen on this:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index_ensm.shtml

Edited February 5, 2010 by Glacier Point

[The Weather Dragon]

Just a little off topic but still relevant - the size of the low pressure system out in the Atlantic on Saturday is amazing the system nearly covers the whole of the North Atlantic.

[NorthNorfolkWeather]

Just a little off topic but still relevant - the size of the low pressure system out in the Atlantic on Saturday is amazing the system nearly covers the whole of the North Atlantic.

What I think is amazing is is not only it's size, how deep it is and yet the block is stopping it going anywhere. It does manage to get some mild air to the west of the UK and over Ireland, and the it just gets pushed back by the cold. Following that the centre splits and just dumbells around itself. In the meantime our cold is looking pretty good thank you.

[Methuselah]

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