Cold Spell - Late November

[martinastro]

A new thread started for discussion, forecasts, warnings etc for a possible significant cold spell during late November. Temps between - 10 and -12 deg C are being forecast for some areas of the UK. Here's an early synoptic discussion from Paul Blight.

Discussion

For some time the longer range GFS and esp the ECMWF have hinted at a cold block becoming established to End November over much of Northern Europe.  The synoptic process for this to occur has now begun.  The synoptic elements for such an event to occur are complex and largely routed on the strength of the block when it becomes established.  Currently on the 300mb chart a rather complex pattern exists.  A broad and filling complex upper and surface low is located over the Bay of Biscay.  Strengthening warm advection assoc with a buckling Jet core is located over the Western Atlantic in response of strong cyclogenesis and warm advection pushing north towards Western Greenland.  Over EUrope pressure is rising over Scandinavia and 300mb heights here rise over the coming 36hrs. 

Thereafter the strengthening block in the Atlantic continues with large scale warm advectiion pushing north into Greenland increasing contour heights in the Mid-Atlantic and at the same time allowing the retrogression of the Scandi High westward as a large scale trough slides south over northern Europe.  Into the Medium range the major questions which exist are the strength and position of this ridge / trough pattern and largely whether the upper ridge creates a cutoff or partial cutoff upper level high over Greenland.  A Omega block which does not cutoff stands a higher chance of collapsing SE into the UK as the whole trough / ridge pattern sinks SE.   However if the high can cut off and retrogress west it allows the extension of the Scandi trough SW'ward towards NW Europe. 

REcent model guidance favours the latter scenario with broad synoptic agreement from the GFS, UKMO and ECM of a cutoff High forming over Greenland with a corresponding positive 500mb anomaly located over Western Greenland and the Davis Straight.  A trough extends SW from Norway and conditions appear right for an arctic low to form or sink SW from Scandinavia in the LEft exit of an extending N Jet rounding the high and diving SW.  THis is very problematic as anything that rounds the arctic and greenland areas is highly data sparse and likely to lead to sig forecasting challenges over the coming days.

The 12Z GEM, GFS and UKMO all are in reasonable agreement that between T_96 and T+144 a low will form over Scandinavia in the LEft exit of the extending NE Jet and sink south across the North Sea allowing some very cold air to move into its ciruclation and influence the weather over the UK.   The timing in the ECM is more questionable.  It seems to develop one low and then spins up a 2nd low which creates a very cyclonic pattern.  There are two possible reasons for this , 1) the higher resolution of the global ECM is allowing it to correctly forecast a type of Meso low which is beyond the resolutions of the GFS and UKMO or it is overspinning and deepening a trough extension which can naturally occur in the form of a pressure weakness extending north from the pre existing low over the SOuthern North SEa.   Given the better agreement from the other models i tend to think the ECM is probably over egging the cyclogensis and tend to prefer the GFS/UKMO solution at T+120/144. 

FOr the next 72 hrs only slow cold advection occurs as the upper pattern does not bring any really cold air into the UK area.  WBFL are around the 400-500M level for the next 3 days though slow cold advection will bring wintry precipitation to the hills of the north esp the Pennines.  (Lower WBFL inland away from N Sea coasts)

Beyond T+96 more steady and progressive cold advection occurs as the sharpening trough extends SW from the NE and the NE Jet forms the low in the Norwegian Sea. 

As the low moves down the North Sea, rain and sleet will move South across the UK, though little in the west, as the cold pool deepens over the N Sea it will allow sig lower WBFL air into the NE which extends down across most of the country late in the week (thurs and Fri)

Showers turn to Snow in the NE later on Weds and extend south and SW on Thurs and FRi - WBFL fall to 100M or lower away from coasts in the SE and E where they stay higher due to latent heat release from the low in the S North SEa.  These Snow showers becoming increasingly organised later in the week with BLizzards and drifting Snow likely to move into the NE and affect much of the Higher ground from the PEnnines northwards by the end of FRiday.

By late Friday and Saturday WBFL are largely cold enough for Sleet and Snow everywhere away from the far SW, accumulations becoming quite significant over High ground in the North and East but showers penetrating inland at times esp during the day but sig accumulations inland are unlikely in the south, however organised troughs will not be able to be depicted at this range.  Very cold in the wind with sharp and penetrating frosts inland.  Daytime temps largely subzero over all highground in the north and only a few degrees above zero further south.

BY Saturday a large cold trough and cold pool is located over N Europe and a large high over Greenland and  the Davis Straight. Very cold air is allowed into N Europe from the Arctic. 850mb temps plunge to -20C over Scandinavia extending into Western Russia.

Both the ECM and GFS ensembles indicate the block may become extended into the early part of December.  It will largely depend on where the Jet Energy is forced to go - IF its allowed to cut south it will largely reinforce the block over Iceland and Greenland.  this would maintain the cold pool over NW Europe.  on the flip side if the JEt is allowed to go north it will topple the ridge south over the UK cutting off the cold pool and allowing the return of milder Westerly Atlantic air. 

Paul

[markt]

It's an exciting synopsis!    Looking at the long range charts on netweather http://www.netweather.tv/index.cgi?action=nwdc;sess= it's gonna be pretty darn wintery, shall have to keep a close eye on the forecasts now to see how the detail evolves.  Fingers crossed for some of the white stuff!!!   

[markt]

There's a video forecast of it here http://www.accuweather.com/video/681364180001/the-cold-train-rides-roughshod-over-europe.asp?channel=vbbastaj

[martinastro]

Thanks for the vid Mark, that was enjoyable. Looks like a major Winter/snow event for S Europe and even some snow for the UK along bitter temps, perhaps things are not as disappointing as I thought. Let's see what happens this week and weekend 

[markt]

Some nice pickies in this article including a nice fog bow.

http://www.dailymail.co.uk/news/article-1331757/UK-snow-fall-Met-Office-warning-Temperatures-plummet-zero.html?ito=feeds-newsxml

[paulster78]

Advisories of severe weather issued already mainly for Scotland and North-East England, Here we go! 

http://www.metoffice.gov.uk/weather/uk/uk_forecast_warnings.html?day=4

[JohnC]

Good link Mark. he makes it interesting doesn't. Looks like it's shaping up to what he predicted in the other link we have here from last week.

[martinastro]

This is tasty...



There was already video footage taken of snow falling over the mountains in S Ireland on Sunday!

[hal2000]

The Sunday Times was talking yesterday about another severe winter on the way. Bu**er, I hate snow.

[martinastro]

That's weird because the experts are going for a milder UK Winter this season, however southern Europe could have an extreme Winter. Latest models are even hinting at Wintry conditions during Dec now in complete contrast to the recent long range forecasts so perhaps the ST are onto something...probably just guessing though and trying to produce headlines...there will be plenty of those later this week with the S word mentioned

[martinastro]

Latest synoptics from Paul Blight...

Tonights 12Z GLobal Models continue with a smilar theme to earlier guidance and my comments from this morning remain valid.
An initial lowering of 850mb Theta-W brings showers to NE Parts starting later Tuesday which turn increasingly to Snow. This colder air filters down the North and East turning Showers to Snow in many upland areas by the end of Weds (Note SSTs on coasts likely to lead to higher WBFL's on coasts in most places over the coming days except until we get to having sub zero Wet bulbs when then even to low levels snow showers are likely. The biggest question marks remain on the handling of the low developing off the NW Coast of Norway and moving SSW. There seems to be an increasing possibility of a surface inflexion developing over NW parts in the form a weak baroclinic zone which forms some kind of cold air occlusion (the main baroclinic thermal zone is running NW / SE west of the main cold air low which is more convective in origin. This brings higher Theta -W across N Ireland into the Irish Sea, Wales and SW England brining outbreaks of Rain here on Friday followed by much colder air, ahead of this the airmass is cold enough for sleet /snow inland. The problem occurs inland on Fri Ni across the SE as the low deepens and night falls precip may well turn more to snow as colder air is engaged from the NE - big question marks on this at the moment.

Into the Weekend good EPS and Determinisitc agreement for low pressure to lie from S North Sea across to N France and Some very cold air (relative to end of Nov) across many parts of the UK. Snow showerrs around coasts exposed to the North and Northeast sunny in the shelter. Temps 4-5C in the Sunshine 1-2C in the cloud and dropping in any heavier Snow Showers. Very cold overnight temps -3 to -6 and well below -8C over the Snow Fields of Scotland by the end of the weekend. Snow accumulations likely to be 10-20cm across many Northern and Eastern Hills by the end of the weekend (drifting leading to higher accumulatrions in places)

GFS runs a filament of Vorticity into SW england beyond that and then there are big question marks on the handling of the Deep low movinmg NE from the Canary Islands (likely to bring Madeira and the Canaries sig Rain) this low moving NE and engaging the cold air - depending on location and depth a sig snow producer on its NW Flank

[paulster78]

Some very cold upper air on that chart Martin and its progged to dip to -10 over the weekend. The consistency of the model outputs and charts is very encouraging for a decent cold spell i think-certainly some headlines and surprises in the week ahead 

[markt]

The next week / 10 days are going to be interesting weather for us, pretty much wherever you are in the UK.  From my perspective the key is the low pressure that is scooting down from Scandinavia, it's exact track is going to be pivotal in how much and where there is snow.  Personally for me in the west midlands I think any showers are going to have to be pretty beefy to make their way from the east coast to my neck of the woods...  However, something that is in my favour is that tucked so well inland the temperatures have the potential to be really quite low, and so any precipitation associated with the aforementioned low, i think, has real potential to fall as snow.  One thing for sure is that I will be keeping a very close eye on developments... 

My parents are currently away on holiday in Anglesey, Wales, and the plan was to travel over on friday night after work to visit them for the weekend, however in light of the current meteorlogical fun and games I will be making a judgement on travel a little nearer the time... 

[Big Dipper]

Thanks for the detailed post Martin. It pretty much confirms the forecasts that I've seen earlier on today.

Bu**er, I hate snow.

Seconded Stevie.
In my case though I'm not at all bothered about the temperatures as much as the slippery conditions which will inevitably follow.

[markt]

Risk of Snow, Ice and Blizzards Later This Week
Issued on 23/11/2010 at 19:06

Valid From: 23/11/2010 at 18:00 until 27/11/2010 at 23:59


Regions affected:


Areas affected in detail:
Mainland UK


Severity level: 4/5



Weather type:
Snow, Ice, Blizzards


Alert risk period:
24/11/10 12:00 until 27/11/10 23:59


Alert details:
This is an advanced warning for the likelihood of wintry weather affecting parts of the UK during the second half of this week and into the coming weekend. Increasingly cold air will take hold across the UK over the coming days, as strengthening north or northeast winds pull cold air SW from Scandinavia. Showers will become increasingly frequent across Scotland and down across eastern England during Wednesday, falling as snow over highground across Scotland and NE England initially, but increasingly to lower levels further south through Thursday and Friday - mainly down the eastern half of England but perhaps further west too.

Increasingly frequent snow showers across northern and eastern parts of Scotland and NE England from Wednesday night onwards into Thursday will likely bring accumulations of 2-5cm inland, with 10cm or more over high ground.

During Friday, further snow showers will affect Scotland and NE England and this risk of snow showers looks to extend further south across parts of eastern England bringing falls of 2-5cm in places with 10cm over higher ground in the north. Strengthening northerly winds in the north may bring the risk of blizzards over higher routes across Scotland and northern England. Snow showers may also penetrate through to parts of the Midlands, north and east Wales aswell.

During Saturday, a wider areas of England and Wales, together with north and east Scotland looks at risk of some snowfall, snow heaviest towards the north and east with further accumulations of 2-5cm in places with over 10cm over high ground in the north.

These snowfall accumulations will bring treacherous driving conditions, and with temperatures falling below freezing overnight - widespread ice is likely to form on untreated surfaces where showers have fallen

Given some uncertainties with distribution of snowfall towards the weekend, this watch will be superceded by alerts over the coming days.



Confidence: 70%
Issued by Nick F for Netweather.tv