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December Severe Cold Spell Thread

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Author Topic: December Severe Cold Spell Thread  (Read 1974 times)
markt
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« Reply #30 on: December 14, 2010, 08:59:49 pm »

Yes!  Its the waiting game now, couple more days till 'the day after tomorrow' hits!  Grin
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« Reply #31 on: December 14, 2010, 09:04:10 pm »

Regression is now beginning to occur over the North Atlantic Sector. The slow moving high with a fair amount of low cloud and clag trapped above a chilly but quite moist airmass is slowly being forced to regress towards Iceland forced by warm advection over the Western Atlantic which is increasing 500mb contour heights in the vicinity of the Davis Straight and Greenland. At the same time a very cold pool of air which has originated in the Canadian Arctic is moving around the top of the high and will be forced south over the NOrwegian Sea towards the North Sea and the UK. This pool of cold air is very cold and has very low DAM heights, reflecting its true Arctic Characteristics. However upper air coverage is very sketchy in the source region so there are still fairly large disrepancies in the model evolution, however the overall synoptic pattern is well forecast and has Mod -High Confidence.

The Arctic front surges south on Thursday, ahead of this less cold air will be present across much of the UK. The frontal system occluding as it moves SE across the UK into Europe. Arctic air undercuts the front as it surges south later on Thurs, the Rain turning to sleet and snow, though really only the back edge, some heavy snow likely in places breifly though accumulations fairly minimal due the speed of the front. However the front will bring a sharp drop in temperature and a serious risk of ice forming on roads which have seen salt washoff. A very cold night into Friday with Snow showers becoming increasingly heavy and prolonged with deep instability in the coastal areas by Friday. Forecast Ascents are deeply unstable with CB's easily achieveable given the very cold air above. Showers blowing a fair way inland on the strong N to NW winds and a marked windchill. Daytime temps away from shelter and decent sunshine will be low and most likely sub zero for most of the day.

Later on Friday another development grabs the interest. This one develops in baroclinic zone NW of Ireland, A tightening of the Thermal gradient interacts with the upper trough and NW Jet to form a wave low which is set to run SE, now the issue is how much this develops. The 12Z GFS is very developmental with this, the UKMO less so and the ECM somewhere in between. The GFS takes a rapidly deepening low across N Wales into Norfolk and then dumbelling into NE England with Snow developing and pushing into many areas (though a more rain/sleet mix for a time in the Far south and SE, though turning to snow even here as the winds swing offshore) The ECM is similar but less surge of warmer air and the UKMO run a low into the Channel into Belgium. The Models struggle with these kind of wave developments and history suggests that they overdo the development and they tend to be weaker than sometimes forecast. I tend to think some thing between the UKMO and ECM is most likely to be correct and that the 12Z GFS maybe overdeeping the low and producing a more rapidly developing low.

Study of the 12Z Postage stamps indicates considerable variability with this feature from very deep lows to shallow waves, indicating the considerable uncertainty. However if the low comes in a period of Mod/Heavy Snow is possible in many places across England and Wales. The GFS produces several Cms across many parts and derived ECM fields from rainfall suggest similar totals crossing many central and southern areas. We will need more runs before concluding too much here. However the period from FRi through Sunday looks very cold with widespread frost and in many places some heavy snow either from Showers or from more prolonged Snow...Credit: Paul Blight.
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« Reply #32 on: December 14, 2010, 09:09:27 pm »

Trouble....

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« Reply #33 on: December 14, 2010, 09:29:05 pm »

Thats a damn scary looking chart  Shocked
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« Reply #34 on: December 14, 2010, 09:36:05 pm »

We are not the only people getting a severe Winter, check out this blizzard in the US mid west captured by Mike Hollingshead recently, he sent this over to me so I thought I would add it here. Look at the inside of his car!

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« Reply #35 on: December 14, 2010, 09:40:51 pm »

Very odd ... baro is 1041 mb & still rising; I wouldn't expect a major weather event in the next couple of days!
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« Reply #36 on: December 14, 2010, 10:04:16 pm »

Very odd ... baro is 1041 mb & still rising; I wouldn't expect a major weather event in the next couple of days!


Met Office has a 1044 mb core off the WNW coast of NI for midnight, over the next 24hrs this slowly rotates, heading off to Greenland - rising still further as it does.  Guess it sets up a nice big pressure gradient for the rip roaring polar flow that is enroute Smiley
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« Reply #37 on: December 15, 2010, 08:17:20 am »

Incredible ... very little wind & substantial clear periods until midnight, overnight min temp +4.8C, baro 1042 mb ... I don't doubt that it's going to change but I think the "perfect storm scenario" is grossly overstated. Met Office forecasts are gradually shifting to a less extreme event too.
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« Reply #38 on: December 15, 2010, 02:47:27 pm »

I saw that too Brian, I noticed the BBC forecasts today playing the whole thing down to 'scattered snow showers' mainly in the 'N and NE' with mostly rain and sleet from the cold front. I think the biggest event will be the cold from this and possible convective activity. I would certainly like to be near the NE, N and NW coasts on Fri night or Sat - deep convection with electrified cbs with thundersnow are possible, the Irish Sea would be the prime place too. Some think the cold front will be convective with a risk of thunder and even a tornado. As for snow...I guess we won't know until it happens...there's alot happening with this system, check out this detailed simulation which is fascinating...

http://www.ukweatherworld.co.uk/forum/forums/thread-view.asp?tid=40628&posts=10
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« Reply #39 on: December 15, 2010, 02:54:30 pm »

From Skywarn UK...

SKYWARN UK SEVERE WEATHER WATCH #012
ISSUED: 0000UTC, WEDNESDAY 15TH DECEMBER 2010.

SKYWARN UK HAS ISSUED A SEVERE WEATHER WATCH FOR THE FOLLOWING REGIONS:
ALL REGIONS IN SCOTLAND
NORTHERN ENGLAND
NORTHWEST ENGLAND
EAST MIDLANDS
WEST MIDLANDS
EAST ANGLIA
WALES
NORTHERN IRELAND
SOUTHWEST ENGLAND

IN EFFECT FROM 00UTC ON THURSDAY 16TH UNTIL 1200UTC ON FRIDAY 17th DECEMBER 2010.

ARCTIC FRONT SURGING SOUTH, CONVECTIVELY UNSTABLE AND WITH VERY COLD AIR BEHIND.

THERE IS A POSSIBILITY OF SEVERE WEATHER AFFECTING THE INDICATED REGIONS IN THE TIME PERIOD SPECIFIED. THREATS WITHIN THIS WATCH INCLUDE, BUT ARE NOT LIMITED TO:
STRONG GUSTS...BLIZZARDS...HAIL...FUNNELS...TORNADOES...SNOW AND SURFACE ICING.

DISCUSSION:
USING A BLEND OF ECMWF, GFS AND UKMO NAE MODEL DATA; REGRESSING HIGH PRESSURE PATTERN IS ALLOWING AN AMPLIFIED TROUGH TO SURGE INTO NW EUROPE FROM THURS 00Z. VERY COLD ARCTIC AIR, WITH 850-1000HPA THICKNESSES DOWN TO 126 AND POSSIBLE 504DAM UPPER GEOPOTENTIAL HEIGHTS. JET STREAM IS NW'LY OVERRIDING A SPLIT SURFACE FRONT CREATING POSITIVE VORTICITY AND 3-400M2S2 0-3KM SHEAR THROUGH SCOTLAND, WALES AND ENGLAND 00-12Z, IRELAND AND THE SW 09-15Z. PRECIPITATION WILL BE RAIN INCREASING AND BECOMING SQUALLY WITH CONVECTIVE ENHANCEMENT, THEN BRIEF HEAVY SNOW AND BLIZZARDS ALONG THE SECONDARY FRONT. SNOW SHOWERS BEHIND WILL ADVECT INLAND AROUND NW AND W SCOTLAND, NORTHERN IRELAND AND WALES AND COASTS OF NW, NE AND SW ENGLAND. WINDS STRENGTHENING TO 40KT ALONG THE FRONT WITH POTENTIAL FOR ENHANCED CONVECTIVE GUSTS, BOWING LINE SEGMENTS AND MID-LEVEL VORTICES LEADING TO POSSIBLE BRIEF TORNADOGENESIS. THERMALLY FORCED CONVECTION AHEAD OF THE FRONT, AND DYNAMICALLY FORCED CONVECTION LIKELY ALONG AND BEHIND IN SHOWER TRAINS, WITH THE ADDED RISK OF HAIL AND LIGHTNING.

SPOTTER ACTIVATION IS REQUESTED AND SPOTTERS ARE REQUESTED TO REPORT SNOWFALL ACCUMULATIONS AND ALL FACTORS EXCEEDING ACTIVATION CRITERIA
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markt
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« Reply #40 on: December 15, 2010, 08:15:29 pm »

It's going to be an interesting couple of days ahead!  I suspect we all may see some surprises in some shape or form.  Scooting around the various threads on the various forums there's still lots of disagreement and variability banding around as to the specifics of the next several days (other than its going to be pretty darn cold!) - sooooooo, I suspect it's going to be a case of watching the radar and the satellite and getting back to a good old bit of now casting Roll Eyes
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« Reply #41 on: December 16, 2010, 12:40:36 am »

Latest from RTE....

3 Day Outlook
Thursday night will be very cold everywhere, with moderate to fresh northwesterly winds. Wintry showers will affect many parts of Ulster, Connaught and west Munster, many falling as snow inland and giving accumulations of several centimetres; elsewhere, however, will be dry and mostly clear, apart from some light snow flurries. Despite the wind, ground frost will be sharp to severe, and ice is likely on many surfaces. During Friday, the distribution of wintry showers will be similar, with further accumulations of snow in the north and west; also, parts of the east and south may see more significant snowfall from showers but there will be a lot of dry, bright weather in those areas. Winds will moderate and back westerly on Friday but it will be an extremely cold day with temperatures barely rising above zero degrees Celsius in many places. These bitter, wintry conditions will persist throughout the weekend in all parts of the country. While winds are unlikely to be strong at any stage, there will be variations in wind direction; these will largely determine the location of snowfall from showers being blown onto land from adjacent sea areas. Initially, the north and west will continue taking the brunt but the focus for snowfall is likely to transfer to southern and, eventually, eastern counties as the weekend progresses. The temperature regime will be exceptionally low for mid-December, with overnight frost and ice persisting by day in many areas in addition to random snowfall.
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paulster78
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« Reply #42 on: December 16, 2010, 12:50:04 am »

Pretty bleak looking outlook, lets hope these big snow showers get electrified out to sea and push inland on the N-NW winds on fri/sat night, as Mark mentioned theres gonna be a lot of radar watching these next few days!
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markt
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« Reply #43 on: December 16, 2010, 06:29:04 pm »

Had an hour or so of snow this afternoon, it settled on 'water free' surfaces, however the sky has gone clear now so I expect the freeze to start.  On an optimistic note there is some indication there could be showers coming through the Cheshire Gap after midnight, these could lead to some accumulations for me! (hurrah!) - In the mean time it's a night down the pub with the lads tonight, so after a few bevvies I reckon i'll be checking the radar before going to bed Wink
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paulster78
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« Reply #44 on: December 16, 2010, 07:31:52 pm »

I like this forecast from Met Eireann

http://www.met.ie/forecasts/regional.asp?Prov=Ulster
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