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December Severe Cold Spell Thread

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Author Topic: December Severe Cold Spell Thread  (Read 4606 times)
Martin Mc Kenna
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« Reply #30 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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