Storm Forecast
Valid: Sat 21 Nov 2009 06:00 to Sun 22 Nov 2009 06:00 UTC
Issued: Fri 20 Nov 2009 22:05
Forecaster: KOROSEC

A level 1 was issued for southern UK across English Channel towards Britanny mainly for severe wind gusts and lesser extent for tornadoes.

SYNOPSIS

A large and deep trough over eastern Atlantic slowly shifts eastwards while a rather rapidly deepening surface low approaches western UK. At surface, a cold front crosses UK during the daytime hours. Over the rest of Europe, an extensive ridging with quite warm and stable conditions persists.

DISCUSSION

... southern UK across English Channel towards Britanny ...

Within the main large trough, several vorticity maxima rotate around its center. During the morning hours a new surface low approaches UK while rapidly deepening later during the forecast period with only slow movement towards east. Its center is expected to be below 970hPa, located close to Ireland by Sunday morning. Quite an impressive jet streak, 50-60m/s at 300mb level, will have its left-exit region right over UK where strong upper-level divergence will allow widespread ascent and quite strong forcing. And with about 20-25m/s at 925mb level, a strong WAA will be pushed NNE-wards. Models simulate weak instability signals along the front and some shallow convection will be possible. Given the impressive wind speeds, deep-layer shear will reach 25-30m/s in the 0-6km layer while LLS near 15m/s should be in place. All this poses a threat for some organized convective storms/lines where the main threat will be severe wind gusts. While taking in account rather strong shear/SREH in the lowest layers as well, non-zero chances for a couple of tornadoes exist. For this reason, a marginal level-1 threat area was introduced over southern UK towards extreme NW France. Strong CAA will overspread the region behind the front and numerous showers, possibly also with weakly electrifies cells, will result. Strong showers with small hail and few funnel clouds/waterspouts cannot be excluded either, given the very steep LL lapse rates in place.

Overnight on Sunday, another short-wave trough will arrive across eastern Atlantic, moving SE-wards towards France. With a very cold mid-levels, some instability should result along the occluded front. Strong jet/shear is expected around the vort max. But as all activity stays over the open waters until the end of the forecast period, no threat level was issued for it. However, the main threat should be severe wind gusts.