Forecast Update
Valid: Fri 27 Nov 2009 20:00 to Sat 28 Nov 2009 06:00 UTC
Issued: Fri 27 Nov 2009 20:07
Forecaster: TUSCHY

A level 1 was issued for extreme SW/S-UK mainly for severe wind gusts and isolated tornadoes.

SYNOPSIS

Please refer to the main outlook, issued at Thu 26 Nov 2009 18:01.

DISCUSSION

... Bay of Biscay ...

Upper wave is in progress just to the SW of Ireland. WV image shows a nice intrusion of dry low-stratospheric air (500-700hPa) just north of the cirrus shield of the baroclinic zone, suppressing deep convection (probably due to entrainment problems and moderate subsidence). Enhanced convection are confined along the warm side of this wave, now affecting the extreme western English Channel, N-Bay of Biscay and Ireland. Nevertheless, strong CAA on the backside is in progress, increasing deep convection north of the dry intrusion.

Band of enhanced convection moves onshore while weakening due to scarce moisture content at low-levels. Nevertheless, a few storms could evolve well inland. Strong to severe wind gusts and marginal hail remain the main risk. The current, more isolated activity over the Bay of Biscay is about to peak, but we had to update this area for the current activity. Increasing isentropic lift /WAA (visible as a cooling cloud shield over the far NW-Bay) will shut off deep convection during the late night hours. Isolated storms still possible over the central Bay, as atmosphere becomes more unstable again during the morning hours. Severe wind gusts are possible with this activity.

... SW/S-UK ...

Aforementioned increase of convection also affects parts of SW/S-UK. Strength of the approaching depression increased somewhat in past model runs and so did the onshore flow with current dewpoint readings between 4-6°C (in-line with GFS). A very potent/sharp streak at 700-500hPa is about to enter the English Channel during the upcoming hours, with its left exit affecting SW/S-UK during the night. Next to some CAPE (even deep convection possible), shear below 3km is on the increase with strong backing forecast ahead of the depression. Tornado probabilities increase in such an environment and a level 1 is needed. Strong to severe wins gusts and marginal hail accompany stronger storms. Storms spread eastward until 06 UTC, probably affecting all of S-UK. Final degree of moisture return dictates thunderstorm coverage, so we decided to stick with lower thunderstorm probabilities. Also, a sharp, coastline-parallel moisture gradient onshore still forecast with a marked decrease in convective activity well inland.

... Central Mediterranean ...

Electrified convection is on the increase over a much broader area than forecast. This needs to be covered in an updated size of the thunderstorm area (next to higher probabilities).

The level 1 over Belgium, the Netherlands, Luxembourg, extreme W-Germany and extreme NE-France was canceled.

A marginal chance for isolated thunderstorms over far east France/central Germany exists in the morning as mid-levels cool, but expected coverage is too low for any area.