Storm Forecast
Valid: Sun 22 Nov 2009 06:00 to Mon 23 Nov 2009 06:00 UTC
Issued: Sun 22 Nov 2009 05:35
Forecaster: SCHLENCZEK
A level 1 was issued for parts of the Bay of Biscay, S UK, NW France and the British Channel mainly for severe wind gusts and tornadoes.
A level 1 was issued for N Ireland and N UK mainly for severe wind gusts and to a lesser extent for tornadoes.
SYNOPSIS
The dominant feature for convective activity on Sunday is a low pressure system over the British Isles. A 965 hPa surface low has developed ahead of a shortwave trough west of Ireland. Another upper trough over the Irish Sea will affect most parts of the UK and the North Sea during the period and is associated with the cold front of the surface low. This cold front stretches from the Strait of Gibraltar via southwestern France and the Netherlands towards S Norway at 06 UTC. West of the Bay of Biscay, a small area of convective clouds is associated with a weak thermal wave and an upper vort-max south of the low pressure system, affecting W France and S UK in the next few hours.
An upper ridge over the central Mediterranean will provide stable and warm conditions in most parts of the Mediterranean and central Europe.
DISCUSSION
...Bay of Biscay, W / NW France, British Channel, S UK...
About 500 J/kg MLCAPE are forecast in the vicinity of an upper vort-max which will cross the area between 06 UTC and 12 UTC from the west to the east. In this environment, strong vertical shear (20 m/s DLS, 15 m/s LLS) is expected given 30 m/s at 850 hPa and up to 40 m/s at 500 hPa. Convective cells will tend to merge into small bowing segments with severe wind gusts as main threat. SRH1 / SRH3 increases to values near 200 / 300 mē/sē over W / NW France and some short-lived mesocyclones are likely. Locally enhanced LLS near the coastlines may allow a few isolated tornadoes with an isolated strong (F2 - F3) tornado not completely ruled out. There is no area with much higher probability of tornadoes in the models which precludes a LVL2 in this case. The associated upper level feature will undergo substantial weakening after landfall and the threat of tornadoes will diminish in the eastern parts of this LVL1 area. Nevertheless, isolated convectively enhanced severe wind gusts will be possible there.
...Ireland, N England, Scotland...
Another area with low-end instability is located in the wake of the bent-back occlusion west of Ireland. Multicell storms may organise into narrow lines with bowing segments that will cross parts of Ireland / northern UK on Sunday morning / afternoon. Tornadoes appear less likely than in the other area of discussion as deep layer shear reaches only values of 10 - 15 m/s which is probably not sufficient for deep mesocyclones but an isolated tornado cannot be ruled out as strong ( >15 m/s ) LLS and about 300 mē/sē SRH1 are forecast. The main threat in this region will be damaging (33 m/s) gusts that are convectively enhanced. Most of the wind should be due to the intense gradient flow with 35 - 40 m/s forecast at 850 hPa. As the surface low occludes, the severe wind gust threat will decrease to the northeast.
...E North Sea, Netherlands, NW Germany, W Denmark, S Norway...
In the vicinity of the upper cold core which is located in the wake of the cold front / occlusion, scattered showers and weakly electrified thunderstorms will develop. Besides an isolated severe gust / brief tornado, severe weather is not expected. The highlighted area represents the region with strongest QG forcing due to an upper vort-max where lightning is most likely.
