Storm Forecast
Valid: Sun 05 Aug 2012 06:00 to Mon 06 Aug 2012 06:00 UTC
Issued: Sat 04 Aug 2012 22:23
Forecaster: SCHLENCZEK
A level 2 was issued for NE Spain mainly for large hail, severe wind gusts and excessive precipitation.
A level 2 was issued for SE/ E France and SW Germany mainly for large hail, severe wind gusts, tornadoes and excessive precipitation.
A level 2 was issued for NE Czech Rep. and S Poland mainly for severe wind gusts, large hail and excessive precipitation.
A level 1 surrounds the level 2 areas for large hail, excessive precipitation and severe wind gusts.
SYNOPSIS
A rather complex situation has evolved over the western and central parts of Europe. High pressure over the E-central Mediterranean and low pressure over the SW UK provide a southwesterly flow at upper levels with several embedded shortwaves along an ill defined cold front. There is a transitional regime between cold air convection in weakly sheared environment over W Europe, strong shear and low CAPE in between and warm air convection in moderately sheared environment over S-central Europe. Severe thunderstorms are most likely in the transition region of moderate instability and strong shear.
DISCUSSION
... E Iberia and SW Mediterranean ...
As the cold front slowly moves southeastwards, parts of Spain will get some 500 - 1000 J/kg of MLCAPE in most places, well overlapping with 30 - 35 m/s of 0-6 km shear and 150 mē/sē SRH3. This will create an environment which provides favourable conditions for well-organised multicellular systems and supercells. One should expect quite a number of large hail events and also excessive precipitation and severe wind gusts. The weak LL shear does not support a noteworthy tornado threat but an isolated event is not ruled out either. Most of the thunderstorms should evolve during the afternoon/ evening hours and diminish in the late night.
... France, S Germany, Alpine region....
Similar conditions as in Iberia are also expected further downstream of the SWerly jet streak. 0-6 km shear is a bit less than further southwest, but 20 - 25 m/s overlapping with 500 - 1000 J/kg CAPE should be fairly enough to allow isolated severe multicells and supercells with large hail and severe wind gusts. Especially along the French Alps, a small channel of augmented 10 - 15 m/s LL shear exists at least in GFS 12Z which will allow isolated tornadoes. Convective activity may continue through the evening and night hours as elevated instability will still be present and QG forcing from an approaching shortwave trough should be sufficient to sustain convection.
... W Hungary, Czech and Slovak Rep., S Poland...
Towards the east, deep layer shear decreases to 10 - 15 m/s but instability is forecast to increase to about 1500 J/kg MLCAPE. In such a moderately unstable environment, multicellular storms with isolated large hail, severe wind gusts and excessive rainfall may develop. Some of them may merge into an MCS which might enhance the severe wind gust/ excessive precipitation threat. AEMET HIRLAM shows a scenario with a developing MCS over E Germany/ W Czech Rep. which intensifies over Poland and moves northwards to the Baltic Sea whereas GFS tends to favor more isolated multicellular convection. In either way, a concentrated area will likely be exposed to an enhanced risk of large hail, severe wind gusts and excessive convective rainfall. This region has to be monitored and an update might be issued if a severe squall line forms.
... UK and N France...
Diurnal heating will create some hundred J/kg of CAPE in a weakly sheared environment. Most of the convection will develop during the afternoon and some storms might produce an isolated large hail event or a weak tornado but overall risk is too low for a LVL1.
...Finland and NW Russia...
Some hundred J/kg CAPE should be present during Sunday afternoon and mostly multicellular convection is likely to form. Due to a decrease in effective PWAT, the risk of excessive convective rainfall is somewhat lower than the day before and no threat level is issued.
