Storm Forecast
Valid: Sun 05 Jul 2009 06:00 to Mon 06 Jul 2009 06:00 UTC
Issued: Sat 04 Jul 2009 22:24
Forecaster: SCHLENCZEK
A level 1 was issued for extreme S France and NE Spain mainly for severe gusts and large hail.
A level 1 was issued for N Italy and SE Austria / N Balkans towards N Poland mainly for excessive precipitation and large hail.
A level 1 was issued for E France and S Germany mainly for excessive precipitation.
SYNOPSIS
The upper flow pattern will not change much on Sunday. An upper low west of Ireland translates eastward and its cold front, initially located from SE England via NW France towards NW Spain, will cross France and the Benelux countries on Sunday. Another frontal boundary is located from S Norway via Denmark and NE Poland towards Ukraine and western Russia, separating cool and dry air in the north from hot and moist air in the south. Except for the convection in western-central and NE Europe, thunderstorms should be diurnally driven and evolve in an environment with weak vertical shear. In the vicinity of the subtropical jet streak over the western and central Mediterranean, GFS and WRF show some instability but no precip - except for NE Spain. Again, excessive precipitation will be the main threat, especially in southern Europe.
DISCUSSION
...NE Spain...
About 1 kJ/kg MLCAPE is expected to be located over NE Spain near the subtropical jet streak. Deep layer shear in order of 15 - 20 m/s and SRH3 around 150 mē/sē should be sufficient for organized thunderstorms, especially supercells which may produce large hail. As the boundary layer is relatively dry, most storms should be high-based with a significant severe wind threat due to evaporative cooling.
...N Italy, Slovenia, N Croatia, SE Austria, SW Hungary...
Diurnal heating will lead to MUCAPEs around 2 kJ/kg in the region discussed here. Deep layer shear stays below 10 m/s which indicates that briefly organized multicells and storm clusters should be most likely. In the late afternoon, strong LL moisture convergence is shown by GFS which will increase the threat of excessive precipitation. Locally, there may be more than 50mm / 24 hr and flash floods should be likely in the whole level 1 region. Additionally, isolated large hail and severe gusts are not ruled out. Although storms should dissipate after sunset, the heavy rain threat will continue into the late night, especially over NE Italy and Slovenia.
...SW Poland, E Czech Rep., W Slovak Rep...
MLCAPEs around 1 kJ/kg in a weakly sheared environment are forecast near the frontal boundary over Poland and in the warm sector. Some storm clusters will develop during the day and heavy rain may lead to flash floods. Isolated large hail may also occur with the stronger storms.
...E France, S Germany...
In the afternoon / evening hours, thunderstorms should also develop in E France and southern Germany in an environment with some hundred J/kg MLCAPE and very weak vertical shear. Storms will likely merge into some clusters that may produce large precipitation sums which will result in local flash floods.
...E-central France, Benelux countries, W Germany...
Ahead of a frontal wave over western Europe, increasing LL moisture and some cooling at upper levels lead to about 1 kJ/kg MLCAPE in an environment with 10 - 15 m/s deep layer shear. Most of the shear is confined to the lowest 3 km and therefore multicell storms will probably organize into a line. Locally, severe gusts and large hail may occur but a threat level should not be warranted at the moment.
