Storm Forecast
Valid: Sun 09 May 2010 06:00 to Mon 10 May 2010 06:00 UTC
Issued: Sun 09 May 2010 05:45
Forecaster: SCHLENCZEK
A level 1 was issued for SE Spain, S France and N Italy mainly for large hail, tornadoes and severe wind gusts.
A level 1 was issued for S-central parts of Germany mainly for large hail and severe wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for NW Ukraine, E Belarus and W / NW Russia for large hail, severe wind gusts and tornadoes.
SYNOPSIS
Between an upper ridge over Greenland and another high pressure system over NW Russia, a low over the Norwegian Sea will move southward towards the North Sea. Another low pressure system remains stationary over the Bay of Biscay. A frontal boundary stretches from western France via central Germany towards NE Belarus with cold polar air in the north and moderately warm and moist air in the south. Especially over France and SW Germany, kinematic conditions should be adequate for isolated severe thunderstorms.
DISCUSSION
...Spain, S France, N Italy...
Ahead of a 1000 hPa surface low, moderately warm and unstable air overspreads large portions of SW Europe. A 35 m/s jet streak at 500 hPa stretches from the Strait of Gibraltar towards the E Pyrenees, leading to 20 - 30 m/s deep layer shear in this region. During the afternoon, some hundred J/kg CAPE should be available and thunderstorms may develop. Near the upper jet streak, instability is reduced and strongly capped (-100 to -200 J/kg CIN). Storms that may develop in this region will profit from strong DLS and locally enhanced SRH. Current thinking is that QG forcing ahead of the upper trough may be sufficient for convective initiation. The main fraction should be organised multicells which may merge into a line, posing a threat of isolated severe wind gusts and marginally severe hail. A few storms may become supercellular and could produce a tornado as LLS is in a range of 7 - 15 m/s and LFC heights are forecast well below 1 km. Convective activity should start in the morning hours and will continue into the late evening as most of the CAPE is surface-based. Later in the evening / night hours, a MCS may cross parts of S France, the Balearic Islands and N Italy with severe wind gusts as main threat. Especially in N Italy, heavy rainfall is possible but the expected precipitation rate is below the threshold for excessive rain.
...E France, S-central Germany...
Near the upper cold core of a weakening upper trough, some instability should be created by diurnal heating. A small 20 m/s westerly jet streak should be centered near Karlsruhe and will lead to a region of enhanced DLS (15 - 20 m/s). GFS expects most of the instability over N Bavaria ahead of the upper vort-max in a region with strong QG forcing. In the late morning / early afternoon hours, showers and thunderstorms will develop over eastern France and the southern half of Germany. Although SRH forecast by GFS is marginal (100 mē/sē), very strong directional shear in the lowest 2 km is indicated by forecast soundings and also recently observed (e.g. 10739 00 UTC sounding). Current thinking is that well-organised multicells and a few supercells will form near the upper jet streak, posing a threat of marginally severe hail and isolated severe wind gusts. If LLS is locally enhanced, an isolated funnel / tornado may be possible. Thunderstorm activity will continue into the late evening hours over parts of Saxony and SW Poland / W Czech Rep.
...E Belarus, NW / W Russia, W Ukraine...
In a warm and dry continental airmass, moderate instability is expected to be created during the day (MLCAPE around 1000 - 1500 J/kg). Deep layer shear has decreased but strong southeasterly winds in the lower levels and southerly winds in the upper levels should create good conditions for organised multicells and some supercells with large hail, severe wind gusts and the possibility of an isolated tornado. The severe threat should be maximised when storms tend to stay discrete. Later on, storms will tend to merge into a linear MCS with a pronounced severe wind gusts threat.
...SW Turkey...
Some low-end instability will be available in the southwestern parts of Turkey in an environment with locally strong shear (20 m/s DLS). An isolated large hail / severe wind gust event is not ruled out as some storms may contain mesocyclones but overall threat is too marginal for a level 1 due to a very low expected storm coverage.
