Storm Forecast
Valid: Tue 08 May 2012 06:00 to Wed 09 May 2012 06:00 UTC
Issued: Mon 07 May 2012 19:29
Forecaster: SCHLENCZEK
A level 1 was issued for W/ NW France and N Iberia mainly for severe wind gusts and tornadoes.
A level 1 was issued for NE Bulgaria, E Romania, parts of Ukraine and W Russia mainly for large hail, excessive precipitation and severe wind gusts.
SYNOPSIS
The upper flow pattern over the North Atlantic has become zonal and numerous shortwave troughs will move across western and central Europe in the following days. A surface high over Poland interrupts the zonal pattern and shifts the frontal zone towards the Mediterranean. Remnants of warm and unstable air are located over the eastern parts of Europe. Thunderstorms which develop in this environment are prone to large hail, excessive rainfall and severe wind gusts as moderate instability and strong vertical Theta-E gradients are expected.
Towards western Europe, instability is very limited but intense shear along the frontal zone will partly compensate this limitation for severe convection. The expected threat is shifted towards severe wind gusts and tornadoes.
DISCUSSION
...Iberian Peninsula and NW/ W France...
Ahead of an approaching surface depression which is coupled with an upper shortwave trough, a southwesterly wind at lower levels will advect warm and moist air into the region of interest. The strong baroclinicity leads to intense winds even in the 850 hPa layer. Current thinking is that some thunderstorms will likely form in the marginally unstable environment with a few hundred J/kg of CAPE. If the updraughts are not instantaneously destroyed by the strong vertical shear, some storms will likely develop a mesocyclone. The shear is almost unidirectional but local orographic effects may allow some degrees of windshift in the lowest ten to hundred meters. GFS shows some 150 - 200 mē/sē of SRH3 over W and NW France combined with 10 - 15 m/s LL shear and 20 - 25 m/s deep layer shear which will favor organised convective storms with a severe wind gust and tornado threat. A marginal level 1 was introduced for those regions where the potential of severe weather is highest.
After sunset, some elevated convection may move across France towards W Germany but decreasing instability will limit the severe weather threat.
...E Romania, Ukraine and W Russia...
In a warm and moist airmass with some 1000 - 1500 J/kg of MLCAPE, numerous thunderstorms will develop during the day with peak-time activity in the late afternoon. Even though deep layer shear is only in order of 10 m/s, a strong vertical gradient of Theta-E and rich BL moisture will lead to an enhanced risk of large hail. Severe wind gusts are also possible due to evaporative cooling. Isolated significant severe events are not ruled out but their possibility is too low to justify a LVL2.
