Storm Forecast
Valid: Fri 07 May 2010 06:00 to Sat 08 May 2010 06:00 UTC
Issued: Thu 06 May 2010 16:55
Forecaster: TUSCHY

A level 1 was issued for Romania mainly for excessive rainfall and large hail.

A level 1 was issued for Bulgaria mainly for large hail.

SYNOPSIS

Large cyclonic vortex over W-Europe remains quasi-stationary throughout the forecast. Numerous disturbances circle that feature and cause unsettled conditions (in respect of deep convection) over most parts of Europe, despite far N-Europe.

DISCUSSION

... Bulgaria and Romania ...

A favorable set-up for scattered to widespread thunderstorm initiation occurs, as cold upper wave overspreads warm and moist LL air mass with attendant more robust MLCAPE up to 800 J/kg, locally even more, especially over Bulgaria. Beneath the trough over Romania, shear remains weak with some backing at mid to high tropospheric levels, so current thinking is that weakly capped air mass and the approaching trough foster initiation over W-Romania already during the morning hours, spreading gradually eastwards. Storms tend to grow upscale into numerous large clusters with an attendant heavy rainfall risk due to the slow storm motion and rapid clustering. However there is neither a real focused influx of moist inflow nor a strong LLJ core, which could inidcate rapid moisture advection towards the ongoing clusters, so the heavy rainfall risk will stay well below level 2 criterion. Next to isolated large hail, an isolated strong/severe wind gust is possible, if a better defined cold pool manages to evolve, which could increase eastward progression of thunderstorms. After sunset, showers/thunderstorms weaken and move off to the east.

Thunderstorms definitively have a better chance to become severe over Bulgaria, next to a strong mid-/ high-level jet core passing to the south over S-Bulgaria. Despite a well separated CAPE/shear environment beneath 5km, stronger winds overspread Bulgaria at the anvil-layer, enhancing the chance for a few longer-lived and better organized storms due to evacuation of precipitation from the updraft area. Large hail will be the main hazard with those storms, mainly during the initiation stage over W/central Bulgaria, before clustering betimes/further east. Again, thunderstorms decay after sunset as they draw off to the east.

... E-central Spain and the Balearic Islands ...

A short-wave rounds the base of the extensive cyclonic vortex over France and crosses central Spain during the afternoon hours, approaching the east coast of Spain during the evening hours. High-level streamline pattern remains diffluent throughout the night in the exit region of a strengthening mid-level jet core over NW Spain. Good LL moisture advection beneath cool mid-levels is forecast with mid-level lapse rates becoming better/steeper during the overnight hours. Models (ECMWF/GFS/NOGAPS and GEM) still disagree substantially in respect of where a potential cluster of storms may evolve, if any. We currently think that GFS is on a good track and we therefore expect a rapid increase in thunderstorm coverage over E-Spain during the evening hours, moving offshore thereafter. If the GFS solution verifies, we may end up with an heavy rain risk for the coast of E/NE Spain beneath the more stratiform rain shield of the cluster and all the way east to the Balearic Islands beneath the active (deep moist convection) part of the cluster. This cluster keeps going well into the night, probably until 06Z.