Storm Forecast
Valid: Wed 27 May 2009 09:00 to Thu 28 May 2009 06:00 UTC
Issued: Wed 27 May 2009 09:16
Forecaster: TUSCHY
A level 1 was issued for Finland mainly for severe wind gusts.
A level 1 was issued for Italy and the Balkans mainly for large hail, severe wind gusts and tornadoes.
SYNOPSIS
A sharp upper trough is situated over central Europe with cool airmass now present north of the Alps. High pressure over SW/NE Europe keeps conditions stable.
DISCUSSION
... Central Mediterranean and the Balkans ...
A cold front, currently situated along the S/SE Alps, moves southwards during the day with widespread initiation forecast over a broad area. Yesterday's very steep mid-level lapse rates weaken somewhat but remain supportive for widespread 500-1000 J/kg MLCAPE with locally higher peaks. Both, southward moving cold front and amplifying upper trough support strong directional shear throughout the atmosphere, compensating the modest deep layer shear of 10-15m/s over the southern level-1 area and 15-25m/s over the northern parts. Right now the best thermodynamic and shear overlap looks like to set up over N/NE-Adriatic Sea.
The main hazard with those storms will be large hail with isolated very large hail possible and strong to severe wind gusts. A swath of severe wind gusts is not ruled out along the N/NE-Adriatic Sea given favorable 20m/s 3km shear and some SRH1. In addition, LL CAPE release is augmented significantly and an isolated tornado can occur. Flash flooding is heavily bound to any developing cluster but models differ in the path and strength of those.
... Poland, W-Belarus, Lithuania, Latvia, Estonia and Finland ...
At 08Z, the cold front was situated over E-Poland with a rapid weakening trend of ongoing convection. Instability tongue ahead of this front becomes weaker betimes but strong forcing and diurnal heating ought to increase thunderstorm chances later on. Modest shear at all levels over E-Poland, W-Belarus, Lithuania, Latvia and Estonia and slim instability release keep severe chances quite low but marginal hail and strong to severe wind gusts can occur. The risk diminishes after sunset.
The cold front passage over Finland excites more concernes regarding more widespread severe wind gusts. GFS keeps up its solution in developing some low-end instability in the prefrontal airmass between 15Z-00Z. Warm EL temperatures and enhanced 0-3km CAPE indicate that a line of low-topped convection is the most likely scenario as the cold front arrives from the SW. Shear is strong with the more relevant 0-3km shear of 20-25m/s and winds at 850hPa at 20m/s. LL shear is also augmented with 15m/s and up to 200 m^2/s^2 SRH1 ahead of the front. A forced line of showers/isolated thunderstorms is forecast with a severe wind gust threat. Dependant on the final strength and coverage of convection along this front, this hazard could become quite widespread. Also, an isolated tornado/marginal hail event can't be ruled out. We went with a non thundery level area as degree of electrification is uncertain.
... Extreme N-Germany and Denmark 00-06Z ...
A strong cold front is the focus for increasing thunderstorm chances but very slim instability release keeps the risk of organized updrafts too low for a level area. However, 20m/s at 850hPa will mix down to the surface so strong to isolated severe wind gusts are possible.
