Storm Forecast
Valid: Tue 04 Aug 2009 06:00 to Wed 05 Aug 2009 06:00 UTC
Issued: Mon 03 Aug 2009 22:13
Forecaster: SCHLENCZEK

A level 1 was issued for the Balkans and SW Romania mainly for large hail, severe gusts and excessive precipitation.

SYNOPSIS

An occluding 980hPa low pressure system west of Ireland will remain almost stationary, affecting parts of Ireland and UK with a strong gradient flow. Its cold front stretches from Scotland via SE Ireland towards the Bay of Biscay and the Azores and it moves very slowly eastward. Ahead of this low, a strong SWerly flow will establish over Iberia and S France and a hot and moderately moist airmass over S Spain will be advected into N Spain and S France during the period. About 1 kJ/kg MLCAPE is forecast for S Spain / Strait of Gibraltar but there is no source for lift like a strong SFC convergence or an upper vort-max and this airmass is strongly capped.

The main focus for convective activity will be a weakening upper trough over the N Adriatic Sea which will move southeastward during the period. Ahead of the trough, a hot and moist airmass (15 - 20°C at 850hPa, 55 - 60°C Theta-E) is present over the Balkans. The left exit region of a 20 m/s jet streak at 500 hPa points at Serbia / Montenegro and N Albania where QG forcing should be strong enough to allow convective initiation in an airmass with moderately capped 1 kJ/kg MLCAPE. Later in the period, shear and instability will decrease as the upper low moves towards the Bosporus.

Another upper trough is present over NW Russia which will move eastward during the period. Ahead of this trough, thunderstorms may develop in an airmass with 500 - 1000 J/kg SBCAPE. Most of the convection will affect W Russia and N Ukraine. Deep layer shear should be in order of 15 - 20 J/kg and some storms may be accompanied by marginally severe hail.

DISCUSSION

...Balkans, SW Romania, SE Hungary...

Ahead of the approaching upper trough, LL moisture advection will increase which results in about 10 g/kg LL mixing ratio. Recent ascents of S Italy (which will be the origin of the hot and moist airmass) show about 1 kJ/kg MLCAPE which is strongly capped. Later in the day, the cap should weaken and an upper vort-max will overspread the region. Deep layer shear will reach values between 15 and 20 m/s which should be sufficient for organized thunderstorms. Locally enhanced 150 - 250 m²/s² SRH3 is forecast as well and discrete cells will probably become supercellular. The main threat should be large hail and severe gusts in the S Balkan region. Further north, the airmass is almost uncapped and numerous multicells will likely organize into a MCS that may produce isolated large hail, severe wind gusts and excessive precipitation. In the late evening / night hours, storms will move eastward into the Balkan Mountains while weakening.