Storm Forecast
Valid: Thu 01 May 2014 06:00 to Fri 02 May 2014 06:00 UTC
Issued: Wed 30 Apr 2014 23:36
Forecaster: GATZEN

A level 1 was issued for the central Balkans mainly for excessive precipitation.

A level 1 was issued for the northern Aegean mainly for large hail.

SYNOPSIS

Weak geopotential is present across Europe, and the jet stream extends from the Bay of Biscay towards the Mediterranean Sea. Two vort-maxima will travel within the jet, one from Italy to Turkey, the other from the Bay of Biscay to the west Mediterranean Sea. Due to limited moisture over the Mediterranean, CAPE will be rather weak. Further north, moist and well-mixed air masses are still present from western into central and eastern Europe. Widespread convection will result during the period. Colder and dry air masses start to spread south from Scandinavia.

DISCUSSION

Greece, Aegean region

A short-wave trough moves east during the day, providing DCVA. The affected air mass is expected to become unstable in response to diurnal heating over Greece and onshore advection of modest moisture. Near the trough axis, initiation will be most likely, but weak vertical wind shear will limit storm organization. The best potential will therefore exist across the Aegean region, where an ageostrophic flow will evolve ahead of the vort-max. More than 20 m/s deep layer vertical wind shear are expected by latest GFS model run in the evening hours. Multicells and supercells may be capable of producing large hail. Given weak low-level vertical wind shear, tornadoes are rather unlikely.

Southern France

At the southern edge of the rather cool but unstable European air mass, a short-wave trough will lead to QG forcing. However, latest models indicate that the air mass will be rather stable due to warm layers spreading over the cooler boundary layer. Additionally, low-level cold air advection is forecast during the day, leading tow weak low-level convergence. Therefore, convective initiation is questionable and thunderstorms are quite unlikely. Every storm that forms will have a rather high chance to organize given about 10 m/s low level vertical wind shear. Severe wind gusts will be the main threat.

British Isles, northern and central France, the Benelux countries, Germany, northern Italy, Poland, and Balkans

Under low geopotential, diurnally driven thunderstorms are expected. The best overlap of moisture and lapse rates is expected across southern Germany and the central Balkans, where CAPE can reach 1000 J/kg locally. Storms that form will be weakly organized due to the weak shear, with a slightly better chance over France and Germany, where mid-level winds will be slightly stronger. Locally large hail will be the main threat. Very weak winds and associated slow storm motion will enhance the threat of excessive rain over the southern British Isles and eastern Europe. The main focus will be the Balkans, where a rather moist air mass is located.