Storm Forecast
Valid: Fri 01 May 2020 06:00 to Sat 02 May 2020 06:00 UTC
Issued: Thu 30 Apr 2020 21:20
Forecaster: PISTOTNIK

Level 1 areas are issued for parts of Bosnia-Herzegovina, Serbia, Bulgaria and SE Turkey mainly for large hail.

SYNOPSIS

An upper-level low is centered over the North Sea. It is flanked by a fairly strong mid-level jet (~40 m/s at 500 hPa) that runs from the Bay of Biscay to Italy. Cool maritime air floods much of W and central Europe underneath and on the cyclonic side of this jet. Further downstream, the jet splits into a northern branch curving far northward to Finland and a southern branch continuing straight to the Near East. In-between, E Europe and Turkey lie under quiescent synoptic conditions with rather warm air.

DISCUSSION

... from Poland and the Ukraine to the Balkans, Turkey ...

Up to 500 J/kg CAPE form in response to daytime heating. Scattered, mostly daytime-driven thunderstorms are expected. Foci for initiation are (1) the cold front from Poland to Croatia which slowly advances eastward, (2) an axis of enhanced moisture across the central Ukraine and Moldova, and (3) orographic features from Croatia, Serbia and Romania southward. These mentioned areas are also overspread by synoptic lift from various vorticity maxima during daytime. Over Turkey, synoptic lift support is neutral or even slightly negative (apart from its SE), hence storms will be more strictly tied to the mountains.
With limited CAPE and weak vertical wind shear, the severe weather risk is low. Only towards the south, somewhat steeper lapse rates and slightly enhanced vertical wind shear (~10 m/s across the 0-3 km layer) under the fringes of the mid-level jet may promote some multicellular storms. Parts of the Balkans and SE Turkey are therefore included into a level 1 for a main risk of (marginally) large hail.

... W and central Europe, Italy ...

Daytime heating also creates low-end CAPE in the cool maritime air, and showers and a few weakly electrified thunderstorms will form. Vertical wind shear increases from 10 m/s across the 0-3 km layer over the British Isles, N Germany and Denmark to 15-20 m/s over S France, N Spain and Italy. In contrast, numerous short-wave troughs cross the northern portions, whereas synoptic lift is largely absent further south. Together with low cloud tops and the small CAPE magnitudes, it is therefore questionable if convection will consume enough of the vertical wind shear to organize and pose an enhanced severe weather risk. Therefore, no threat levels seem necessary for now.