Storm Forecast
Valid: Fri 07 May 2021 06:00 to Sat 08 May 2021 06:00 UTC
Issued: Thu 06 May 2021 16:04
Forecaster: PISTOTNIK

A level 1 is issued for NE Italy, parts of Slovenia and Croatia for large hail, severe convective wind gusts and excessive convective precipitation.

A level 1 is issued for parts of Germany, the Czech Republic and SW Poland mainly for severe convective wind gusts.

A level 1 is issued for small parts of Tunisia and NE Algeria for excessive convective precipitation and large hail.

SYNOPSIS

An unseasonably strong zonal flow influences most of Europe. It is accompanied by a long and wavy, rather diffuse frontal zone near 50N. A progressive short-wave trough amplifies from central Europe towards the Balkans, making the frontal boundary act as a cold front in this section.
Warm and mostly calm conditions prevail in the Mediterranean region. Late in the forecast period, strong warm air advection will also overspread the British Isles ahead of an Atlantic storm cyclone.

DISCUSSION

... central to E Europe ...

Marginal CAPE forms and allows scattered shower activity across wide areas as the cool maritime air on the cyclonic side of the jet stream is exposed to daytime heating. Some showers may grow deep enough to produce limited lightning and thunder, but probably not deep enough to organize better under the influence of enhanced 0-6 km shear beneath the jet stream. Nonetheless, strong to isolated marginally severe wind gusts will accompany this convection. A low-end level 1 is issued for parts of Germany, Poland and the Czech Republic, where lift from the short-wave trough may be sufficient to compensate the overall background of cold air advection and allow slightly deeper convection. In case of multicellular convection with deviant motion away from the long and straight hodographs, updrafts might briefly acquire rotation and an isolated tornado is not completely ruled out.

... NE Italy into Slovenia and Croatia ...

As the mid-level trough crosses the Alps, strong lift and increasing 0-6 km shear overspread a marginally unstable air mass. Scattered storms are expected to initiate in NE Italy and to organize into multicells and perhaps the odd tail-end supercell. Marginally large hail and severe downbursts are possible. Storms will quickly be undercut by the southward moving cold front in Slovenia and Croatia, which terminates the severe weather risk apart from a possible period of heavy to excessive rain.

... Level 1 area in NE Algeria and Tunisia ...

Forecast models agree on afternoon storms over a confined area in an environment of moderate CAPE (up to 1000 J/kg), weak vertical wind shear and faint sources of lift (thermally driven onshore and upslope flow over hilly terrain, weak vorticity advection aloft). One or two flash floods along with plentiful and marginally large hail are the main risks.