Storm Forecast
Valid: Mon 21 Aug 2023 06:00 to Tue 22 Aug 2023 06:00 UTC
Issued: Sun 20 Aug 2023 22:15
Forecaster: PISTOTNIK

A level 1 is issued for parts of Estonia, Latvia, Belarus, the Ukrain, Poland and the Czech Republic for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

SYNOPSIS

Low 500 hPa geopotential and cool maritime airmasses stretch from the North Atlantic towards Sweden, with the most prominent cyclone NW of Scotland. A zonal mid-level jet stretches along its southern flank with an axis from the Celtic Sea to Denmark and Lithuania. Two embedded short-wave troughs translate from Denmark to the Baltic States and from Ireland to the central North Sea, respectively, in the 09 to 18 UTC time frame.
On the anticyclonic side of the jetstream, a diffuse, inactive and stationary frontal boundary runs across France, Germany, Poland and Belarus. Everywhere further south, synoptic conditions are quiescent and the current heat wave goes into its second week. A weak and diffuse cut-off low wobbles from S Italy towards Tunisia, whereas a mid-level ridge stretches from Spain to the Alpine region. (NB: The freezing level reaches record-breaking heights around 5200m in the range of this ridge.)

DISCUSSION

... Baltic States, Belarus, Poland, Slovakia, Czech Republic, SE Germany ...

Cooler low-level air slowly slips southward in the range of the inactive frontal zone, creating a rather pronounced capping inversion around the 850 hPa level which suppresses vertical mixing. Converging winds and evaporation accumulate moisture beneath the cap and create a zone with 2m dewpoints around 20C, which allows CAPE up to 1000 J/kg or locally more despite rather meager mid-level lapse rates. Vertical wind shear is rather weak, but rises to 10 (15) m/s across the 0-3 (0-6 km) layer towards the north, courtesy to the fringes of the mid-level jet.
A travelling short-wave trough should initiate scattered to widespread thunderstorms in the Baltic States and Belarus until noon or in the early afternoon. Further southwest, synoptic lift is weaker and earlier and will likely give way to synoptic subsidence until the peak heating hours, hence a breaking of the strong cap is more questionable. Convective initiation will therefore probably be only isolated or scattered at best, though some models predict an upswing in the evening is well.
The combination of moderate CAPE and moderate vertical wind shear allows mainly multicellular storms, with perhaps a few temporary supercells. A few excessive rain and marginally large hail events are expected and isolated severe downbursts not ruled out.
Convection will further spread eastward into Russia and the Ukraine overnight while it becomes elevated and weakens.

... S and SE Europe ...

A few afternoon storms are possible over orographic features in an environment of moderate CAPE and weak, only in S Italy moderate vertical wind shear. Isolated heavy rain and marginally large hail are not ruled out, but considering the low storm coverage, this threat appears to be too low for a level 1.

... Ireland, Ulster, Scotland ...

Some daytime heating and strong lift ahead of a vorticity maximum create marginal CAPE, and showers with limited lightning activity are possible in the afternoon. Vertical wind shear is pretty strong across all layers, but mostly towards the south, where the depth of the unstable layer will likely be too shallow to allow precipitating or even electrified convection. Nonetheless, isolated severe wind gusts and/or a short-lived tornado cannot be ruled out in case the overlap of non-zero CAPE and strong vertical wind shear turns out to be better than anticipated.