Storm Forecast
Valid: Fri 24 May 2024 06:00 to Sat 25 May 2024 06:00 UTC
Issued: Thu 23 May 2024 23:33
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for the Netherlands, central to E Germany mainly for excessive convective rainfall and to a lesser degree for tornadoes.

A level 1 and level 2 are issued for the Czech Republic and Austria mainly for excessive convective rain, large hail and to a lesser degree for severe convective wind gusts and tornadoes.

A level 1 and level 2 are issued for Poland, Slovakia, Hungary and W Romania mainly for excessive convective rain, large hail and to a lesser degree for severe convective wind gusts.

A level 1 and level 2 are issued for NE Spain, S France, N Italy, Slovenia, parts of Croatia, parts of Bosnia-Herzegovina and North Macedonia for large hail, excessive convective rain and severe convective wind gusts.

A level 1 is issued for N Finland and NW Russia for severe convective wind gusts, large hail and tornadoes.

A level 1 is issued for central Turkey mainly for excessive convective rain.

A level 1 is issued for E Turkey and Iran for excessive convective rain, large hail and severe convective wind gusts.

SYNOPSIS

A channel of low 500 hPa geopotential and surface pressure stretches from the British Isles across central Europe to Turkey, opposed by a blocking high between W Russia and the Norwegian Sea. The subtropic jet runs across the Mediterranean region, the polar jet across Iceland and N Scandinavia.
Most of Europe is covered by moist airmasses - rather cool in the range of the strongest cut-off low over the British Isles and France, rather warm elsewhere.


DISCUSSION

... central Europe between central France, BeNeLux, Germany, Poland, the Baltic States, Romania and the Alps ...

Despite rather poor lapse rates, the rich low-level moisture is "recycled" into low to moderate CAPE (up to 1000 J/kg, locally more along convergence zones and/or delayed vertical mixing) and allows another round of scattered to widespread, daytime-driven thunderstorms. Under mostly weak vertical wind shear, single cells, multicells and loosely organized clusters are the expected convective modes, and excessive rain is clearly the main hazard. The highest storm coverage is expected at the forward flank of a small cyclone over central Germany, where faint warm air advection and positive vorticity advection create synoptic lift that overlaps with moderate CAPE from Hungary into the Netherlands. This belt is therefore covered by a level 2. In contrast, clouds and rain as well as low-level stabilization quench thunderstorm activity at the cyclone's rear flank across the SW half of Germany.
Marginally large hail, copious amounts of small hail and localized severe wind gusts are secondary hazards, especially from Austria, the Czech Republic and central Poland east- and southward, where lapse rates are slightly steeper and capping is slightly stronger, allowing higher CAPE and therefore more discrete storms. The same applies to the eastern level 1 area across the W Ukraine, W Belarus and NE Poland, where a lack of synoptic lift in vicinity of the blocking anticyclone keeps thunderstorm coverage lower.
Low cloud bases and pronounced low-level convergence can also allow the spin-up of a few short-lived tornadoes, most notably at the nose of the warm air advection regimes across the Netherlands and NW Germany, where low-level wind profiles are best with 0-1 km shear around 10 m/s and pronounced low-level veering. A lack of insolation may be a limiting factor, though. Slightly enhanced tornado possibilities also stretch further along the most pronounced convergence zone across central Germany and Bohemia into E Austria.

... NE Spain, S France, N Italy, W Balkan states ...

The southern fringes of the CAPE reservoir overlap partly with the subtropic jet, which enhances deep-layer shear to values around 15 m/s, in NE Spain even to 20 m/s and beyond. Limited to absent synoptic lift keeps convective initiation isolated to scattered and mostly confined to orographic features. The best chances for more widespread storms into the night and possible upscale growth into one or two clusters are foreseen for N Italy, where a more pronounced short-wave trough arrives in the afternoon. The same trough crosses NE Spain and S France before, but forecast models are more reluctant to simulate convective initiation there. It is difficult to identify a clear reason, but a compensation of the trough's lift by cold air advection as well as strong entrainment by rather dry environmental air are possible detrimental factors.
The combination of moderate CAPE and moderate vertical wind shear allows well-organized multicells and a few supercells which can produce large hail, severe wind gusts and heavy to excessive rain alike. The rain hazard becomes dominant in N Italy in the evening and overnight with expected clustering of storms, while the other hazards diminish.
A few elevated and non-severe storms are also possibly over Corsica, Sardegna and surrounding sea waters overnight, when the mentioned short-wave trough extends its influence further SE-ward.

... NE Sweden, N Finland into NW Russia ...

On the warm side of a frontal boundary that is stationary first and starts slipping southward as a cold front overnight, a few hundred J/kg CAPE along with synoptic- and mesoscale lift allow scattered thunderstorms. However, forecast models do not simulate convective initiation before late afternoon and evening. Strong deep-layer shear around 20 m/s allows multi- to supercellular organization with a possibility of severe wind gusts and marginally large hail. Tornadoes are not ruled out, either, especially in the Russian part of the level 1, where the 0-1 km shear well exceeds 10 m/s. It is a bit unclear (1) if surface-based convection can form and (2) whether its motion can deviate far enough off the long and straight wind profiles to ingest enough streamwise vorticity for rotation. These limiting factors do not allow more than a low-end level 1 at the moment.
A few elevated and probably non-severe storms can travel quite far eastward overnight, reflected by a long extension of the low-probability thunder area.

... central and E Turkey into Iran ...

Scattered, daytime-driven thunderstorms are expected in an environment of moderate CAPE in the range of the Turkish cut-off low and at its forward flank. Near the center of the cut-off low over central Turkey, rather moist air with poor lapse rates and weak vertical wind shear keeps storm organization poor and the main hazard confined to excessive rain. Further east at the forward flank of the low, moderately enhanced vertical wind shear along with steeper lapse rates and drier air could also support some hail and wind events.