Storm Forecast
Valid: Tue 16 Jul 2024 06:00 to Wed 17 Jul 2024 06:00 UTC
Issued: Mon 15 Jul 2024 17:54
Forecaster: PISTOTNIK

A level 2 and level 1 are issued for Poland, parts of Slovakia and parts of Hungary for excessive convective precipitation, large hail and severe convective wind gusts.

A level 1 is issued for parts of Austria and N Italy for excessive convective precipitation and large hail.

A level 1 is issued for NE Germany for severe convective wind gusts and to a lesser degree for tornadoes.

SYNOPSIS

An extensive mid-level low filled with cool maritime air extends between Scandinavia, Iceland and the North Sea. The most active surface cyclone within this low-pressure complex is centered over the North Sea. Its almost stationary warm front runs east into Belarus, whereas its cold front swiftly crosses north-central Europe before its tail becomes stationary along the Alps and Pyrenees as well. Very hot air is present south of this long frontal boundary under weak geopotential / pressure gradients. The heatwave in E and S Europe therefore continues.

DISCUSSION

... Poland, Slovakia, Hungary, Ukraine, Belarus ...

Rich low-level moisture accumulates on the warm side of the frontal boundary and creates CAPE mostly on the order of 500 to 1500 J/kg, as it partly overlaps with steep lapse rates further south in the warm sector. The highest CAPE is expected over SE Poland and will likely exceed 2000 J/kg. Scattered to widespread thunderstorms will form from noon onward across Poland, where the cold front and a mid-level vorticity maximum create sufficient lift. Storm coverage will be somewhat lower further south in Slovakia and Hungary, where this lift is weaker. Deep-layer shear is mostly around or below 10 m/s, hence single cells, multicells and loosely organized clusters are the expected convective modes, and excessive rain is the dominant hazard. However, some discrete storms can organize a bit better and can also produce localized large hail and severe wind gusts, owing to particularly steep lapse rates to the south and to somewhat stronger deep-layer shear (10-15 m/s) towards the north. Strong to severe wind gusts driven by cold air pools may also occur at the leading edge of mesoscale storm clusters.
Convection will continue to move NE-ward into the Ukraine, Belarus, Kaliningrad and onto the Baltic Sea in the evening and overnight, gradually weakening and loosing its severe weather threat.
To the south of the almost stationary warm front, a belt of similarly high CAPE also extends eastward along the Belorussian-Ukrainian border. However, forecast models show very little precipitation signals there. Synoptic subsidence beneath a short-wave ridge along with a lack of orographic features makes convective initiation indeed unlikely. Nonetheless a careful monitoring of observational data is recommended in this area as well, as isolated cap-breaking storms could also produce heavy rainfall and large hail under this CAPE magnitude.

... Austria, N Italy ...

Scattered storms are expected at the trailing cold front along and near the main Alpine crest. CAPE is confined to a few hundred J/kg over high terrain, but increases to 1000 J/kg and locally beyond over the Alpine forelands. Vertical wind shear is low to moderate, but may rise into the 15-20 m/s range north and east of the Alps, where upvalley circulations are opposed to the W-erly to SW-erly mid-level flow. Storms that manage to detach from the mountains can therefore organize into multicells or perhaps the odd transient supercell and produce large hail, but this scenario is quite questionable due to a fairly robust cap. Otherwise, localized excessive rain is the main hazard, also over high-Alpine terrain. The coverage of convective storms is still somehwat unclear, as it depends on the track and strength of a subtle vorticity maximum aloft. Most forecast models show rather sluggish convective initiation and a peak of the activity not before evening, hence the time window for surface-based storms could also be limited. A level 1 seems to suffice to cover the combined rain and hail hazard.

... Netherlands, Germany ...

The strongest short-wave trough crosses BeNeLux and the N half of Germany well behind the cold front and interacts with a marginally unstable, maritime airmass, though its lift is partly compensated by cold air advection. Scattered and partly thundery showers will form in the course of the day. The majority of this convection will be too low-topped and/or too far on the cyclonic side of the jetstream to benefit from enhanced 0-6 km shear, therefore severe weather is unlikely.
However, right beneath the jet stream, a belt of enhanced vertical wind shear (~20 m/s across the 0-6 km layer, 5-10 m/s across the 0-1 km layer) runs straight from SW to NE across Germany. While convective initiation is unlikely across the southern half of Germany due to a lack of synoptic lift, the majority of the ICON-D2 ensemble members indicate scattered convective initiation from north-central Germany (roughly the province of Saxony-Anhalt) northeastward towards the Polish border and the southern Baltic Sea. This belt is therefore upgraded to a level 1, as low-topped multi- to supercells can produce isolated severe wind gusts or maybe a brief tornado.

... SE Europe, Turkey, NE Spain ...

The hot airmass features steep lapse rates, but is often deprived of low-level moisture, keeping CAPE mostly on the low side (apart from coastal areas, where it is strongly capped). In the absence of synoptic lift, only isolated afternoon thunderstorms are expected where upvalley and upslope circulations feed enough moisture towards prominent mountains. These pulse storms can pruduce localized excessive rain and large hail. However, their expected coverage appears too low for a high-probability thunder or level 1 area.