Storm Forecast
Valid: Sat 02 Sep 2023 06:00 to Sun 03 Sep 2023 06:00 UTC
Issued: Fri 01 Sep 2023 23:20
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of Morocco, parts of Algeria, Portugal, Spain and S France for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts and tornadoes.

A level 1 and level 2 are issued for Turkey, Armenia and Georgia mainly for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for E Slovakia, S Poland and the far-west Ukraine mainly for large hail and excessive convective precipitation.

A level 1 is issued for parts of Lithuania and Latvia for excessive convective precipitation and severe convective wind gusts.

SYNOPSIS

Two pronounced cut-off lows and their attendant surface cyclones are placed over the central Baltic Sea and over Portugal, respectively. A moderate mid-level jet with up to 20 m/s at the 500 hPa level rounds their flanks from SW-erly direction over Spain and France, W-erly direction over central Europe and finally S-erly to SE-erly direction over far-West Russia and Finland. Near the surface, the main frontal zone stretches near 50N across central Europe and is stationary there before it is pushed NW-ward as a warm front over France and the Bay of Biscay ahead of the Portuguese low. It separates subtropic air to the south from modified, maritime polar air to the north.
The ingredients for partly organized storms come together at the forward flank of these cut-off lows, as well as of a third, weaker cut-off low over W Turkey.

DISCUSSION

... Portugal, Spain, S France ...

Steep lapse rates from diurnal heating in inland areas and rich maritime moisture pushing inland overlap enough to create CAPE mostly on the order of 500-1000 J/kg, up to 2000 J/kg along the Mediterranean coasts and offshore (where it is strongly capped, though). Thanks to plentiful lift ahead of the cut-off low, scattered, partly clustered and embedded convection should already be active in the morning. After some daytime heating, more storms are expected to iniated and spread NE-ward, also reaching Catalonia, the Basque Country and finally towards evening S France.
Deep-layer shear is mostly in the 10-15 m/s towards the west and rises to 20 m/s towards the Mediterranean Sea. Early, discrete storms can therefore organize into multicells and probably some supercells and pose a hazard of large hail and perhaps isolated severe wind gusts. With a strong clustering tendency, heavy rainfall soon becomes the main hazard.
While convection should gradually weaken after sunset in inland areas, a strengthening easterly low-level jet from the Mediterranean Sea sets the stage for multiple and even more intense rounds of storms along the Spanish east coast overnight. Rainfall accumulations can well exceed 100 mm within a few hours, and flash floods become an increasing issue, possibly including some life-threating events. Updrafts of tail-end storms that manage to stay discrete and benefit from unimpeded moist inflow can still rotate and produce large hail, severe downbursts and perhaps one or two tornadoes in addition. This dangerous weather situation continues into Sunday.

... Turkey, Armenia, Georgia ...

A similar setup with steep lapse rates over the elevated inland terrain and moist sea breezes is in place ahead of the weak cut-off low over Turkey, also yielding CAPE up to 1000 J/kg (or more near the S coast) that overlaps with 10-15 m/s deep-layer shear towards the west and 15-20 m/s towards the east. Synoptic lift is more subtle, therefore convective initiation will likely be later and less widespread than in Spain. However, models largely agree on upscale growth into several large storm clusters in the evening that can persist well into the night. Likewise, large hail and isolated severe downbursts are the main hazards in early stages of discrete storms and towards the east, shifting to heavy rain as soon as clustering sets in.

... central Europe ...

Low-level moisture accumulates on the warm side of the frontal boundary and allows CAPE up to 1000 J/kg despite rather modest lapse rates, which overlaps with 15-20 m/s deep-layer shear under mostly unidirectional wind profiles. Forecast models agree on scattered thunderstorm activity towards the east that turns more isolated further westward, but agreement on timing and placement is rather low. The CAPE-and-shear space suggest mostly a multicellular storm mode with a few temporary supercells.
A level 1 is introduced for E Slovakia, S Poland and the adjacent Ukraine, where a first round of surface-based storms appears likely during peak heating hours. Forecast models somewhat agree on a second round of storms overnight mainly over the Czech Republic and again Slovakia, but these will likely be elevated. Additional convective initiation is also expected over orographic features further south of the frontal boundary, namely the Alps (especially in Austria) and the Carpathians in Romania, but will struggle to detach from the mountains under lacking synoptic lift support.
A few large hail and isolated excessive rain events are possible wherever storms form and persist, especially surface-based ones in the afternoon and evening, though the spread of forecast model simulations is too large to issue larger level 1 areas for now.

... central Baltic Sea and surrounding coastlines ...

Heating from the comparably warm Baltic Sea creates marginal instability within the cool airmass near the center of the northern cut-off low and supports widespread and partly thundery shower activity throughout the forecast period. Some heavy rain events are possible at onshore coasts of the Baltic States and perhaps S Finland. Parts of Lithuania and Latvia are upgraded to a level 1, as the strong background wind field plus ~10 m/s vertical wind shear across the 0-3 km layer could suffice to produce isolated severe wind gusts next to the heavy rain hazard.
It is not ruled out that a few non-mesocyclonic tornadoes might spin up right in the cyclone's center that wobbles around the SW tip of Finland.