Storm Forecast
Valid: Tue 29 Sep 2026 06:00 to Wed 30 Sep 2026 06:00 UTC
Issued: Mon 28 Sep 2026 20:27
Forecaster: TUSCHY

A level 2 was issued across parts of S France mainly for excessive rain.

A level 2 was issued across far NE Spain mainly for excessive rain.

A level 1 surrounds the level 2 areas for heavy rain but also for a few coastal tornado events.

A level 1 was issued for the Caucasus Escarpment mainly for heavy rain and an isolated tornado.

SYNOPSIS

An explosive cyclogenesis occurs over the far NE Atlantic, which keeps a strong ridge steady over S/CNTRL to NE Europe. An extensive upper trough over SE Europe features numerous embedded vortices, which bring scattered thunderstorms over a broad area.

DISCUSSION

... Far NE Spain during the daytime hours ...

Ongoing from the previous day, we see a plume of moist and unstable air streaming onshore due to a 20-25 kt southerly LLJ (which veers betimes). This activity roots into a very moist and unstable airmass offshore with MUCAPE in the 2000 J/kg range. Plenty of coastal convergence exists in the BL wind field for repeated training convection during the day, which builds E betimes.

The main concern is the anomalous high moisture and CAPE content of this airmass. Deep warm cloud layers point to very efficient collision coalescence processes with hourly rainfall rates in the 40-80 mm range possible (including a few 100 mm/h (plus) events). A flash flood risk exists and a few significant/life threatening events are forecast (e.g. in urban areas). Hence the level 2 upgrade. A few coastal tornado events are also forecast and even some hail cannot be ruled out with broadening CAPE profiles during the day.
The veering low-tropospheric wind profile support a decreasing risk from the evening hours into the overnight hours, although some lingering/slow moving convection could persist all night long on an isolated scale.


... S France during the evening and overnight hours ...

A classique episode cevenol or Marin event unfolds for the level 2 area with a significant flash flood risk forecast including a risk of a few damaging/life threatening events.

During the day, BL pressure responds to the explosive/RI cyclogenesis over the far NE Atlantic and lowers on a constant rate, while a surface ridge is anchored from Italy NE towards E Europe. Resulting S to SE-erly winds atop the Gulf of Lion increase during the day and especially during the overnight hours.

This low level jet advects a very moist and unstable airmass from the W-CNTRL Mediterranean towards the French coast. This jet roots into SST anomalies of 2-3K with MUCAPE in the 1000-2500 J/kg range with improving TPW anomalies into the 200% range to the background climatology.

This onshore streaming plume of unstable and extremely moist air (TPWs in the 35-40 mm range) in combination with deep warm cloud layers up to 4km and impressive WAA driven looped hodographs point to a growing risk during the night for training convection, as this airmass meets the orography of the Massif Central/Cevennes. Very efficient rainfall production/warm rain processes are forecast and improving IVT values point to ongoing moisture supply from the S for mentioned DMC activity.

EPS data (ICON) has high probs. for more the 70 mm/12h along the Cevennes but areas with repeated passage of intense convective cores or beneath training convection see 12h amounts probably more in the 150-300 mm range (with local 40-80 mm/h amounts possible). This area will see a significant nocturnal flash flood risk and a few life-threatening/damaging events are possible.

Further S (towards the coast) it remains a bit unclear, if we get a back-building V-shaped MCS going, wich builds into the strong LLJ. This scenario is shown by EZ data, but for now we think that evolving offshore convection decreases this risk somewhat. We expanded the level 2 a bit south however to account for some backbuilding.

An isolated coastal tornado risk is possible and this risk could extend a bit inland also.

This will be a multi-day rainfall event which keeps going beyond 06Z.

... E Black Sea ...

A lifting upper pool of cold mid-levels exits the area of interest during the day and leaves behind a disorganized/broad marine vortex. This vortex could see some better organization ahead of another cold-core vortex, which approaches from the N during the night. Temporal deepening of the vortex over the E Black Sea is forecast, which is encompassed by warm/moist and unstable air and a temporal deepening shallow warm core anomaly is possible. Most DMC activity occurs E/NE of the vortex'center but a temporal "subtropical" stage is not ruled out during landfall along NW Georgia. Heavy rain and an isolated tornado is possible next to strong gusts.

... NW Europe ...

A broad low probability thunderstorm area was added to account for spotty elevated convection along the N-ward surging warm front and in the capped warm sector with some elevated MUCAPE.