Storm Forecast
Valid: Tue 12 Sep 2023 06:00 to Wed 13 Sep 2023 06:00 UTC
Issued: Mon 11 Sep 2023 21:27
Forecaster: PUCIK/KUZMENKO

A level 2 was issued across E Spain mainly for large to very large hail, severe wind gusts and heavy rainfall.

A level 1 was issued across Algeria, the Balearic Sea mainly for large to very large hail, severe wind gusts and heavy rainfall.

A level 1 was issued across N Spain, S to Central France mainly for heavy rainfall.

A level 1 was issued from NW France through S Belgium, Luxembourg, N Germany to NW Poland mainly for severe wind gusts, large hail and heavy rainfall.

A level 1 was issued across the Alps mainly for heavy rainfall and to the lesser degree for large hail.

A level 1 was issued across Finland mainly for severe wind gusts and tornadoes.

SYNOPSIS and DISCUSSION

Majority of thunderstorm activity across Europe will be associated with a cold front that will stretch from France across N Germany and S Sweden into Finland and a short-wave trough that will move from Spain towards S France during the forecast period.

The most pronounced overlap of CAPE and shear (and thus the highest probability of severe given initiation) is forecast across E Spain. Here, models simulate MLCAPE > 1500 J/kg with 0-6 km bulk wind shear reaching 25 m/s. Isolated to scattered initiation is forecast with upslope flow. Several supercells capable of large to very large hail, isolated severe wind gusts and heavy rain are forecast. While models disagree on the coverage, well-organised and severe convective storms may persist well into the overnight and morning hours over the Balearic Sea, travelling towards S France and Corsica.

Further N and NW (N Spain, S France, central France) will experience will see scattered to widespread initiation with numerous storms already existing in the early morning hours. Here, the major severe weather will be excessive rainfall as the strength of the storm motion speeds decrease during the day.

Across NW France, Belgium, Luxembourg, Germany and NW Poland, stronger vertical wind shear is forecast, reaching up to 15 m/s both in 0-6 and 0-3 km layers. The highest CAPE values are forecast on the interface of the well-mixed boundary layer with high LCLs originating from the Alps and moister airmass along the front. Several clusters of storms may travel along this high CAPE corridor. High res models suggest first cluster in the morning hours (in elevated form), followed by late evening to overnight storms. Isolated large hail, severe wind gusts and heavy rainfall are forecast with the more intense storms.

A marginal Lvl 1 is introduced over Finland. Here, models agree on skinny CAPE profiles, 0-1 km bulk shear above 10 m/s, curved hodographs and rather strong mean flow in the lower troposphere. However, the lapse rates in the boundary layer will be poor. Combined with weak CAPE, it is not convincing that strong updrafts or downdrafts will be present in the lower troposphere. Nevertheless, isolated severe wind gusts or a tornado can not be ruled out.