Storm Forecast
Valid: Thu 24 Aug 2023 06:00 to Fri 25 Aug 2023 06:00 UTC
Issued: Wed 23 Aug 2023 21:53
Forecaster: PUCIK/KUZMENKO

A level 2 was issued from France through N Switzerland, S Belgium towards S Germany mainly for large to very large hail and damaging wind gusts.

A level 1 was issued across S-Central France mainly for damaging wind gusts and large hail.

A level 1 was issued for NW France, SE UK, Netherlands, W Germany mainly for large hail, severe wind gusts and to the lesser extent for heavy rain.

A level 1 was issued over the Alps mainly for large hail, heavy rain and severe wind gusts.

A level 1 was issued over Romania, Bulgaria, S Serbia, Montenegro, Albania, N Macedonia, N Greece mainly for excessive rainfall and large hail.

SYNOPSIS

At mid to upper troposphere, a deep low resides over the northern UK and a ridge will stretch across the W Mediterranean. In between, 25+ m/s of 500 hPa flow is simulated by the models. Further SE, a cut off low remains south of Sicily. A short-wave trough is forecast to move over Belarus and E Ukraine during the day. Closer to the surface, a wavy cold front will stretch from central/N France through BENELUX into N Germany. A warm front will move over Poland. The highest potential for convective storms is forecast in a belt of abundant low-level moisture along and ahead of the cold front. A plume of steep mid-tropospheric lapse rates that overlaps with at least modest low-level moisture will be confined to S/Central France and the Alpine region.

DISCUSSION

... SE UK, France, BENELUX, W/S Germany, Switzerland, Austria, N Italy ...


Due to the large differences among the NWP model simulations, this is a highly uncertain forecast. Decided to go for a broad Lvl 2 to cover an area where the environment is most favorable for intense and well-organised convection.

Morning hours will likely start with one or more elevated clusters over N France and BENELUX. Moderate shear above the boundary layer and 1000+ J/kg of MUCAPE may result in isolated instances of large hail. The remains of the elevated clusters may later become foci of surface-based storm development as the heating destabilizes boundary layer. This scenario is depicted by some high-resolution, convection allowing models, yielding convective windstorms over S Germany.

Towards the afternoon hours, models simulate a pronounced overlap of high CAPE (2000+ J/kg) and strong shear (25+ m/s in 0-6 km layer) acrross central to N France, perhaps reaching up to S Belgium. The highest CAPE is forecast along the boundary between a very well mixed and rather dry airmass advected from S and more abundant low-level moisture to N. The exact position of this boundary and its evolution differs from one model to another. Therefore, this has to be tracked during the day to establish more precise location of the highest severe weather corridor. The lack of clear forcing from either the cold front or other synoptic-scale features makes it difficult to predict exact timing, location and coverage of storms.

Some of the high resolution models show initiation of storms over Massif Central and their progression towards the E with high risk of severe wind gusts due to the well mixed boundary layer combined with 0-3 km bulk shear around 15 m/s. Towards the evening hours, isentropic lift due to the warm-air advection will likely initiate further storms N of the boundary. These will rapidly become elevated supercells or bow-echoes with threats of large to very large hail and severe wind gusts.

Towards BENELUX and NW Germany, CAPE will decrease and mostly skinny CAPE profiles are foreast, reducing the threat of large hail to some degree. Still, the risk will be present with elevated supercells, as well as isolated risk of severe wind gusts. The highest coverage of storms is forecast during the overnight hours. Both here, and over N France, 0-1 km shear is forecast to increase towards the night, but the storms will likely be elevated, reducing the tornado risk.

... Romania to N Greece ...

Forecast profiles show abundant CAPE combined with 0-6 km bulk shear between 5 and 15 m/s. The shear will increase towards N. Primary hazards will be isolated large hail, due to the abundant buoyancy in the cold part of the cloud and exessive rainfall due to the slow storm motions.