Storm Forecast
Valid: Fri 25 Aug 2023 06:00 to Sat 26 Aug 2023 06:00 UTC
Issued: Thu 24 Aug 2023 21:17
Forecaster: GROENEMEIJER

A level 2 was issued across a large part of Central Europe for large hail, severe winds, and heavy rainfall.

A level 1 was issued across across and area surrounding the level 2 for the same risks.

A level 1 was issued across southern Italy mainly for large hail.

DISCUSSION

A strong mid/upper-level southwesterly flow between a ridge extending from the Baltic States southwestward and a closed low across Scotland is translating eastward. As an associated trough moves eastward, quasigeostrophic forcing for lift will overspread NW Germany. At lower levels, forcing is weaker, but as low-level temperatures gradually fall, CIN weakens which gives the opportunity for deep-convection from the boundary layer to form.

As on Thursday, storms are expected across a very large area. Within this area it is difficult to pinpoint the regions with the highest risk.

Across Austria, Czechia, and southeast Germany, relatively high temperatures imply a risk of strong winds through the formation of evaporatively cooled downdrafts and strong cold pools with any well-organized storm system. Ample CAPE of 2000 J/kg and locally higher also implies a considerable risk of hail, which may locally be very large (> 5 cm). These hazards will occur with storms that will likely take the mode of supercells transitioning into bowing systems.

Across Poland, Central and northern Germany, the eastern Benelux and NE France, CAPE should nevertheless also be considerable (1000-1500 J/kg). In combination with moderate 15=20 m/s deep-layer shear across Poland, increasing to 30 m/s towards France/Benelux, a number of well-organized supercells and bow echoes are expected here as well. Probably the risk of very large hail is lower here, courtesy of the weaker instability. There is a small chance that a tornado could develop given marginally supportive low-level shear. The storm activity should gradually also affect Denmark and southern Sweden, where a lower risk will exist, but some marginally severe hail, wind and strong rainfall are possible as well.

Across southeastern France. there is a comparatively good model consensus on the development of a slowly-moving storm system across the lower Rhone Valley, that could bring not only large hail, but also very high precipitation accumulations and an attendant risk of local flash floods.

Across the Pyrenees and Northeast Spain, isolated storms may develop during the afternoon, that could produce large hail given the supportive instability and CAPE.

Southern Italy...

Isolated storms are expected here, which can produce large hail given the very high ~3000 J/kg CAPE in place.