Storm Forecast
Valid: Wed 12 Jul 2023 06:00 to Thu 13 Jul 2023 06:00 UTC
Issued: Wed 12 Jul 2023 00:04
Forecaster: GROENEMEIJER

A level 3 was issued across northwest Italy and far southern Switzerland mainly for very large hail, and to a lesser extent for severe wind gusts, very heavy rainfall and tornadoes.

A level 2 was issued across the Alps and their vicinity for large and very large hail, damaging winds, and for very heavy rainfall.

A level 1 was issued across an area surrounding the level 2 for large and very large hail, damaging winds, and for very heavy rainfall.

SYNOPSIS

A ridge with an axis initially from the Baltic Sea to the northwestern Balkans moves eastward to reach a line from Lithuania to Romania by Thursday morning. Upstream a strong southwesterly flow in the mid- and upper troposphere creates a strongly sheared environment that stretches across the Alpine region where a diffuse cold front moves slowly moves southeastward. Together with moderate instability on the northern forelands, and high instability on the southern, this sets the scene for another round of severe storms. Further south, the air-mass is capped by a deep elevated mixed layer.

Level 2 area...

Numerical weather prediction models suggest that little quasigeostrophic forcing for upward motion will affect the area during the day. Hence, lift will be primarily created by mesoscale processes. These will include orographically induced circulations and upward motion along low-level boundaries. Convection-allowing models suggest that diurnally created upslope flow will lead to convective initiation across the Alps and its northern, eastern, and southern flanks at several locations as well as downstream of the Massif Central and other lower mountain ranges. Pinpointing the locations is complicated, however initiation should be widespread. The environment, however, is clearly supportive of severe weather including supercells with a risk of large hail and severe winds.

Convection-allowing models suggest that storms will initiate by late morning or early afternoon and may soon grow upscale posing a threat of severe and locally extreme (>32 m/s) wind gusts downstream across much of Austria, South Tyrol, and possibly into adjacent countries such as Slovakia, Hungary, and Slovenia. However, this scenario is not supported by all models and hence not enough confidence is present to issue a level 3. Besides the wind threat, these storms will be capable of producing large to very large (> 5 cm) hail and locally very heavy rainfall.

Across far southeastern France, northwest Italy, and southern Switzerland more certainty exists that one or multiple rounds of storms will develop. As they do, they will encounter particularly strong 1-3 km shear that suggests strong storm-relative inflow and particularly vigorous updrafts in a high-CAPE environment, at least across southern Switzerland and northwest Italy. This is ideal for storms producing very large hail of 5 cm or possibly even 10 cm in diameter. Furthermore, severe winds and very heavy rainfall are important risks. As a result, a level 3 was issued for this area.

Across Northeastern Italy and Slovenia, there is high confidence that convective storms will spread to the lowlands during the late evening, into an environment where wind shear has substantially increased to nearly 20 m/s in the 0-3 km layer and abundant CAPE. These storms are likely to include some very severe hailstorms as well with an attendant wind and heavy rain risk.

Across Southern France, northern Switzerland, and southern Germany, shear will be as strong, but CAPE a little lower. Still, this should be more than sufficient to sustain very large hail of 5 cm or larger, locally damaging winds, and severe wind gusts.

West and Central Poland...

Remnants of an elevated mixed layer will be removed across Poland during the day as cooler air flows in from the west. In an environment of moderate CAPE and moderate wind shear, a few organized storms may form, including short-lived supercells with a risk of large but likely not very large hail, and possibly some heavy rain.

Southeast Poland, East Slovakia, East Hungary, Northwest Romania, extreme southwest Ukraine...

In a warm air advection regime across the region, both the ECMWF and GFS models suggest that an intense convective system may develop that translates along the warm from Poland to Romania. Given that 15-20 m/s of deep-layer shear are in place, an organized system seems well possible. That said, moderate CAPE of about 1000 J/kg, and some uncertainty about whether it will actually develop reduce the confidence to an extent that a level 1 seems adequate.