Storm Forecast
Valid: Fri 12 Jul 2024 06:00 to Sat 13 Jul 2024 06:00 UTC
Issued: Thu 11 Jul 2024 22:59
Forecaster: PUCIK/ENGERT

A level 2 was issued from N Italy through SE Germany, Austria, Czechia, W Slovakia into NE Germany and Poland mainly for large to very large hail, severe wind gusts, heavy rainfall and tornadoes. Read the text description for more specific placement of hazards in different locations.

A level 1 surrounds the level 2 for lesser coverage of severe weather.

A level 1 was issued across the E Baltics, W Russia and Belarus mainly for heavy rainfall, large hail and severe wind gusts.

A level 1 was issued across Turkey mainly for heavy rainfall.
DISCUSSION

... extreme E France / N Italy ...

A borderline Lvl 2/Lvl 3 situation is forecast across the area with numerous uncertainties obvious from the models. What all models agree on is the presence of strong vertical wind shear, ranging from 20 to 30 m/s in the 0-6 km layer and up to 20 m/s in the 0-3 km layer, supportive of supercells and also bow echoes should the linear segments develop. All models also show abundance of storms already in the morning hours.

While already this activity may bring severe weather, large uncertainty remains concerning its impact on the main wave of storms that should come in the afternoon. Compared to the 00 UTC runs of Thursday, 12 to 18 UTC runs have decreased the CAPE values and also 2m temperature close to the Alps. This resulted in weakening of supercell structures simulated by e.g. ICON-D2. There's a large difference in the CAPE field between the local and global models. Global models show the highest CAPE to the NE of the Apenines, whereas local models produce fohn and drying in their wake. The third source of uncertainty is the presence of dry layer at the mid-troposphere especially over W Italy in the afternoon to evening hours.

The most intense storms should appear after 15 UTC, initiating over the Alps and spreading also over the lowlands/foothills of N Italy. Expect several supercells capable of large to very large hail and severe wind gusts. Towards the evening, increase in the low-level shear will enhance the tornado threat over NE Italy. While Lvl 3 was considered for parts of NW Italy, the effect of morning storms and expected disruption in the diurnal heating leave the coverage of extremely severe weather a bit too uncertain at the moment.


... Austria / S Germany / Czechia / W and NW Slovakia / S-central to NE Poland ...

A complex scenario is forecast across the area. Same as for N Italy, numerous storms may be ongoing across the region already in the morning to early afternoon hours. These may influence the pre-convective environment for the later storms.

The main activity is forecast to start on the N flanks of the Alps around 12 UTC across S Germany and spread towards N Austria and Czechia in the afternoon hours. At first, supercells may form as the 0-6 km bulk shear reaches 20 m/s. As a strong low-level forcing overspreads the area, it is likely that storms will grow eventually upscale. Question remains when and where will this happen. Several ICON-D2 EPS members suggests isolated supercells reaching even E Austria towards the evening, while CLAEF members mostly point at quick upscale growth already from the beginning. Nevertheless, the first activity will be capable of large to very large hail and severe wind gusts.

Towards the evening hours, further upscale growth is forecast with increasing strength of WAA, arrival of the cold front and also increasing wind/wind shear near the ground owing to the cyclogenesis along the frontal wave. This will also increase the potential for severe wind gusts and heavy rainfall. The severe wind gust potential will be heavily influenced by the previous convection and cloudiness. Should the N Austria - E Czechia - S Poland - extreme W Slovakia receive abundant diurnal heating, there will be potential for numerous swaths of damaging wind gusts in high LCL environment. Should LCLs remain rather low and convection more discrete, the increase in the low-level shear and hodograph curvature may result in an increase in tornado threat instead. Mesoscale evolution of the pre-convective environment and convective mode during the day needs to be closely monitored.

During the night, large convective system(s) will travel from Czechia and NW Slovakia to Poland and further towards the Baltic Sea. Driven by low-level jet and powerful WAA, more widespread heavy rainfall and some severe wind gust threat will persist as the systems become elevated.

... Switzerland to W/central/NE Germany to W Poland ...

Storms are forecast to develop along the cold front surging E. Severity will increase from W to E owing to the increasing values of CAPE and reduction in the dry layer at the mid-troposphere. Vertical wind shear will reach 15 - 20 m/s in 0-6 km layer, allowing for a mix of multi and supercells. While environment will not be extremely conducive to any type of major hazard, a mix of large hail, severe wind gusts and heavy rainfall is expected with more intense storms. High expected coverage warrants a Lvl 2.

... E Baltics, W Russia, Belarus ...

Scattered to widespread storms are forecast along the cold front. Compared to yesterday, shear is forecast to be weaker and storms will move slower. Expect mostly heavy rainfall threat with large hail and severe wind gust threat increasing towards the S/SE, where higher CAPE values are simulated.

... NW Turkey ...

Flash flood risk is forecast with basically stationary storms as mean wind is close to 0 m/s.