Storm Forecast
Valid: Thu 08 Aug 2024 06:00 to Fri 09 Aug 2024 06:00 UTC
Issued: Wed 07 Aug 2024 22:07
Forecaster: BAART DE LA FAILLE/ADAMOVSKY/PUCIK
A level 1 was issued across parts of Austria, Slovenia, Italy and Switzerland for heavy rainfall and large hail.
A level 1 was issued across parts of Poland, Czech Republic, Slovakia for marginally severe wind gusts, heavy rainfall and possibly locally large hail.
A level 1 was issued across Italy (central/south) for heavy rainfall and large hail, to a lesser extent also wind gusts.
A level 1 was issued across parts of Russia for heavy rainfall, in the S/SE part also for wind gusts.
SYNOPSIS
A low S of Iceland induces strong mid- and upper-level winds over Norway and the British Isles. The associated cold front over W-Czechia and W-Poland into Sweden moves east into E-Poland and Slovakia. More to the south, the frontal boundary is trailing in the zonal flow over NW-Austria and Switzerland. South of the front, a rather moist airmass is present over the southern Alps and the central Mediterranean/Adriatic. A short-wave upper-level trough (visible at 500 hPa), initially near the NL-GER border, moves east, approaching the frontal boundary. The associated increased flow in the mid- and upper levels enhances bulk shear over the SE Alps from Thursday late afternoon.
A less pronounced low remains almost stationary over western parts of Russia.
DISCUSSION
... Austria, Slovenia, NE-Italy ...
Just south of the cold front, CI is expected over the mountains in MLCAPE > 1000 J/kg. 0-6 km shear is increasing during the late afternoon due to the shortwave trough, up to around 15 m/s. Model guidance is in good agreement on convective initiation with the highest coverage expected in southern Austria into Slovenia. However, the simulated storms by high resolution ensembles are rather weak, both concerning updrafts and their rotation. This could be caused by the presence of drier airmass around the mid-troposphere during much of the day. Main threats will be heavy rainfall rainfall and large hail.
... Poland, Czech Republic, Slovakia ...
Prefrontal and frontal convective initiation is expected especially in the afternoon, with some isolated storms possible already in the morning. Shear increases later in the afternoon, reaching up to 15 m/s in the 0-6 km layer. MLCAPE will reach up to 1000 J/kg, with mostly skinny profiles. Stronger storms may be capable of isolated heavy rainfall, marginally large hail or marginal severe wind gusts events. All threats are limited to some degree. Hail by the presence of only skinny CAPE, wind gusts by low LCLs and heavy rainfall by mean wind around 10 m/s resulting in rather fast moving storms. This barely warrants a low-end level 1. There's also a question if morning convection will limit the thermodynamic profiles.
... Central/south Italy and the Balkans ..
Lift will be provided by local orography with isolated to scattered convection developing in the early afternoon hours. Sea breeze is forecast to increase moisture along the Italian coasts, but doesn't seem to act as a forcing. Models are disagreeing concerning the low-level moisture content over the Apenines. Depending on the absolute moisture content, the threat over both regions may either shift towards heavy rainfall and some hail (given high content) or towards severe wind gusts (given lower content and high T-Td spreads near the surface.
... Russia ...
Models simulate an environment conducive to widespread initiation of slow-moving cells in weak mean wind. Forecast soundings show skinny CAPE but relatively moist profiles, warranting a flash flood risk. To the southeast, boundary layer will be drier, indicating a risk of severe wind gusts.
