Storm Forecast
Valid: Tue 11 Jul 2023 06:00 to Wed 12 Jul 2023 06:00 UTC
Issued: Tue 11 Jul 2023 00:01
Forecaster: PUCIK
A level 3 was issued across central-E France, NW Switzerland and SW Germany mainly for severe to extremely severe wind gusts and very large hail.
A level 2 surrounding the level 3 is issued for the same threats, but with slightly lower likelihood.
A level 1 was issued across NE Spain mainly for severe wind gusts and large hail.
A level 1 was issued across England mainly for severe wind gusts and marginally large hail.
A level 1 was issued across SW Ukraine and Romania mainly for large hail and severe wind gusts.
A level 1 was issued for NE Turkey and Georgia mainly for excessive rainfall.
SYNOPSIS
At the mid to upper troposphere, a pronounced pressure gradient has developed between a deep low over the UK and a ridge over the W part of the Mediterranean. 25+ m/s of 500 hPa flow is forecast across much of France, BENELUX, and the Northern Sea. Towards the E, a large cyclone will cover Russia with its trough extending towards the Caucasus and a short-wave rotating towards E Estonia and E Latvia. Strong mid-tropospheric flow is wrapped around the base of the cyclone. Closer to the surface, a cold front will accelerate eastwards in the evening hours, crossing France and W half of Germany by the end of the forecast period. Ahead of the cold front, a plume of steep mid-tropospheric lapse rates will have advected over parts of W and Central Europe from the Sahara. Along the front, moisture pooling is forecast.
DISCUSSION
... S France through Switzerland, the Alpine range and S Germany ...
*** Very large hail and several swaths of severe to extremely severe wind gusts are forecast across the area ***
Models are in good agreement concerning the evolution of 1500+ J/kg of MLCAPE (locally even 3000+ J/kg) across S France, Massif Central, NW Switzerland, and N Italy thanks to the steep mid-tropospheric lapse rates. CAPE will overlap with strong vertical wind shear, which will increase towards the evening hours and will decrease from W to E. Across the Massif Central, models simulate around 25 m/s of 0-6 km and 20 m/s of 0-3 km shear. Initiation is expected in the afternoon hours due to the orographic lift as the front will still be far W. First storms will rapidly become supercells with almost certainty of large hail occurrence. Given substantial amounts of CAPE in the hail growth zone and rather straight and very long hodographs, 8+ cm hail may occur in the strongest storms.
The highest severe weather risk will be due to the severe to extremely severe wind gusts. High-resolution, convection-allowing, models consistently show one or more swaths of severe wind gusts across the area, including pockets of gusts exceeding 32+ m/s. The exact track with the most intense gusts is not clear. With the lack of strong linear forcing, the upscale growth of convection will depend on details such as storm mergers of individual supercells. Upscale growth will be supported by progressively deeper boundary layer towards E with Inverted-V profiles and high potential for strong cold pools.
While some of the models show an increase in the low-level SRH towards the evening hours across E France and SW Germany, the predominance of linear convective mode and high LCLs may hinder the tornado threat. This threat needs to be monitored especially if isolated supercells manage to persist into the late evening hours.
Lvl 3 is introduced for the area with the anticipated highest likelihood of extremely severe weather (very large hail in the W and extremely severe wind gusts in the E). Lvl 2 covers also parts of N Italy, where some of the models depict an evolution of supercells or another system with severe wind gusts in the late night to early morning hours. The severe weather threat would be very high given the degree of CAPE/shear overlap.
... NE Spain ...
Models show only weak signals for convective initiation in the area confined primarily to the mountains. Forecast Skew-Ts show fat CAPE profiles with deep Inverted Vs. Hodographs support supercells, especially near the Pyrenees. If storms manage to form, they will likely be severe with threats of both damaging wind gusts and some large hail.
... S BENELUX towards N Germany ....
Storms are forecast to develop along the advancing cold front. Forecast hodographs show shear favorable for both supercells and some linear segments. The most likely threat will be isolated severe wind gusts, but that could depend on whether the storms would already be isolated as they move over the area overnight. The severe weather threat will be increasing from N to S towards the areas of more abundant CAPE and a higher likelihood of surface-based convection.
... England ...
Scattered well-organised storms, including supercells, are forecast to move across the area. Isolated severe wind gusts and marginally large hail will be possible with stronger storms. That said, each of the threats will be limited to some degree: hail by skinny CAPE profiles and wind by lack of stronger low-level shear or absence of deep boundary layer.
... Romania ...
Vertical wind profiles will allow for well-organised storms, including supercells. These may be capable of large hail and severe wind gusts. The severe wind gust threat will be maximised towards the south, where the deep boundary layer will yield Inverted-V profiles. Models are not in good agreement concerning the coverage of storms and also the extent to which storms move away from the Carpathians. Thus, a broad Lvl 1 is introduced without any specification about a more precise severe weather track.
... NE Turkey, Georgia ...
Prolonged onshore flow will result in multiple waves of storms in environment conducive to high rainfall amounts.
