Storm Forecast
Valid: Mon 25 Jul 2022 06:00 to Tue 26 Jul 2022 06:00 UTC
Issued: Sun 24 Jul 2022 22:49
Forecaster: TUSCHY

A level 2 was issued across parts of Germany mainly for damaging wind gusts and large hail.

A level 2 was issued for parts of SE France/NW Italy to S Austria mainly for large hail and severe wind gusts.

A level 1 surrounds all level 2 areas with similar hazards but less coverage. A low-end tornado risk is possible over N-CNTRL Germany.

A level 1 was issued for NE Spain mainly for large hail and severe wind gusts.

SYNOPSIS

The progressive wave pattern keeps going over N into CNTRL Europe with another wave crossing UK until 18Z before affecting Denmark / N Germany during the overnight hours. Subtropical ridging covers the CNTRL Mediterranean although gradual thickness decrease occurs over W-CNTRL Europe and a more noticeable drop of 850 hPa temperatures is forecast ins this area.

A 995 hPa depression in GFS (also tightly clustered between 995-1000 hPa in IFS-ENS) crosses the sea area Fair Isle into Viking/Utsire while weakening before re-forming in the lee of the Scandinavian Mountains. This low pushes a warm front E over Germany into Poland before multiple cold fronts follow from the NW. In addition, a pronounced convergence zone takes shape in the warm sector so no surprise: the main reason for ongoing uncertainties of this setup emerges of this amount of boundaries. Additionally, there remains a zonal spread regarding the cold frontal placement (e.g. at 12Z) in numerous models with GFS a tad slower than ICON/IFS. IFS-ENS spread is reasonable for this time-frame (covering Benelux) with similar magnitude in CMC-GEPS but less than NCEP-GEPS (which improved since yesterday).

The 500 hPa geopotential height field is stable in all models with just a slight decrease in amplitude of the approaching trough during the past few runs. GFS/GEM have some subtle indications for lifting leading short waves ahead of the main trough, which could also play some role in CI.

There are weak signals of an evolving LL depression over NE Germany next to the convergence zone but large ensemble spread keeps this scenario uncertain for now. In fact the latest ENS came in less aggressive and supports more a meridionally aligned surface pressure channel.

Overall the NWP guidance improved compared to yesterday and the cold front placement converged in most models.


DISCUSSION

... S/E Denmark, S Sweden and Germany ...

Not much change for Denmark into S-Sweden. Main focus for a few better organized DMC events converges to SE Denmark into S Sweden. Anafrontal-type character of this cold front with weak prefrontal capping should focus CI along the backside of the LL convergence zone, where weak LL lapse rates and modest MUCAPE overlap. Main focus for severe gusts remains from far SE Denmark to extreme S-Sweden (mainly offshore). Hence we kept the level 1 as yesterday. Heavy rain and isolated hail can occur with stronger cores.

Germany remains puzzling with NWP guidance still offering extreme solutions with either a near zero event or an high impact event.
ID2 remains on the very conservative side with a progressive prefrontal convergence zone racing E over N Germany, bound to a sharp surface trough. Resulting lowering BL T-Td spread remains manageable with ongoing impressive LL lapse rates around 10 C/km/ 3km and DCAPE around 1 kJ/kg. In addition, some better LL moisture flux converges emerges over NE Germany with some meager BL moisture input from the S. Hence we expect isolated CI along the backside of the convergence zone/leading edge of a moisture surge over NE Germany late in the evening with adequate MUCAPE. Damaging gusts are likely with isolated hail and some ensemble support for near hurricane-force gusts along the German/Polish border still exists.

CI along the cold front over W-CNTRL Germany during the afternoon hours remains suppressed in some models like ID2. However, point-source forecast soundings even in ID2 indicate a virtually uncapped air mass on a regional scale with MUCAPE in excess of 1000 J/kg. Even with bullish negative background support (cold front affected by CAA) mesoscale forcing along the orography should still support at least isolated CI with this kind of environment. ID2 a bit less aggressive with BL moisture next to LL confluent flow regime, so the decision was done to stick with the more aggressive model suite, neglecting the drier ID2 including its ensemble. A potential outcome could be a few regions with spotty CI, which grow upscale into small clusters betimes before morphing into one or two dominant bow echo events. The environment is supportive of rapid cold pool intensification and accelerating convection poses a damaging wind gust threat besides hail and locally heavy rain (mainly an issue during cell merging).

In the end we have to issue a rather broad level 2 to summarize all potential NWP outcomes along the convergence zone and cold front. Currently the highest confidence (relatively speaking) for such a wind-driven event exists over CNTRL into E-CNTRL Germany.

Not sure about CI over NW into N-CNTRL Germany with less diabatic heating and lower probabilities for improving BL conditions behind the convergence zone but kept a level 1 for isolated CI, which could become severe - even with a low-end tornado risk due to lower LCLs (but iffy looking shear conditions).

... SE France to Alps into SE Germany and N Italy ...

Not much change for this area despite a regional upgrade into a level 2. Better mid to deep layer shear over SE France may support a mix of multicells/isolated supercells with severe gusts and large hail.
Similar DLS but less 0-3km shear exists further E over S Switzerland, S Austria into N Italy. Widespread CI of organized multicells / temporal supercells should offer numerous large hail and severe wind gust reports in the upgraded area. Of main concern is the region Turin-Milan, where MLCAPE in excess of 2000 J/kg and discrete cells may produce very large/damaging hail next to severe wind gusts.
The severe risk diminishes during the night towards NE Italy but level 1 conditions continue with isolated hail issues (capped with modest MUCAPE and 15 m/s DLS).

... NE Spain ...

A grazing upper thermal trough affects this area during peak heating, when inland moving sea breeze undercuts very steep mid-level lapse rates, resulting in capped 2 kJ/kg MUCAPE. CI is bound to the orography, but with 20 m/s DLS, developing updrafts may bring large to very large hail and damaging gusts on a regional scale, probably matching local level 2 conditions. However, spotty CI and mixed model signals precluded such a significant upgrade for now.