Storm Forecast
Valid: Thu 26 Sep 2024 06:00 to Fri 27 Sep 2024 06:00 UTC
Issued: Wed 25 Sep 2024 21:55
Forecaster: TUSCHY

Numerous level 2 areas were issued over S-CNTRL Europe mainly for excessive rain.

Level 1 areas surround the level 2 areas for similar hazards with lower probabilities.

A level 1 extends from UK/France all the way to the Baltic States with variable risks. An augmented tornado/gust risk extends from France into Germany/Benelux with a more dominant excessive rain risk affecting S UK and the coastal areas. The level 1 becomes more marginal towards the Baltic States with an isolated gust/heavy rain and tornado risk.

SYNOPSIS

A deep positive tilted Rossby wave extends from the Norwegian Basin/Sea to the North Sea, bending SW into the West European Basin with the deepest negative mid-level height anomalies just W/NW of the Bay of Biscay. Not much change of placement and geometry of this wave is forecast as it shifts E during the forecast with its axis forecast to approach the German Bay into the Bay of Biscay.
Despite its slow motion, numerous waves with shorter amplitude circle the main trough and add a more dynamic component to this pattern. In addition, embedded steering vortices within this trough weaken/re-form like a decaying vortex over Irland with a new one evolving E/atop the North Sea. This has some impacts on enhanced DMC activity on a regional scale. The Rossby wave gets framed by positive height anomalies over Russia and next to Greenland but also down into N Africa with rather healthy subtropical cells present.

This anomalous pattern features rather impressive moisture fields, flowing from the E-CNTRL N Atlantic E/NE into the SE flank of the main trough. Layered TPWs for this plume of gradually modifying tropical air show deep anomalous moisture in all levels including the 700-500 hPa layer, where PWs just shy below 20 mm are comparable to the ITCZ activity. Blended TPW anomalies show values in the 40-55 mm range, which is in the 150-200 % range regarding background climatology. This atmospheric river is certainly a rather impressive one as IVT values exceed 1000 kg/ms over most of W into CNTRL Europe, also highlighted by EFI water vapor flux showing peak EFI values from N Spain into the far W Russia (including embedded SOTs up to +2). As impressive as the vapor flux is the EFI precipitation parameter with an extensive region showing values up to 1 and SOT of 1 and more over W/CNTRL Europe.

Next to the moisture, EFI CAPE/shear is also enhanced from France into Germany and highlights a rather impressive CAPE/shear space for this time of year. Despite CAPE being on the low to modest side, a deep/intense hyperbaroclinic zone with intense mid/upper-level winds (+3 sigma at 250 hPa) impacts most of the base and SE fringe of the Rossby wave. This assists in an impressive corridor with augmented severe from W into CNTRL Europe.

Numerous embedded synoptic-scale fronts/waves within this atmospheric river will serve as foci for enhanced thunderstorm activity. The main eye-catcher for organized DMC activity evolves between a wave, which moves from CNTRL France into Germany and a second cold front over far NW France, which shifts E/SE during the forecast and encompasses parts of CNTRL into NE France, Benelux and W/N Germany but also parts of UK into the S- North Sea.

Please keep in mind that ESTOFEX only covers the rain risk, which is accompanied by DMC. Please refer to the local national weather services regarding the more general QPFs.

DISCUSSION

... W-CNTRL to NE/E France, Benelux into W/NW Germany ...

An overlap of constantly cooling mid-levels (500 hPa dipping into the -20C range over far N-France) atop an unseasonable moist low-tropospheric airmass results in a broad belt of low to moderate instability with MUCAPE in the 200-500 J/kg range. Forecast soundings show effective ELs of 6-7 km AGL, so thunderstorms should take profit of more or less all of the extreme 30-45 m/s DLS. 0-3 km shear remains constant in the 15-20 m/s range with patchy 0-1 km shear peaks of similar magnitude.

Forecast hodographs are very long but also curved in the ongoing WAA regime with LCLs in the 200-600 m range. LLCAPE will be on the order of 100 to patches of 200 J/kg, so the main question remains if shear could be too much for this amount of CAPE. However, merging cells should be able to attain a broader/more robust updraft status and interaction with the background shear/dominating linear pressure perturbation dynamics should push storms away from the hodograph, causing rather impressive SRH values with lots of streamwiseness but also enough streamwise vorticity in the inflow layers.

Another uncertainty is how discrete thunderstorm activity remains with EZ driven guidance being more aggressive in clustering, whereas others supporting more semi-discrete cells. Q vectors indicate numerous waves with compact/enhanced forcing within a broad belt of subsidence and hence a still substantial spread in potential convective activity along this corridor is noted in NWP guidance.

For now a broad level 1 was issued to highlight the general risk of strong to severe gusts with more robust cells. Amount of 0-3 km shear also points to bowing line segments with an enhanced gust risk.
A tornado risk is certainly present with any longer-lived thunderstorm and rapid translation of the cells would also support a significant/long-tracked event. No upgrade to a level 2 was performed as it remains unclear, if a more clustered tornado event next to discrete thunderstorm activity materializes, caused by ongoing but important issues of the MSLP and geopotential height geometries. Some models would support a regional outbreak potential of rotating updrafts e.g. for far NE France to Belgium/Netherlands into NW Germany with nowcast becoming an important issue!

... Far SE France to S-Switzerland into the far S Alps and NW Italy ...

Interacting moisture plume with the orography causes scattered thunderstorms from the afternoon into the overnight hours, partially embedded in the more stratiform rain. A 50-60kt 700 hPa jet with 0-3 km shear in the 20-25 m/s range points into the orography, assisting in a very effective inflow of the unseasonable moist airmass. Combined with warm cloud layers in the 3-4 km range, an efficient collision coalescence process assists in impressive 12-h rainfall amounts within the level 2 areas, where 50-150 l/qm are forecast with local higher peaks. The saving grace is the progressive nature of the trough and atmospheric river, which should keep the peak amounts more or less in check. Nevertheless, these amounts still support flash flooding in the highlighted areas with a few significant events possible along the complex orography.

... S-UK into the Germany Bay and coastal areas of the Netherlands and NW Germany ...

The already mentioned reforming upper vortices over NW Europe result in a broad/zonally aligned channel of low pressure. The available airmass along this broad confluent flow regime features MUCAPE in the 200-500 J/kg range but also deep/moist profiles. Once again the final geometry of the pressure/height fields determines the magnitude/outcome for this event. In addition, passing weak vortices increase LL convergence on the regional/temporal scale.
The expectation is a gradually southward sagging zonally aligned belt of enhanced precipitation with embedded convection, which induces 24 h rainfall amounts in the 40-80 l/qm range - a few higher peaks are possible. Flash flooding a possible and if this band sets up next to larger urban areas, a few significant flash flood events cannot be ruled out.
In addition, augmented LLCAPE next to the convergent flow regime and low LCLs could support a few tornado events.

Another QPF peak occurs in the Newcastle area but this is probably more a cold jet/TROWAL event, interacting with the orography, so limited upright convection precludes our level issuance for now.

A rather potent setup could evolve over/E of the Strait of Dover to the IJsselmeer/Markermeer into far NW Germany during the overnight hours. A structuring vortex atop the S Germany Bay consists more of a stretched channel with numerous embedded vortices. Influx of warm/moist air assists in shallow warm core signatures and NWP guidance continues to support various scenarios with spin-ups of one of those vortices. IFS-ENS indicates an elongated area with clustered 990 hPa members. Any more dominant vortex could result in a subtantial wind risk on a regional scale due to the thermal core structure.

Rather lively DMC activity within a slightly unstable airmass is forecast with convection posing a severe gust threat next to an isolated tornado risk, especially along the frictionally forced coastal convergence zone. Dependant on where any convergence evolves, repeated thunderstorms could bring heavy rain along the coasts - some models keep this activity more offshore, others push it ashore, so no upgrade is possible with ongoing uncertainties.


... Baltic States ...

A broad level 1 was added for this area. It resides in the strong/deep WAA regime with the advection of a moist/unstable airmass well N. A few embedded thunderstorms are forecast and they could organize in a favorable shear environment with 15-20 m/s DLS. Forecast hodographs feature rather impressive curved hodographs, but lowest 500 m AGL cloud stay rather stable, so most of the activity could be more elevated. Not much BL modification is needed for a few better organized thunderstorms, so a level 1 was expanded all the way into Russia.