Storm Forecast
Valid: Wed 12 Jun 2024 06:00 to Thu 13 Jun 2024 06:00 UTC
Issued: Tue 11 Jun 2024 21:09
Forecaster: TUSCHY

A level 3 was issued across parts of N-Bulgaria and S-Romania mainly for very large hail and extreme wind gusts.

A level 2 surrounds the level 3 for similar hazards but lower probabilities. An isolated strong tornado event is possible.

A level 1 surrounds both level areas with hail/gust hazards and heavy rain.

A level 1 was issued into N Italy for large hail, severe wind gusts, heavy rain and an isolated tornado risk.

A level 2 upgrade was performed over parts of NE Italy mainly due to higher probabilities of severe hazards.

A level 2 was issued for SE Spain mainly for large hail, heavy rain and a tornado risk.

A level 1 surrounds the level 2 for similar hazards but lower probabilities.

A level 2 was issued for far NE Ukraine into Russia mainly for very heavy rainfall.

A level 1 surrounds the level 2 for similar hazards but lower probabilities including a large hail risk over SE Ukraine.

A level 1 was issued for NE Algeria mainly for an isolated large hail event.

SYNOPSIS

A quasi-stationary wave pattern continues over Europe with a deep trough placed from Scandinavia into Spain and an intensifying subtropical ridge arching into SE Europe. This pattern supports a broad overlap of high shear and CAPE with numerous foci for enhanced severe.

Along the surface, an extensive and wavy boundary extends from CNTRL Europe into Russia and separates an unstable prefrontal airmass from cooler/more stable conditions to its W.

DISCUSSION

...Bosnia and Herzegovina, Serbia into parts of Romania and Bulgaria ...

An outbreak of numerous long-tracked supercells with destructive hail and hurricane-forced wind gusts is forecast!

Phasing of the polar front and subtropical jet results in a vast area with DLS in excess of 20 m/s with SOT CAPE/shear >1 overlapping with a plume of unseasonable moist air with SOT IVT > 2. The result will be a favorable environment for organized DMC activity.

The main focus for CI (on the synoptic scale) will be a rapidly E-ward traversing low-amplitude wave, which crosses the Adriatic Sea at around 06Z before moving into Romania around noon.
Expect scattered CI over Croatia into Bosnia and Herzegovina before noon along the Dinaric Alps/Vlasic Mountains. Initial more elevated morning storms turn more surface based especially over the E with better BL moisture and dewpoints in the mid/tens (as measured yesterday). Main uncertainty is if convection stays more clustered with semi-discrete cells or if a more dominant and discrete storm mode materializes, which also depends on when/how fast storms move off the orography. Modest negative Q vector magnitude and rapid eastward motion of the wave could support scattered CI but with weakening background forcing betimes, a more discrete storm mode could already emerge over E Bosnia and Herzegovina.
Forecast soundings over the E are supportive of large to isolated very large hail with elongated and looped hodographs, weakened LL shear and a deep effective inflow layer with lots of SR inflow and increasing moisture along the top of the inflow layer. The only limiting factor seems to be moderate CAPE and no top-down weighted CAPE distribution due to the rather warm mid-levels. Severe to extremely severe wind gusts and heavy rain will become another issue next to an isolated tornado risk along the orography, where LCLs remain lowered.

Beyond noon, a corridor with very favorable CAPE/shear space for long-lived supercells evolves from Serbia into S Romania and N Bulgaria, which gets roughly framed by the Dinaric Alps/Balkan Sredna Groa to the south and the Southern Carpathains to the N.
A mixture of bowing structures with healthy 0-3 km shear (15-20 m/s) and mature supcercells crosses CNTRL Serbia around noon, before racing E/SE. A weakly capped airmass with weak background forcing along the Romanian/Bulgarian border could also support more discrete cells. A diurnal driven enhanced T-Td spread of 15K or more assists in intense cold pools and swaths of gusts in excess of 120 km/h. Locally extreme SRH within a deep inflow layer next to 0-1 km shear around 2 m/s and better mid-level CAPE profiles indicate a chance for very large and destructive (wind driven) hail. AR-CHaMo supports this with enhanced values.
During the evening, mature bow echoes/supercells interact with an inward moving sea breaze, where dewpoints increase into the mid-tens. As storms interact with this vorticity-rich boundary with somewhat lowering LCLs, an isolated tornado event cannot be ruled out. However, even further W into N-Bulgaria a strong tornado event cannot be ruled out with intense mesocyclonic acticity (despite large LCLs).

This activity moves offshore atop the Black Sea with a gradual weakening trend as capping increases.

A level 3 was issued despite modest CAPE forecasts in some guidance. EZ/GFS are rather aggressive with CI and cyclonic flow aloft due to the passing wave all pushed confidence level into the 3 category. Any supercell will prodcue swaths of severe and extremely severe within this setup. Otherwise similar hazards exist in the level 2, but lowered confidence in storm mode and CAPE magnitude kept this area in a 2.

Further N along the Apuseni Mountains and E to the Carpathian Mountains, scattered CI occurs with modest CAPE and 20 m/s DLS, so large hail and severe wind gusts become an issue before storms weaken E of the Carpathians due to drier BL with vanishing CAPE.

... N-Italy ...

Onshore and W-ward advection of better 2m dewpoints in the mid-tens continues between the Apennines and the S-Alps along the Po river. Beneath weak/modest mid-level lapse rates, CAPE resides in the 400-800 J/kg range (MLCAPE) with high-resolution models indicating regionally even higher peaks. Placed next to the main trough axis, a prolonged period with strong background forcing impignes on this region with SW-erly flow pointing into the orography. Hence expect scattered to widespread CI all day long in a weakly capped airmass.

Long/straight hodogrpahs for most of the time assist in splitting supercells, which f.ex. move off the Apennines to the NE. Large hail and heavy rain will be an issue combined with a few strong to severe gust reports. This activity continues on and off well into the night and affects areas like Emilia-Romagna, Lombardia and Veneta into the N-Adriatic Sea. We pushed this area into the level 2 category as some models indicate clusters of well organized supercells affecting this area on and off well into the night with all kind of hazards including a tornado risk. The final magntiude of this event also depends on the timing/track of a diffuse/eastward moving surface low into the Adriatic Sea, where shear profiles regionally improve substantially. This region needs to be monitored closely during the afternoon into the overnight hours.

Further W into W-Lombardia/Piemonte, relaxing shear turns the activity into a mess with a mixture of multicells/semi-discrete supercells and regions with BL cooled more stratiform rain. A broad 1 was issued as any more organized cells could produce hail/gusts/heavy rain (main issue) and a tornado or two.

Finally during the morning hours (13th), a few supercells could impact the coastal areas of Croatia with all kind of hazards including a tornado risk. Models diverge in coverage with most showing only isolated CI until 06Z.

... SE Spain ...

Placed along the base of the main upper trough, a deep cyclonic westerly flow affects the area of interest. With diurnal heating into the mid/upper twenties over SW Spain, a thermal low (also dynamically driven beneath coupled jets) results in onshore flow from the Mediterranean with LL mixing ratios well above 10 g/kg. As this moisture advects beneath a plume of steep mid-level lapse rates, MUCAPE in the 400-1000 J/kg range evolves and becomes weakly capped. DLS remains in the 15-20 m/s range so initiating cells pose a large hail and gust risk, before the main risk turns more to heavy rain with colliding/clustering convection. Any more dominant cell could acquire supercell characteristics over SE Spain with a tornado risk E of a line Valencia to Murcia and S, where dewpoints increase.

We expanded the level 1 well E to the Balearic Islands for a sporadic risk of multicells/supercells with an isolated tornado and hail risk along a quasi-stationary LL confluence zone well into the night.

... E Ukraine into Russia ...

A slowly eastbound moving frontal boundary with a broad meridionally aligned LL vortex shifts gradually E/NE during the forecast. Prefrontal airmass features CAPE in the 500-1000 J/kg range over NE Ukraine, increasing to 2 kJ/kg towards the Black Sea.

Initiating storms over the S-Ukraine pose a large hail threat with mentioned CAPE magnitude which turns more into a heavy rain threat betimes with DLS around 10 m/s.

Further N from N-CNTRL Ukraine into Russia, the risk for convectively enhanced rain with flooding issues becomes a major issue as BL moisture soars above 15 g/kg (mixing ratios) along the LL confluence zone. Weak shear but ample upper divergence induce a favorable setup for training convection during the forecast with models showing QPF in the 50-150 l/qm range/24h with local peaks in excess of 200 l/qm. Major flooding becomes an issue and hence a level 2 was added.

... NE-Algeria ...

An isolated hail risk from an elevated thunderstorm exists in this highlighted area. This activity peaks during the evening hours.

... Baltic States ...

A few strong storms are possible during the day mainly along the western parts, where 200-500 J/kg MUCAPE overlap with strong DLS. A few splitting cells are forecast with isolated hail and strong gusts but the risk seems too low for a level 1 upgrade.