Storm Forecast
Valid: Fri 17 May 2024 06:00 to Sat 18 May 2024 06:00 UTC
Issued: Thu 16 May 2024 16:46
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for E Austria into central Hungary for large hail, severe convective wind gusts, excessive convective precipitation and to a lesser extent for tornadoes.

A level 1 and level 2 are issued for S Hungary, Serbia and parts of Bosnia-Herzegovina mainly for large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for NE Italy mainly for large hail.

A level 1 is issued for N Spain and far-south France mainly for large hail and excessive convective precipitation.

A level 1 is issued for central into NW France mainly for excessive convective precipitation.

A level 1 is issued for far-west Germany and parts of the Netherlands for excessive convective precipitation.

A level 1 is issued for central Turkey mainly for large hail and severe convective wind gusts.

SYNOPSIS

An omega pattern remains in place with a blocking anticyclone over the Baltic Sea, flanked by cut-off lows over the Bay of Biscay and over SW Russia. The mid-level jet stream is therefore divided into a subtropic branch, which runs across the Mediterranean region and the Balkans, and a polar branch, which only influences Iceland and far-North Scandinavia. Most of Europe is filled with seasonably tempered and moist airmasses of Atlantic (or Mediterranean) origin. Warmer and drier conditions prevail in the range of the blocking anticyclone.
The western low extends its influence also into Germany (where a daughter cyclone has formed and starts filling up), the Alpine region and the Balkans (which are crossed by its occluding frontal system), plus Italy and France. Low to moderate CAPE builds in response to daytime heating and/or synoptic lift processes in large areas under this cyclonic influence. The southern fringes of the CAPE reservoir also overlap with the subtropic jet stream, allowing another round of somewhat better organized storms to form.

DISCUSSION

... Austria, Hungary, Serbia, Bosnia-Herzegovina ...

The occluding frontal system with a rain band and possibly some leftover embedded convection will depart to the NE in the morning, and skies should partly to largely clear for at least some hours. Daytime heating and synoptic lift ahead of another travelling short-wave trough then allow the build-up of CAPE on the order of 500-1000 J/kg, locally more in case of favorable mesoscale modifications (to be discussed below). Deep-layer shear increases from 15 m/s towards the NW to 25 m/s towards the SE, owing to the mid-level jet. Aided by synoptic and mesoscale lift, at least scattered thunderstorms are expected to initiate around noon and soon organize into multi- and some supercells.
Storm coverage will be highest from E Austria into central Hungary, where the associated hazards seem evenly distributed between large (2-4 cm sized) hail, a few severe downbursts and locally excessive rainfall. Further south, i.e., from S Hungary into Serbia and parts of Bosnia-Herzegovina, stronger shear and higher cloud bases shift the main hazard more to large hail (isolated very large hail not ruled out), but weaker lift and drier environmental air make convective initiation less likely and will keep it more isolated.
Interestingly, high-resolution models agree on the formation of a mesoscale cyclone in the wake of the Eastern Alps somewhere near the Austrian/Hungarian border, driven in parts dynamically by the flow across the mountains and in parts thermally by maximized insolation. The near-surface wind field responds by tightening and further backing to E-erly or even NE-erly directions ahead of this feature. In contrast, enhanced SW-erly winds in its wake will act as Foehn when pushing from the Dinaric mountains and the Slovenian Alps into the Pannonian Plain in the course of the day. As a result, a dryline boundary will move NE-wards into the Pannonian Plain and could become quite pronounced by afternoon, separating 2m dewpoints of 14-16C ahead of the boundary from 5C or even lower on its rear side. Depending on the strength of this meso-low and its additional effect on moisture concentration, a confined belt just ahead of the dryline could see an unusual overlap of CAPE around or above 1000 J/kg, 0-1 km shear exceeding 10 m/s, and an impressively helical flow with 0-3 km (0-1 km) storm-relative helicity in the 200-400 m^2/s^2 (100-200 m^2) range. Since the Adriatic origin of the airmass and the plentiful overnight rain keep the cloud bases around or below 1000m, any supercell forming at and keeping track with the dryline could even become tornadic.
Convection will decouple from the surface around sunset, weaken and decay while it travels further E- to NE-ward into Slovakia, E Hungary and Romania.

... France, N Spain, N Italy ...

Daytime heating creates a few hundred J/kg CAPE also in the moist maritime air behind the frontal system and near the center of the cut-off low. Some synoptic lift on the cyclonic side of the jetstream is discernible over Spain and France but stays rather diffuse. Scattered, rather low-topped, daytime-driven thunderstorms may produce localized excessive rain. Large amounts of small hail and isolated severe wind gusts are additionally not ruled out with the strongest pulse storms. It is not clear yet how far north the fringes of the subtropic jetstream will extend, but convection over NE Spain and far-south France could just tap into deep-layer shear up to 20 m/s and produce some larger hail.
The same goes for the Italian Alps, though subtle synoptic subsidence, as well as the still plentiful snow cover at higher altitudes, will keep the number of storms on the lower side.

... far-West Germany into the S Netherlands ...

In the warm air advection regime on the northern flank of the cyclone, very moist and neutral to marginally unstable air is exposed to strong synoptic and mesoscale lift. Given enough insolation on the NE side of the occlusion, a few storms are possible mostly in the afternoon and evening. They will soon become embedded into the cloud and rain band, but could enhance rainfall enough to create some flooding. This is reflected by a small level 1, also considering the recent high-rainfall history.
(NB: Non-convective excessive rain may also occur near the cyclone center SE of the level 1 area, not reflected by ESTOFEX.)

... W- to S-facing coasts of Scotland, Wales, England, Ireland, and Brittany ...

In an environment characterized by low CAPE, weak vertical wind shear, moist air and an overall cyclonic background wind field, sea breezes pushing inland might produce a few funnel clouds or brief, non-mesocyclonic tornadoes.

... central Turkey, NE Algeria, as well as N Sweden and N Finland ...

Ironically, despite the large geographical distance, these three regions face quite similar environments of a few hundred J/kg CAPE, strong shear beneath the respective branches of the jetstream (in Turkey even exceeding 25 m/s), but neutral to downward vertical motions due to the anticyclonic curvature of the flow. Convective initiation is therefore expected to be isolated at best, and will probably be delayed to the evening. It is not clear whether convection will manage to be surface-based in an environment that would otherwise allow excellent organization. In case storms form already in the afternoon, large hail and severe wind gusts are well possible. Small level 1 areas are added for Turkey and for NE Algeria, where this conditional scenario seems a little more likely, also owing to the more prominent (and snow-free) mountains.