Storm Forecast
Valid: Thu 22 May 2025 06:00 to Fri 23 May 2025 06:00 UTC
Issued: Wed 21 May 2025 15:17
Forecaster: TUSCHY
A level 2 was issued for parts of NE Algeria into N Tunisia mainly for large to isolated very large hail and severe wind gusts.
A level 1 surrounds that 2 for similar hazards with lower probabilities (including flash flooding).
A level 1 was issued for parts of NE Bulgaria into SE Romania mainly for hail, gusts and heavy rain on a local scale.
A level 1 was issued for parts of the E Baltic Sates into far NW Russia mainly for an isolated tornado and heavy rain/hail risk.
SYNOPSIS
The final stage of the long lasting and probaly stratospheric-driven blocking event is underway with positive height anomalies extending from the Azores N towards Iceland as NAO bottoms out. Downstream cyclonic gyre over Scandinavia dives S towards S Sweden with modest negative height anomalies for this time of year.
A lead impuls (low-amplitude wave) over France moves ESE and starts to phase with the main trough to the N. This feature remains progressive during the forecast as it crosses N Italy east.
Another more focused area with geopotential height falls materializes over Spain so most of Europe will be influenced by the main/complex upper trough structure.
Rising heights occur over Russia while pogressive waves embedded in the subtropical jet affect parts of the Mediterranean.
Driven by the amplifying trough an extensive wavy front along its E fringe shifts gradually E and gets disturbed by a structuring lee cyclogenesis over N Italy. This configuration results in a broad area with CAPE build-up and enhanced thunderstorm probabilities.
DISCUSSION
... Far NE Algeria into N Tunisia ...
By far the most interesting area DMC-wise exists in the level 2 area, where a marine airmass advects ashore beneath rather steep mid-level lapse rates. The net result will be MUCAPE in the 1500 J/kg range.
The phasing of the polar front and subtropical jet stretches out an extensive area with anomalous strong mid/upper level winds, also affecting our area with aoa 20 m/s DLS and with similar peaks in the 0-3 km layer. As strongest LL pressure drop remains far N, not much MSLP gradient and LL shear is forecast. All that supports a large hail threat although rather subtle cooling at mid-levels hampers the overall threat a bit.
Nevertheless, expect scattered multicells/supercells to erupt ahead of the approaching cold front with large hail and severe gusts the main risk - a very large hail event cannot be ruled out. A deep moist profile (e.g. at mid-levels) also points to effective rain rates and flash flooding is possible especially with clustering cells.
... SE to E Europe ...
An extensive area with low to moderate MUCAPE exists during this forecast with varying foci.
E/NE Bulgaria into SE Romania will see somewhat ehanced BL moisture but with expected strong diabatic heating, a deeply mixed BL airmass will be in place. Weakly capped 400-800 J/kg MUCAPE with roughly 10 m/s DLS point to a gust and hail threat with stronger storms. In fact, ICON peaks with more than 1000 J/kg MUCAPE, weak shear but effective profiles for wet downbursts and a few severe events (localized) are possible.
E Baltic States into far NW Russia will have an overlap of more than 10 m/s 0-3 km shear and up to 15 m/s DLS, LCLs around 500-700 m AGL and good LL curvature in forecast hodographs. A low-end tornado risk exists with stronger cells next to small hail, gusts and locally heavy rain.
Both areas were upgraded to a level 1.
Otherwise either shear or CAPE or both parameters remain too marginal for anything severe. Spotty instances of gust/rain or hail are possible but mainly driven by the mesoscale/orography.
... Denmark to N Germany ...
An active day with numerous CAA driven thunderstorm events is forecast. Placed along the SW fringe of the deep cold-core vortex over S Sweden, rather impressive NWP discrepancies exist at 500 hPa regarding the placement of the coldest mid-level airmass, which creates issues with the final geopotential height geometry. This results in rather different jet positions with GFS/UK10 offering more background shear for strong downward impulse, whereas others like ICON come in less bullish. An offshore peak of SBCAPE causes strongest convection to move ashore with a gradual weakening trend. Strong gusts and graupel will be the main risk with patches of heavy rain along the coasts (where repeated convection affects the same places).
