Storm Forecast
Valid: Tue 25 May 2010 06:00 to Wed 26 May 2010 06:00 UTC
Issued: Mon 24 May 2010 23:05
Forecaster: KOROSEC

A level 1 was issued for south-central France, NW Switzerland and SW Germany mainly for large hail and excessive convective rainfall.

A level 1 was issued for parts of Ukraine mainly for severe wind gusts and marginal hail.

A level 1 was issued for eastern Austria, SW Slovakia and much of Hungary mainly for large hail and lesser extend for strong wind gusts.

SYNOPSIS

A quite complex scenario unfolds across Europe on Tuesday. There is a large deep trough over northern Europe while a kind of omega like pattern over southern Europe slowly wipes out while making eastwards progress. There, two upper lows are located on both of its flanks. The western upper low moves into western Europe while merges with the main trough towards the Wednesday morning. The second, weaker upper low moves NE-wards from extreme southern Mediterranean into extreme SE Europe.

At surface, a cold front associated with the northern large trough moves further SE across east-central Europe while slowly weakening. The frontal boundary stays quasi-stationary over central Europe where its western flank transforms/merges with the warm front advecting across France. The frontal system pushes the cold front into western France.

DISCUSSION

... south-central France, NW Switzerland and SW Germany ...

Ahead of an approaching upper low/trough/frontal system, strong WAA pushes the warm front across France during the day. Behind the front, quite rapid moisture recovery with upper 10s surface dewpoints overspreads France as well as south-western Germany. Moderately strong instability release with MLCAPEs reaching near 1500 J/kg is quite likely. Over central France, GFS pushes the values even higher, while ECMWF stays slightly above 1000 J/kg. It seems that storms initation will begin in early to mid afternoon hours when strong surface heating weakens the capping inversion across the warm sector. With forming weak surface low and increasing southerly flow, a few near-surface convergence zones can be seen besides the main cold/warm frontal boundaries, which will trigger the convection. Only marginal mid-level jet streak will be present across France/S Germany until late afternoon when deep-layer shear gradually increases up to 15m/s. Despite rather limited shear, organized storms can be still expected with the main threat for large hail given the quite high instability release. Rather slow moving storms could enhance the flash flood threat as well.

Later towards the evening, models are confidently simulating convective rainfall threat mostly along the stationary warm front from eastern France into NW Switzerland and SW Germany. One or two large convective clusters are quite probable in the evening hours trailing ENE-wards overnight due to the increased LLJ ahead of the surface low over east-central France. Therefore, excessive convective rainfall threat could lead into local flash floods.

... parts of Ukraine ...

A short-wave trough on the SE side of the main large vortex will cross Ukraine/Belarus during the first half of the forecast period. In the morning hours, Ukraine lies right under the nose of relatively strong mid-level jet where a decent forcing leads into convective initiaton along the rapidly eastards moving cold front. With moderate 20-25m/s shear in place, organized storms are expected. A couple of well organized multicells as well as supercells can be possible. Posing a threat mostly for strong to severe wind gusts and marginal hail. Enhanced helicity is partly seen over narrow warm sector which could lead into LL rotating updrafts, but this threat ramains limited.

... eastern Austria, SW Slovakia and much of Hungary ...

An aforementioned weakening cold front over central Europe becomes quasi-stationary during the day. It lies E-W oriented from western Ukraine across Slovakia/Hungary border westwards into southern Germany. A rather impressive moisture pooling is expected south of this boundary as healthy moisture recovery takes place over Hungary and Austria from the Balkans. Especially the GFS model is probably slightly overestimating the near surface dewpoints, but 0-1km mixing ratios of 10-11 g/kg are still quite likely. Expected clear skies will lead into strong boundary layer heating and combined with steep mid-level lapse rates, it will result in high instability between 1500 and 2000 J/kg of MLCAPE, maximized over Hungary. A relatively strong mid-level jet streak on the back side of the main trough is still placed over central Europe, which will provide moderate to strong sheared environment across the level 1 area. Its southern flank (15-20m/s in 0-3km/0-6km layer) will well overlap with instability.

The capping over eastern Alpine flank/Hungary gives some uncertainities to the forecast, but it seems that convective temperature (near 26°C) will be easily reached in the late afternoon hours to initiate the convection. Organized and rather isolated severe storms are expected across the level 1 area, where quite a few well organized multicells and supercells will be possible. Given the high instability release, large to very large hail (3-5cm in diameter) will be the main threat, while severe wind gusts cannot be ruled out either, but remain limited. This threat should diminish in the evening hours, when instability vanishes.

The current thinking is that no widespread activity is expected over these areas, but the activity and the next model runs will be closely monitored for a possible upgrade/adjustments later tomorrow if conditions change significantly.