Storm Forecast
Valid: Fri 25 May 2007 06:00 to Sat 26 May 2007 06:00 UTC
Issued: Thu 24 May 2007 19:44
Forecaster: TUSCHY

SYNOPSIS

Leisurely eastward progressing upper-level trough north of Ireland and Scotland and a weak upper-level disturbance over Spain and Portugal will merge during the forecast period, which will cause a channel of lowering geopotential heights over western Europe.

Hot and humid conditions in a weak-gradient environment prevail during this forecast period. Numerous convergence lines over SE and south-central Europe will be conducive for scattered to widespread thunderstorm development. Weak shear but instability in the order of 1000 J/kg and steep lapse rates will be present. The only reason against issuing a large level-1 area over SE Europe was the displacement of the best instability fields and global models' convective precipitation forecasts.
Anyway...each stronger, pulsating storm will have the chance to produce isolated large hail and strong wind gusts.
An update will be issued, if 12Z soundings reveal a more aggressive environment.

Parts of NW Italy were included into a level-1 due to an enhanced large hail threat.

DISCUSSION

Confidence rises that a streak of strong to severe thunderstorms will evolve over parts of France, Germany and Poland.

....Central France....

Weak pressure gradients and no anyhow more robust forcing signal over France will shift the main focus for thunderstorm initiation at lower levels.
Like yesterday, moisture advection charts indicate maximized convergence over central France, where averaged mixing ratios will reach 11-13g/kg. Yesterday's thunderstorms / showers and additional evapotranspiration should all help to bring dewpoints up to 19-21°C.
Atmosphere at mid- and upper-levels will unergo tentative cooling, which should also foster steep lapse rates of at or above 7°C at mid-levels.
Although GFS keeps lower-/mid-levels pretty dry, confidence is high that robust thunderstorm evolution will take place.
Modified soundings indicate moderate to high instability release ( MUCAPE and SBCAPE around 2500 J/kg and ICAPE also above 2000 J/KG ) and this looks reasonable.
Initiation will be at 12Z onwards, when diabatic heating will help to overcome the cap at lower levels.
Despite the absence of shear, large hail will be a distinct possibility, given the large amount of CAPE in the favored hail growth zone ( also supported by modified soundings, which indicate hail of 2-4cm in diameter).
High LCLs and pretty dry / well mixed sub-cloud layer also favors strong downbursts with severe wind gusts, matching our severe criterion.
A weak steering flow and repeated thunderstorm evolution in this high energetic environment will pose an excessive flash flood risk !

During the late afternoon / evening hours, thunderstoms should also initiate further towards the south, where strong diabatic heating will overcome the cap. The main difference will be more isolated thunderstorm initation and constantly increasing SRH3 values up to 150J/kg, which supports an enhanced large hail / severe wind gust threat, too.

The threat, although temporarily diminishing between 18-00Z will last the whole night.

...N-Spain, SW France ....

Strengthening upper-level divergence and a northwards traveling upper-level vort max will create a nice set-up for scattered thunderstorm development around the Pyrenees ( around 18Z onwards). Marginal shear and instability should limit the severe weather threat and main risk looks like to be heavy rainfall with gusty winds and locally marginal hail.

...NW Germany...

Aforementioned swath of high moisture content boundary layer will stretch from west-central to northeast Germany.
Weak mid-level lapse rates, early initiation and hence limited diabatic heating should confine the severe weather threat over NW Germany. This will also limit instability release which is forecast to be marginal at best.
Despite the somewhat enhanced DLS, we decided to exclude this area from the level -1 region, because of all those facts .
Early thunderstorm initiation can be expected with gusty winds and marginal hail being the main threat.

...Central Germany....

The main risk for a few strong to severe thunderstorms should stay confined to the level-1 area. Strong diabatic heating to 25-30°C, moderately steepened lapse rates and dewpoints between 15 and 20°C will be in place and up to 1000 J/kg can be expected. Main risk will be an isolated large hail and severe wind gust risk, but expected low coverage and available energy/shear should justify a low-end level-1 for those regions.


...Extreme NE Germany, N-Poland and the southern Baltic Sea...

High - resolution GFS maps constantly hint on a possible weak LL depression evolution over NE-Germany during the afternoon hours, moving slowly towards the east / northeast. The same does BOLAM and NOGAPS and confidence is high that a mixture of an open wave / weak LL depression will evolve.
A combination of 700 -1000 J/kg instability, enhanced LL and DL shear and low LCLs will be in place and an isolated tornado report over NE Germany and N-Poland can't be excluded.
Stronger storms will also pose a large hail / severe wind gust risk.
Strengthening mid-level flow will help those storms to shift rapidly towards the NE, crossing the southern Baltic Sea during the night hours.