Storm Forecast
Valid: Thu 24 Apr 2008 06:00 to Fri 25 Apr 2008 06:00 UTC
Issued: Wed 23 Apr 2008 19:42
Forecaster: TUSCHY

SYNOPSIS

A strengthening high pressure area over SW Europe builds northwards as geopotential heights continue to rise over Portugal and Spain. This helps to tighten the geopotential height gradients over NW Europe, resulting in a strong 70m/s speed max., crossing Ireland, Scotland and UK during the forecast period from the west. Weak pressure gradients over the rest of Europe prevail, resulting in thunderstorm activity which is mainly diurnal driven.
Another outbreak of cold air grasps N-Sweden / Norway.

DISCUSSION

... Ireland, Scotland and United Kingdom ...

Latest IR / WV-9 RGB composite exhibits the airmass which will affect those areas during the day. Behind an eastward drifting and disbanding occlusion just west of Ireland (18Z), a broad sector with enhanced convection spreads eastwards. Models and satellite data all indicate that a cold airmass at mid-levels continues to overspread a well mixed and quite humid BL which results in currently seen enhanced convection. This airmass will cover those regions during the forecast period and as diabatic heating starts, widespread showers and thunderstorms can be anticipated.
We went with steepest mid-level lapse rates, best BL moisture and favorable timing to evaluate the area with the highest thunderstorm chances which runs from Ireland east / southeastwards, also including central UK and S-Scotland.
The main storm mode will be pulsating thunderstorms with strong wind gusts and marginal hail being the main threat. Central UK ( roughly Sheffield - London ) has the best chance to see an isolated large hail and funnel / tornado report as LL CAPE values are enhanced and DLS stays in the range of 15-20 m/s.
No level area was issued as neither parameters nor expected coverage convince us.
Both intensity and coverage of thunderstorms will diminish after sunset.

... S-Germany, W-Austria and Central / N-Italy ( 12Z - 18Z ) ...

and

... S-Italy ( 12Z - 00 Z ) ...

Dewpoints up to 10°C over S-Germany and ~ 10-14°C over Italy coupled with moderately steepened mid-level lapse rates ( also increasing slightly as mid-levels cool down ) result in an extensive area of scattered thunderstorm development. Shear is weak, instability release not too strong and missing forcing should all preclude organized thunderstorm development. Diurnal driven, pulsating thunderstorms will evolve with strong wind gusts and marginal hail.
At the moment, S-CNTRL Italy looks like to have the most extensive thunderstorm coverage and as LCLs stay low , concuring gust fronts could support a short-lived spin-up but too much nowcast / uncertainty in that scenario to augment severe probabilities.
Again, thunderstorms wane after sunset although they could go on until midnight over S-Italy.

... The Netherlands, N / central / E - Germany and W - Czech Republic ( 18Z - 06Z ) ...

A sharp but well structured through crosses those areas from the NW, resulting in scattered thunderstorms over the area of interest. Instability release is limited and no organized thunderstorm risk is anticipated despite gusty winds / marginal hail in isolated stronger updrafts.

Scattered to widespread thunderstorms are also forecast to evolve over parts of SE Europe but again, no severe weather threat is expected.




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