Storm Forecast
Valid: Mon 09 Mar 2009 06:00 to Tue 10 Mar 2009 06:00 UTC
Issued: Sun 08 Mar 2009 16:52
Forecaster: TUSCHY

SYNOPSIS

A progressive SE-ward moving upper trough dictates the weather over most parts of Europe as unsettled conditions prevail. The atmosphere remains cold/stable over NE-Europe and warm/stable over SW-Europe.


DISCUSSION

... Germany, Switzerland and parts of N/E-France ...

Latest enhanced IR images (15Z-16Z) show a field of enhanced convection now entering Ireland/Scotland and UK from the NW. This activity nicely coincides with a strong UVV anomaly, embedded in the NW-erly flow along the southern fringe of the main cyclonic vortex. 12Z soundings from Ireland also capture the environment well with steep lapse rates and well mixed LLs. However they also show, how shallow the moist 3-4°C Td layer is and not much mixing is needed to lower Td values to at or below 0°C. Dewpoints of past few GFS runs over the area of interest look reasonable, increasing confidence in attendant CAPE output of that model. Eventually the first lightning reports came in from UK at 15Z/16Z.

Strongest convection will be bound heavily to the pocket of coldest mid-level air and the SE-ward moving forcing in terms of an healthy short-wave. The airmass is uncapped so dependant on local topography, mesoscale boundaries,regional better BL moisture content or better diabatic heating, isolated thunderstorms can pop up everywhere over NE France, Belgium, the Netherlands, Luxembourg, Germany and Switzerland. Better LL moisture over NW/N-Germany and the Netherlands but stronger forcing further towards the south make numerous, general thunderstorm area necessary. Favorable timing, coldest mid-level air and position of the UVV field were responsible for the placement of those regions. Stronger showers/thunderstorms bring strong wind gusts and marginal hail/sleet.

Belgium and central Germany were excluded as trajectories emanate from UK, so shadowing and therefore lower BL moisture values could keep thunderstorm chances very low.

... Parts of N-Italy, Adriatic Sea and the S-Balkans ...

A rapidly SE-ward advancing upper trough crosses the area from the NW, affecting Italy during the afternoon hours and N-Greece after midnight. Strongest convective activity will stay confined to the pocket of very cold mid-level air and to the left exit of a strong 50m/s mid-level streak. Intense shear and marginal CAPE remain separated most of the times although some overlap occurs over N-Greece during the night hours. An isolated large hail/severe wind gust event over N-Greece and mainly a low-end large hail risk over the N-Aegean Sea are possible between roughly 03Z-06Z . Indications are that a stable, nocturnal boundary layer will be already present and convection remains slightly elevated, so despite strengthening LL shear, we won't include a tornado threat in the reasoning for the issuance of the level-1.