Storm Forecast
Valid: Fri 12 May 2006 06:00 to Sat 13 May 2006 06:00 UTC
Issued: Thu 11 May 2006 19:08
Forecaster: TUSCHY

SYNOPSIS

Weak pressure perturbations continue to affect most parts of Europe and hence, many places of TSTM development can be expected.
All over synoptic pattern will be nearly unchanged to yesterday, although parts of western and CNRTL Europe will see a slow shift towards a more zonal flow and in a row the effects of an approaching upper-level disturbance during the evening hours.
Eastern Europe will again see a well mixed and pretty dry airmass which should suppress TSTM development... Main focus for isolated to scattered storms will be the area under an eastward shifting trough // parts of Turkey, Bulgaria, Romania and Moldovia//, but weak wind shear and instability should restrict severe TSTMs....although don't want to exclude a marginal hail risk with maturing storms.
Northern Europe will be influenced by another trough, slowly shifting eastward over Norway and Sweden... Accompanying low-level depression will help to shift a cold front towards the SE, crossing the southern part of the Gulf of Bothnia during the afternoon and evening hours... DLS up to 15m/s and SBCAPE up to 300 J/kg should be enough for an isolated, marginal hail risk...otherwise, storms will stay sub-severe.
Increasing upper level divergence and forcing ( approach of upper-level trough and slowly southward moving frontal system ) combined with up to 400 J/kg instability should yield an favorable environment for scattered TSTM development...TSTMs, which develop next to southward moving frontal system will have up to 15m/s DLS, posing a risk for marginal hail, but otherwise storms should not reach the severe TSTM limit.

DISCUSSION

...France , Belgium, the Netherlands and extreme NW Germany...

Difficult to pinpoint exact areas of TSTM development over such a broad area, but there are some indications that TSTM development looks more promising than yesterday... Despite yesterday's very dry soundings, models indicate a veering of the wind in the lower atmosphere with an establishing southerly flow.... A weak frontal boundary, which caused widespread TSTM development over Spain yesterday evening, will move towards the NE and should be responsible for soggier but also slightly warmer mid levels ( also indicated in marginal weaker mid-level lapse rates)... This, combined with a more humid boundary layer and steep low level lapse rates should be enough for at least isolated TSTM development... Main concern will be possible limited insolation due to frontal clouds/decaying TSTM anvils.
Weak shear but about 500 J/kg SBCAPE will yield a marginal hail risk with maturing storms..otherwise TSTMs should mostly stay sub-severe.

....N-Spain and SW France...

Main concern for organized storms arises over parts of SW Europe during the evening and night hours.. Ejecting upper level speed max //out of an east-Atlantic trough // will help to create a favorable environment ( left exit region and increasing upper-level diffluence) for a developing upper-level disturbance over W-Europe.
In spite of rapidly decreasing instability values, increasing forcing should help for scattered to widespread TSTM development over the mountainous areas of N-Spain and extreme SW France.
DLS will increase to 15-20m/s and a line of storms should develop with an isolated
severe wind gust / large hail threat.
If more instability can be materialized than expected, an upgrade could become necessary!