Storm Forecast
Valid: Fri 13 Apr 2007 06:00 to Sat 14 Apr 2007 06:00 UTC
Issued: Thu 12 Apr 2007 19:57
Forecaster: TUSCHY

SYNOPSIS

A well established split flow regime will persist over the western / central half of Europe with one branch affecting Iceland and Norway and the other one N-Africa.
Strong ridging over central Europe provides a continuance of the unseasonable warm conditions, while an upper-level disturbance over Spain will bring unsettled conditions for most parts of SW Europe.
A broad cyclonic vortex over western Russia continues to steer cold air far towards the south/southeast.

DISCUSSION

...NE Algeria and N-Tunisia...06-15Z...

An intense baroclininc zone has already set up over central Algeria during the past 24 hours and this will assist a strong jet on the base of the upper-level trough. This jet will race eastward and will foster a negatively tilted trough axis to shift towards the E.
At lower levels, global models indicate what could be a closed LL circulation, which will cross N-CNTRL Tunisia from the WSW between 9 and 15Z.
Keeping in mind that data input of this region is pretty sparse, models could have problems with exact strength / track of the depression and latest synop data indicate numerous possible smaller low centers over central Algeria.
Extremely dry air towards the south and strong WAA downstream of the depression will confine the area of the most robust TSTM threat under the base of the trough and along the eastward sliding frontal boundary over NE Algeria and N-Tunisia.

Intense vor max, coming out of the base of accompanying trough will cross the area from the SW and compact UVV field and just slowly weakening upper-level divergence will favor scattered thunderstorm development.
Capped areas downstream of the depression will see best LL shear / backing but favorable shear could stay enhanced along the eastward moving frontal boundary.
DLS of 20-25m/s and good lapse rates will favor mainly a large hail / severe wind gust risk although an isolated tornado can't be ruled out.

During the latter part of the forecast period, the main thunderstorm activity will shift / evolve further towards the north under the base of the NW-SE-erly elongated upper-level trough. Diminishing shear and weak lapse rates will preclude any storm organisation.

...SW / W France and SW-UK ...

After a few days of continual subsidence and strong diabatic warming at lower levels, atmospheric stratification over France was characterized by pretty steep lapse rates at lower and mid-levels and hence up to 300J/kg SBCAPE release was detected.
This fine stratification will get modified from the south by the influx of a warmer and humid airmass ( thick Ci-shield already present over most parts of France at 12th April 18Z ).
This will help to weaken lapse rates significantly.
In addition, GFS indicates rising geopotential heights over W / NW-France, which should also suppress any significant storm acitivty mainly during the evening hours and onwards.
Numerous vort maxima will cross the areas from the SE, weakening gradually.
A few storms can be expected to evolve mainly in the highlighted areas but shear will stay too weak for any storm organisation.

An exception will be SW France, where LL shear will be enhanced, but right now environmental conditions won't justify higher probabilities for more than an isolated thunderstorm threat

A few isolated but sub-severe thunderstorms can be expected over SW-UK, too.