Storm Forecast
Valid: Fri 18 Apr 2008 06:00 to Sat 19 Apr 2008 06:00 UTC
Issued: Thu 17 Apr 2008 21:51
Forecaster: TUSCHY

SYNOPSIS

Synoptic pattern aloft features an high over low pattern just west / northwest of Europe. An ample low pressure complex centered over the Bay of Biscay steers numerous disturbances over Portugal, Spain and the western Mediterranean towards the east. Integrated in this complex a strong depression ( central pressure at or below 980 hPa ) develops over the S - Bay of Biscay coming onshore over SW France during the midday hours. A cold front surges far southeastwards, creating a rapid cool-down over extreme NW Africa. An outbreak of cold air affects parts of N-Sweden / Norway during the forecast period.

DISCUSSION

... Portugal and Spain ( apart from NE Spain ) ...

A well mixed postfrontal airmass overspreads Portugal and Spain and mainly diurnal driven convective activity is anticipated. Current thinking is that most of the activity will occur over the northern parts of the highlighted area, where mid-level lapse rates are steeper and instability release somewhat stronger. Shear is weak and beside a stronger pulsating thunderstorm with a marginal hail / gusty wind risk, no thunderstorm organisation is expected. LL shear increases over NW Spain during the afternoon / evening hours and shear could become supportive for shallow LL mesocyclone development although decaying instability should limit overall threat.
Finally another disturbance approaches Portugal from the west during the early morning hours and if thunderstorms develop until 06Z they could also pose an isolated tornado risk. Right now we think that the main activity will occur after 06Z and hence not in our forecast period.

... Central Italy and the Adriatic Sea (06Z - 12Z) ...

A rapidly northeastward shifting short - wave trough affects those areas and a cluster of thunderstorms will go on during the morning hours. DLS of 35m/s and enhanced LL instability release over central Italy overlap and hence there is a risk for large hail / an isolated tornado with stronger storms. Activity should weaken during the midday hours and so does the severe weather threat.

... NE-Spain, S-France and N-Italy ...

Yesterday afternoon / evening, a disturbance crossed the western Mediterranean and was responsible for the advection of a cooler / somewhat drier aimass towards the southeast. Latest GFS brings this airmass all the way to the Balearic Islands.
This has an effect on today's thunderstorm evolution as airmass recovery over NE Spain / SW France is uncertain and possibly incomplete as main surge of better moisture should affect SE-France and N-Italy.
Nevertheless, forcing is quite strong and uncapped 100 - 500 J/kg MLCAPE will be present. Rapidly steepening lapse rates and enhanced storm relative helicity ( 3km at or above 150 m^2/s^2 ) support a large hail risk.
Regarding the tornado risk GFS brought out NE Spain during the past few runs with strong LL shear of 15m/s and storm relative helicity of 150 - 200 m^2/s^2 so there is an enhanced tornado risk. Another area with augmented tornado chances is SW France, where dynamics ( left exit region of a strong mid-level jet ) and modest instability release could support more vigorous updrafts in a moderately sheared environment.
The main risk will occur between 12Z and 18Z.

As thunderstorms build / develop eastwards, they reach an area with better LL moisture as a 15-20m/s LLJ points northeastwards and a rapid thunderstorm increase both in strength and coverage will occur over SE-France and just offshore. Right now we expect an upscale growth into a well structured MCS, affecting mainly the coastal areas of SE France with severe weather between 16Z and about 02Z. The best environment for surface based thunderstorms will occur along the coastal areas and offshore and those could result in an isolated large hail / strong wind gust report. The risk for tornados also increases over SE France and eastwards, as LL shear becomes stronger.
After 00Z, a cluster of thunderstorms ( likely a V-shaped MCS ) approaches N-Italy from the SW and if more discrete storms develop ahead of the line, the environment would be favorable for supercells with an attendant risk for large hail, tornadoes and severe wind gusts. Otherwise the main risk will be mainly severe wind gusts as the MCS progresses rapdily northeastwards.
The most obstructive parameter is the instability and this was the main reason for not issuing a level - 2. Latest GFS runs also made a quite significant shift to the south regarding the main thunderstorm area which indicates that there are still some uncertainties left. We will monitor the area if conditions become more favorable for more widespread supercell development than currently anticipated.

... E-France and SW Germany ...

In the range of a weak low pressure channel at upper-levels, a few thunderstorms should evolve during peak daytime heating. Gusty winds and small hail will be the main threat with strongest storms. Activity diminishes rapidly after sunset.