CONVECTIVE FORECAST
VALID Mon 25 Jul 06:00 - Tue 26 Jul 06:00 2005 (UTC)
ISSUED: 24 Jul 17:54 (UTC)
FORECASTER: TUSCHY

There is a slight risk of severe thunderstorms forecast across Switzerland and southern Germany

SYNOPSIS

Weather pattern change underway for most parts of central Europe. Broad southwesterly flow now established downstream of vertically tilted trough west of France...another trough over the North Sea supports this flow regime and unstable airmass spreads over most parts of central Europe....Strengthening WAA further weakens the faint trough over northeastern Europe. Southward, ridging should suppress deep moist convection in most parts of the Mediterranean area.

DISCUSSION

...Switzerland and southern Germany...
Feature of interest will be a cold front, which moves southeastward during the forecast period....being nearly parallel to the background flow, no significant propagation expected during the forecast period....Ahead of this front, a humid and warm airmass spreads northeastward from the central Mediterranean area, where stations reported dewpoints abut 17-19°C over extreme south France and about 15°C over the rest of France (15UTC)... Tongue of high Theta-E forecasted to infiltrate ahead of the front...Current thinking is that instability forecast of GFS too optimistic [ over 1000 J/kg ] and will go with more reasonable 200 - locally 500 J/kg... Main inhibiting factor should be limited insolation forecasted from the models...Current WV loop (16UTC) shows realtively dry airmass over the area, but weakly capped atmosphere should be conducive for early TSTM initiation...Some forcing of the North Sea trough and additionaly forcing along the front should support scattered to widespread TSTMs in the risk area...Models don't agree very well with the exact position of the front, but most of the storms should develop in an area with 20m/s deep layer shear and 12m/s low level shear...Main severe weather threat would be an isolated severe wind gusts, but combined with low LCLs, there could be a marginal tornado threat with each storm.

Later in the period, during the evening hours, storm coverage should increase again. Right rear entrance region of strong upper level jet, northeastward moving short wave and forcing along the front in a weakly capped environment should be more than adequate for widespread storm initiation. Kinematic parameters won't change that much, so main threat will be again isolated severe wind gusts and a marginal tornado threat. Because of high PWAT values and a slow moving front, training storms possible with an attendant threat for flash flooding....enough severe weather potential for issuing a marginal SLGT risk.

...Northern Spain, parts of Austria, Czech Republic and parts of Poland...
Strengthening upslope flow in northern and central Spain should help to produce isolated to scattered TSTMs. Inverted-V forecast soundings and very dry low levels should be conducive for strong downbursts and main severe weather threat will be an isolated severe wind gust,a marginal hail threat and dangerous lightning strikes, which could arouse new fires.
Rest of the SEE TEXT area will see weakly capped environment and scattered to widespread TSTM initiation, but weaker windfield should allow a pretty fast clustering of storms...Isolated strong to severe wind gusts and locally heavy rain will be the main threat.

...Northeastern France, Belgium, the Netherlands and parts of Germany ...
Main feature of interest will be a depression which could develop somewhere over the English Channel...Models don't agree very well how strong this system will be and how far east it will move...Global models show strong modification of low level wind field and up to 15m/s low level shear over parts of Belgium, Netherlands and north west Germany.... Steep low level lapse rates and slightly cooler mid levels could provide about 200 J/kg of instability and scattered TSTMs should develop. Deep layer shear between 15 and 20m/s will be enough for some storm organisation and main risk will be a locally severe wing gust threat. Exact development of depression important for tornado threat but low LCLs,0-1km SRH values up to 200 m**2/s**2 and those strong shear values would be conducive for a few tornadoes...Uncertainness of system and insolation inhibit issuing a SLGT ATM.

...Parts of Ukraina...
Strong forcing under base of weak upper level trough in a moderate unstable airmass will be enough for widespread TSTM development....Weak kinematic parameters not good for storm organisation and TSTMs will cluster pretty fast... main threat should be locally heavy rain.

#