Storm Forecast
Valid: Fri 22 Jun 2007 06:00 to Sat 23 Jun 2007 06:00 UTC
Issued: Thu 21 Jun 2007 20:57
Forecaster: TUSCHY

SYNOPSIS

Impressive EML over the west-central Mediterranean will slowly shift eastwards with 850hPa temperature readings between 20-30°C .
Intense heat wave will therefore go on mainly for central and also for the eastern Mediterranean.
A strong upper-level trough over NW Europe moves slowly eastwards, advecting a cooler but still humid airmass over western and central Europe.
Another trough over western Russia will slowly shift towards the east, inducing a strong surface depression on its downstream side. The main activity of this depression will be out of our forecast area and hence no further investigations were done.

DISCUSSION

.... N-Italy, Slovenia, N-Croatia, parts of Austria and Hungary...

Conditions become fine for an overlap of the remnants of the impressive EML and intense shear over the area of interest with an attendant risk of severe thunderstorm evolution.

The main player is the strong upper-level trough over NW Europe, sending an intense and compact vort max NE-wards, crossing the area from the SW during the daytime and early night hours.
Latest synop datas ( 21st June 18Z ) show dewpoints of 20-25°C over most parts of central Italy, decreasing only minimal towards the north.
Broad surface pressure fall south of the Alpes and a constantly backing wind field will help to keep dewpoints at this level during the next 24 hours.
Lapse rates should weaken somewhat ( mainly during the early morning hours as mid-level pool of warmer air will cross the region ), but should re-sharpen during the day.
In combination with excellent BL quality, at or above 1kJ / kg MLCAPE look attainable.

Main concern right now is the intense wind field. Very strong 40m/s streak at 500hPa will cross central Italy during the morning hours, while weakening and so does the 50m/s jet streak at 300hPa, which reaches the western Alpine region around 12Z....all from the SW. GSF splits the broad jet in numerous small-scale streaks, crossing N-Italy and Croatia during the afternoon hours.
Despite the weakening trend, DLS will reach 25-30m/s and nice veering at mid-/upper-levels also helps to bring SRH3 values up to 150-250J/kg.
A well mixed layer at lower-levels, nice instability values and strong shear in the favorable hail growth zone all point to potential severe hailstorms with a few significant hail events possible.
During the late afternoon / early evening hours, LL shear constantly strengthens and the tornado threat will be on the increase (e.g. eastern/southeastern Austria, where surface temperatures should stay below 30°C and T-Td spread won't be that strong ).

Intense DLS will also contribute to an enhanced severe wind gust threat, especially if storms line-up / have bowing structures.

We went a bit further towards the south with the level area over central Italy. Stout cap which was observed during the past few days will weaken and weak forcing / very high dewpoints could support a few storms to break the cap. Any thunderstorm would become rapidly severe with an attendant severe hail / wind gust risk.

....SE-Germany and the Czech Republic...

Same vort max, discussed above will cross the area from the SW mainly between 12-21Z.
Modified soundings indicate low to moderate instability release and if at least modest diabatic heating can be realized up to 500 J/kg MLCAPE will be a distinct possibility.
DLS is significantly enhanced ( 25-30m/s ) and EL analysis has clear signals of deep convection over SE Germany.
Latest thinking is that early / scattered thunderstorm initiation starts over the western Alpes of Austria, moving rapidly towards the northeast, while organizing. Expexted hodographs should be straight and severe wind gusts are likely if a line of storms can evolve.
Although DLS should slowly decrease during the afternoon hours ( around 20m/s ), this will still enough for a continual support of an organized line of storms, moving rapidly towards the NE, reaching the Czech Republic during the later afternoon hours.
Higher probabilities will be issued if later model runs keep shear at least that strong and if an assessment about the final instability's strength can be made.

...France and central Germany....

Low-end instability release looks reasonable in a weakly sheared environment. Scattered thunderstorms can be expected with gusty winds and marginal hail.

A few severe wind gusts over central France will be possible due to a 20m/s speed max at 850hPa. In addition, steep LL lapse rates should help for a nice 0-3km CAPE release with some veering in this layer. Low LCLs will be present, too.Therefore an isolated tornado report can't be excluded.

....United Kingdom...

A weakly sheared / moderately unstable environment will be present during the next 24 hours. Nice mixing ratios and no dominating steering flow will help for scattered to widespread thunderstorm development with slow moving storms. Low LCLs and various outflow boundaries could enhance the threat for a few funnels clouds / an isolated and short - lived tornado, but main risk will be excessive rainfall with an attendant flash flood risk ( with regard to the latest active convective period ). No level will be issued due to the overall marginal set-up.