Storm Forecast
Valid: Mon 04 Aug 2008 06:00 to Tue 05 Aug 2008 06:00 UTC
Issued: Sun 03 Aug 2008 23:32
Forecaster: TUSCHY

SYNOPSIS

The stage is set for a more autumn - like storm system to develop rapidly over the Baltic Sea during the forecast period. Due to its strong and extensive wind field and its interaction with a moist and unstable airmass, numerous areas over central Europe could see organized thunderstorms. This rate of intensification is supported by a favorable coupled jet structure, resulting in intense upper divergence over the central Baltic Sea and central Sweden. In addition, this strong depression includes a warm and humid airmass into its core, resulting in a strengthening baroclinic feature. Latest model data show a central pressure somewhere between 984hPa and 990hPa just west of Lithuania. Another potent depression evolves over the extreme NE Atlantic Ocean although its forward speed is very low.

Hot and stable conditions continue for the Mediterranean and SE Europe.

DISCUSSION

... NE - Italy, parts of Austria, Slovenia, parts of Hungary, Slovakia, E - Czech Republic, parts of Poland, parts of Belarus and Ukraine ...

Latest WV / IR composite loops already indicate a well structured and very compact upper trough over Ireland and the British Isles with a pronounced intrusion of stratospheric air along its southern tip. It is interesting to see this, as GFS cleaved to this evolution during the past few runs and models bring this feature next to the current area of interest during the late morning / midday and afternoon hours. Sat derived winds and sounding data also show a broad and quite intense upper jet, supporting a rapid tilt of this very dynamic feature from a neutral in a negative position, so forcing along its leading edge will be very strong. Keeping this very dynamic system in mind we will have a look at the surface as a cold front plows through those areas from the W / NW, way more aggressive over the northern forecast areas due to the more favorable background flow compared to the Alpine regions. There, it gets more complicated due to Alpine blocking and various convergence zones S and SE of the Alpes and the fact that atmosphere is quite uncapped. Anyhow, forcing at all levels will be more than adequate to support thunderstorm initiation in those areas.

Thermodynamics are the best S / SE of the Alpes as rich BL moisture and a tender rim of the SW - European EML / spanish plume overspread those areas. Somewhat meager moisture but very steep mid - level lapse rates just E / NE of Austria will also help to establish a tongue of MLCAPE values at or above 1000 J/kg ahead of the approaching cold front all the way up to NW Slovakia with SBCAPE values > 1 kJ/kg running all the way to NW Belarus. Instability values ease during the early night hours although intense forcing could temporarily offset this negative trend.

As mentioned above, the wind field is very strong as a 35m/s mid level streak approaches S - Poland during the midday hours and NW Ukraine during the evening hours. DLS along the instability axis increases to 20 - 30 m/s 0-6km bulk - shear and 10 - 15m/s LL shear over W Belarus / Ukraine and SE Poland. A broad shield of sign. enhanced SRH - 1 values over this area and very strong winds at 850hPa overlap, too.

We think that along this arc shaped and rapidly eastward racing cold front, scattered to widespread thunderstorms will develop. Aforementioned parameters and their dispersal point to a large hail risk over NE - Italy, Slovenia, E / SE Austria, Hungary and Slovakia, but also to a severe wind gust risk as conditions for strong cold pool development are good. Despite the chance for a few significant large hail events, wind field is not strong enough to raise the confidence for a level - 2 area all the way to N - Italy, but either the hail risk or an extensive and more internal controlled squalline with a more widespread severe wind gust risk could result in an update / upgrade. It is interesting to see the strong signals of the STP over Hungary, which could be a result of abundant LL CAPE release so we also have to include the chance for a few tornadoes over this area.

Intense forcing, very strong winds at LL, better directional shear and low LCLs favor an enhanced tornado risk from Slovakia NNE-wards and even strong tornadoes can't be ruled out. In addition, a strongly forced line of thunderstorms should evolve with a swath of strong to damaging wind gusts. Steep lapse rates are also supportive for large hail reports and we have to issue a broad level - 2 area to reflect all those risks.

Thunderstorm activity will go on during the night along the cold front, although a slow decrease can be expected during the morning hours.

... S / central Germany, W / central Czech Republic ( 06Z - about 12Z ) ...

A quite complex forecast situation for those areas as a cold front slowly fades away betimes. Latest ( 22Z ) radar data has a more messy appearance over the Low Countries and NW Germany with a few discrete cells ahead of the main front . Forecast sounding data for this limted time - frame indicate that little diabatic warming is needed for some low - end instability release and forecast EL temperatures indicate the chance for deep convection and despite its very localized appearance, we have to mention that due to a favorable sheared environment ( 10 - 15 m/s LLS and enhanced SRH1 up to 200 J/kg ) so an isolated tornado / severe wind gust can occur with each stronger / more discrete cell. The strong dynamic nature of the whole system and some low - end instability release were the reason to expand the level - 1 area far westwards, including W-Poland and W - Czech Republic and even parts of S - Germany. The risk for those areas should diminish rapidly from west to east.

... N - Germany ...

Perfect timing of the main upper trough axis and attendant compact vorticity lobe give another opportunity for organized thunderstorms between 12Z and 18Z. Rapidly cooling mid - levels and hence steepening mid - level lapse rates, combined with still adequate BL moisture should allow for a broad area with low - end instability release. Low LCLs and some LL shear overlap with those instability values and stronger storms could contain marginal hail and produce strong wind gusts and an isolated tornado. We issued a small and marginal level area over N - Germany, where all those parameters overlap.

... Central Baltic Sea ...

The winds increase during the evening and night hours as the depression deepens and winds of 25 - 30 m/s at 850hPa start to develop in the SW / S quadrant of the depression, where atmosphere is more unstable. Severe to damaging wind gusts can be expected during the night hours, but confidence is low that deep convection will be sufficiently involved to introduce any level - areas.