Storm Forecast
Valid: Fri 11 Apr 2008 06:00 to Sat 12 Apr 2008 06:00 UTC
Issued: Thu 10 Apr 2008 19:24
Forecaster: TUSCHY

SYNOPSIS

Finally persistent blocking pattern for most parts of Europe weakens as main upper-level trough over western Europe moves slowly eastwards. Ahead of this feature, very warm air spreads north / northeastwards and a well structured EML overspreads Sicily, S-Italy and parts of SE Europe. All those areas will stay capped during the forecast period.
Otherwise the focus for scattered thunderstorms will be an eastwards shifting boundary / cold front, covering a broad area from the central Mediterranean all the way to Belarus.

DISCUSSION

A very difficult forecast unfolds for most parts of central Europe and the degree of uncertainty is quite high.

A warm and humid airmass spreads northwards just downstream of the broad upper-level trough over western Europe. Model pool agrees that an extensive surface low pressure system evolves out of this constellation, running from the Strait of Gibraltar to Belarus. Embedded in the strong speed max of a far northward expanding subtropical jet, numerous disturbances are forecast to develop / decay along this baroclinic zone. Comparing run - to run consistency of the past few model cycles of GFS, no real trend can be deteced regarding exact placement and strength of all those disturbances although this is crucial regarding shear / instability dispersal.

We decided to highlight a few areas where signals for initiation and organisation were persistent / strong enough to enhance the faith in severe thunderstorm evolution.

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

Downstream ( NE ) of the Alpes, isohypses reveal a constantly more cyclonical curved pattern with persistent pressure fall at the surface. A combination of modest steepened mid-level lapse rates and forecast surface dewpoints at or above 10°C ( yesterday 17Z already about 7-9°C ) should yield modest instability release of 100 - 300 J/kg MLCAPE over SE Germany, increasing to 500 - 800 J/kg MLCAPE over most parts of the Czech Republic and eastern Germany. DLS is weak with about 10m/s DLS framed by 15-20m/s over E-Germany / NW Austria and any developing thunderstorm should be of pulsating nature. Isolated large hail during the mature phase of strongest storms is a distinct possibility. Thunderstorms could see somewhat better shear conditions over eastern Germany / NW Austria where DLS and / or helicity signals are stronger. A broad level-1 area was issued due to the isolated large hail risk.

... N-Italy ...

Although instability release is less than north of the Alpes, a stronger sheared environment could offset this more negative fact. The main inhibiting evolution for a more robust thunderstorm risk looks like to be a nearly uncapped environment which should result in widespread showers / thunderstorms, inhibiting the diabatic heating. However, a few strong to severe thunderstorms are expected and DLS of 25 - 30 m/s and strong helicity at lowest levels / up to 3km hint on an isolated large hail risk if a more discrete thunderstorm could evolve.

We also issued a level-1 for parts of the Ukraine, Romania, Hungary, Slovakia, Poland and Belarus as shear and instability will be sufficient for an isolated large hail report.

In addition, if thunderstorms can evolve south / southwest of a line Romania - Hungary they would be in an environment favorable for large hail / severe wind gusts but capping was in most model runs too strong for initiation.