Storm Forecast
Valid: Fri 28 Sep 2007 06:00 to Sat 29 Sep 2007 06:00 UTC
Issued: Thu 27 Sep 2007 16:55
Forecaster: TUSCHY

SYNOPSIS

It's a complex upper-air streamline analysis over Europe, as highly amplified weather pattern continues.
An upper-level trough over Greenland gains influence on the evolution downstream over Europe as its strengthening WAA west of Ireland, Scotland and Norway attenuates the CAA over Norway and Sweden, which in turn sustained the large upper-level trough over central Europe. In addition, geopotential heights continue to rise over Norway and Sweden ( as a result of the aforementioned WAA ) and results in a cut-off system , centered somewhere over NE Germany at the end fo the forecast period.

An ill defined feature from the subtropics will cross parts of SW Europe from the southwest, while warm and dry conditions prevail over eastern Europe and hot conditions can be found over the eastern Mediterranean.

DISCUSSION

...Strait of Otranto, northern Ionian Sea , Albania, parts of Macedonia and Montenegro and Serbia...

A concentrated but fairly long lasting severe weather event unfolds over the area of interest during the next 24 hours.

The focus for thunderstorm initiation will be a slowly eastward / southeastward shifting cold front which is forecast to become stationary / diffuse during the night hours.
Ahead of this boundary, mixing ratio averaged over 0-1km, increases significantly, reaching more than 12g/kg offshore and 10g/kg well inland. Dewpoints in the upper-tens look likely, even reaching at or above 20°C along the coastal areas and offhore.
At the same time, a pronounced EML over Tunisia and Algeria builds northeastwards . This airmass is charactericed by quite warm low-/ mid-levels while the upper troposphere stays cool.
Steep lapse rates at mid-levels are the result and up to 1.5 kJ/kg capped MLCAPE south of Italy will be the result, decreasing gradually towards the north.
Numerous vort maxima either cross or streak the area of interest and provide a prolonged period of enhanced lift. Due to the slow forward propagation of the frontal boundary, repeated thunderstorm development over nearly the same area looks likely.
A fluent transition from a moderate instability- high DLS shear environment over Albania and Montenegro to a low instability - high LL shear environment over Serbia and Macedonia necessitates the release of a small level-2 area.
Enhanced SRH-3 in combination with steep mid-level lapse rates support a large hail risk and even hail larger than 5cm is likely , especially over southern Serbia, eastern Montenegro and northern Albania.
LL shear stays significantly enhanced with values of more than 15m/s and as SRH-1 gets stronger during the day and the LCL stays low a risk of a few tornadoes exists.

High Theta-E values, a slow moving front, strong LL inflow from the SE, a mid-level flow near normal to the mountain range of Montenegro and Albania and expected long-lasting convection will result in an enhanced flash flood risk.

...Eastern Austria, the Czech Republic, Poland, western Romania and the western Ukraine...

As the result of the approaching cold airmass especially at mid-/upper-levels from the west, an intensifying jet helps to lift the upper-level low slowly northward.
At lower levels, a LL depression gets better organized over eastern Germany during the day while moving to the N / NNW.
A weak frontal boundary, centered over Poland down to Hungary gets re-activiated, as the cold air arrives at lower levels from the SW. This front will be the focus for some enhanced convective activity.
The threat for severe weather is somewhat diffuse, as best instability and strongest shear do not overlap.
An axis of better instability release and deeper convection is forecast to develop over NE Austria, the Czech Republic and western Poland and shear should be strong enough for a few multicell storms. DLS of 10-20m/s and steep mid-level lapse rates could support an isolated large hail report.
The tornado signals are bimodal with a broad area of about 10m/s LL shear, but very weak storm relative helicity values. As LCLs stay low and local modifications of the BL wind field due to the topography could enhance the shear an isolated tornado can't be excluded.

The same for the western Ukraine.

A possibly more pronounced tornado threat should evolve over northern and eastern Poland. LL shear and SRH-1 values hint on a few tornadic storms. EL data don't support any deep convection scenario and hence the severe wind gust threat should stay limited.
Stronger DLS over SE Poland and Slovakia will be present, but convective activity should ease rapidly during the day from the SW.


....Areas along the northern / eastern Adriatic Sea...

A level-1 was issued mainly for the morning hours, when thunderstorms from the night will go on. A gradual decrease in activity should start during the later morning hours.
Strong DLS favors a few severe wind gust reports.