Storm Forecast
Valid: Thu 06 Sep 2007 06:00 to Fri 07 Sep 2007 06:00 UTC
Issued: Wed 05 Sep 2007 21:32
Forecaster: TUSCHY

SYNOPSIS

A strong upper-level low stalls over SE Europe and weakens somewhat during the forecast period.
An active polarfront jet continues to direct disturbances over far northern Europe and will preclude any changes from the unsettled and cool weather conditions.
A high pressure area affects western Europe, where warm and dry conditions prevail.

DISCUSSION

....Gulf of Venice, Adriatic Sea and the Strait of Otranto....

Overall large-scale synoptic weather pattern remains persistent during the forecast period, as the huge cyclonic vortex slowy drops southward over SW Europe. Horizontal displacements of such features are marginal during a 24-hour period and the attention therefore turns towards the streak allocation, which drives smaller-scale features like short-waves / thermal troughs.
There is no exeption in this case and the attention will be a strong mid-/upper-level streak over the Gulf of Venice, which drops southward. A broad upper-level trough axis is forecast to cross the areas of interest from the north during the next 24 hours. Attendant cool-down especially at lower and mid-levels will overspread the warm sea surface ( 24-25°C ) and should significantly enhance lapse rates at lower levels.
Pronounced LL cape release and weak shear will be present and numerous spouts are forecast to develop. Airmass is very dry and could limit coverage along the coastal areas ( especially where a persistent offshore flow can be found ), but rapid mixing will help to support scattered thunderstorms and we won't exclude any areas along the eastern coast of the Adriatic Sea.

...Tyrrhenian Sea...

The same story for this area. SSTs are a tad higher ( between 24-26°C ) compared to aforementioned areas, but cool-down of the atmosphere is alleviated at the same time, which should result in the same environmental conditions like further towards the east, so spouts could easily spin up.
Further towards the east ( north of the Strait of Messina ), DLS gets stronger and will reach more than 25-30m/s during the early evening hours. This would lower the spout threat, but would enhanced the risk for a few organized storms at the same time with an augmented severe wind gust threat.
The same for the Ionian Sea.

....Moldova and western Ukraine....

Scattered thunderstorms are expected to evolve ahead of a slowly eastward moving cold front, which will stall somewhere over Moldova.
Shear is weak and will signifcantly suppress storm organisation although locally more robust instability release and DLS of 15m/s could support multicells with gusty winds and small hail. A very moist PBL, slow eastward movement of the boundary and upper-level divergence could result in local intense rainfall with attendant flash flooding.

GFS indicates a possible organized line of storms over western Ukraine during the later afternoon and evening hours, moving from the south towards the north. Latest thinking is that indeed a small area exists, where instability and shear could support a few severe thunderstorm events, mainly as a severe wind threat, but we also don't want to exclude a locally large hail report, as mid-level lapse rates remain quite steep and SRH3 values are somewhat enhanced.

... NE Austria and Slovakia...

Model pool is consistent in advecting a tongue of higher moisture content in form of the wrap-around process as far west as E/NE Austria during the evening and night hours. Impressive upper-level divergence in the right rear entrance region of a strong 300hPa streak, persistent signals of deep convergence, adequate mixing ratios and mid-level lapse rates about 6°C/km will be present and could support embedded convection in the broad shield of intense, stratiform rain ( especially as forcing gets stronger during the evening and night hours ).
We therefore extended the highlighted areas that far towards the west.
No severe weather threat can be expected with those storms, but they could produce excessive rainfall and enhance the flood risk over eastern Austria.