Storm Forecast
Valid: Fri 28 Jul 2006 06:00 to Sat 29 Jul 2006 06:00 UTC
Issued: Thu 27 Jul 2006 18:04
Forecaster: TUSCHY

SYNOPSIS

Downstream of an intense upper-level longwave trough over the eastern Atlantic, ongoing, but weakening WAA will continue to infiltrate a very warm airmass over most parts of central Europe.
Another well developed trough is still affecting parts of NE Europe, where cool and mostly stable conditions prevail.
The Mediterranean area will stay dry and hot, although a few TSTMs will be possible

DISCUSSION

....Most parts of France, Switzerland, Germany, the Low-Countries and parts of Austria...

The condition for a few organized storms will be favorable over a very broad area.

A pretty tricky forecast ahead, because of many factors, which finally could play an important role in exact TSTM placement and organisation.

The first question will be, where exactly the main TSTM activity can be expected.
A weakening and opening upper-level trough will slowly slide NNE-ward over eastern France / western Germany during the forecast period and it looks like that it will split in 2 pieces....one of them affecting Switzerland and S-Germany and the other one NW Germany and the Low-Countries.
Therefore, we have to deal with a diffuse upper-level wind field, although there is an over-all tendency of a weak SW-erly flow with embedded speed maxima.
More problems will arise when we go down to the lower - levels .... A still noticeable and waving baroclinic zone over CNTRL France will slowly shift towards the east and should be the focus for enhanced TSTM activity.
Of yesterday storms over France and Germany also helped establishing various moisture convergence zones / outflow boundaries, which also should support widespread TSTM initiation.

Final point will be the development of a weak low-level depression over N-Germany.... It is impossible to pinpoint the final position of this system that far out, but I can't ignore the development of a broad low - pressure channel, also affecting parts of eastern and NE Germany, which was calculated during the 12Z GFS model run.

Current thinking is, that ongoing convection will be mostly sub-severe during the first hours of the forecast period.... Re-intensification / widespread development in a nearly uncapped environment can be expected over various places.

Two areas could see a more concentrated severe weather coverage:

An area of concern will be central and eastern Germany, where still very steep lapse rates can be found...The wind field will become somewhat stronger compared to yesterday and expected ~1000 J/kg CAPE, combined with the background flow yield an enhanced possibility of large to isolated very large hail and a few severe wind gusts.

Another area will be the Lower Countries and NW Germany, where model-calculated shear parameters look fine.... Limited insolation should restrict the instability release to 600, locally up to 900 J/kg, but developing storms will have an attendant severe wind gust / isolated large hail risk..... The area will be monitored regarding the chance of a low-end tornado potential if some backing of the low-level wind field could be established.

Otherwise, each TSTM in the broad level-1 area will have the chance to produce large hail and severe wind gusts during its life-time... High PWAT values indicate the possibility for intense rain with stronger cells and attendant flash flooding ( especially if indicated storm clusters over NW / SW Germany develop ), but this threat is not considered in the choose of the risk level.

.... eastern parts of Sweden....

Broad area of moderate instability release can be expected over the area of interest.... Main reason for including this region in the level-1 area was the possible development of a stronger southerly low-level wind field.... Latest soundings reported a pretty humid boundary layer and hence low LCLs.... The main risk for an isolated tornado report will be over the CNTRL Gulf of Bothnia and along the eastern coast of Sweden and the extreme SW coast of Finland.

...Austria...

Scattered TSTM development can be expected mainly over western and central Austria, due to an approaching upper-level wave and a nearly uncapped environment.... The background flow should stay weak and there will be mainly an marginal hail risk.
Models like BOLAM indicate a possible cluster of storms, developing during the evening hours over N-Italy and affecting eastern Tyrol, Salzburg and Carinthia... Main threat would be an isolated severe wind gust and marginal hail risk.... locally intense rainfall could cause flash flooding.