Forecast Update
Valid: Fri 15 Jun 2007 10:00 to Sat 16 Jun 2007 06:00 UTC
Issued: Fri 15 Jun 2007 09:39
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Thu 14 Jun 2007 22:47 !

DISCUSSION

....Level 2 area over parts of W-Russia....

Some modifications to the level-2 area. Early arrival and limited instability has probably contributed to the weak thunderstorm activity along the cold front, but fast moving thunderstorms have already evolved ahead of this feature.
Strong WAA and diabatic heating helped to warm up surface temperatures to around 25°C ahead of the cold front and this will be enough for moderate instability release.
Expect a constant increase in thunderstorm activity with an enhanced severe - damaging wind gust / large hail risk !
Strong veering at lower levels also poses a risk for a few tornadoes with a strong one being a distinct possibility.

... Switzerland, Germany, parts of Austria and Poland...

A complex weather situation seems to evolve over the area of interest.
Run to run consistency is not that bad but all the more the inconsistency between the different models.

Latest surface analysis has numerous wind shift zones over E-France and an approaching cold front from the west ( placed over central France at 7UTC).
Clearing over most parts of central / eastern Germany and Austria helped the surface temperatures to already reach 20-24°C and during the day, around 25°C over S-Germany, 25-30°C over SE Germany and above 30°C over eastern Austria and NE Germany will be likely.
This should help to strengthen the cold front over SW Germany.
IN addition, strengthening mid-level PVA spreads towards the NE and will reach SW Germany during the next few hours.

Impressive upper-level forcing still forecast and a cluster of storms ( already ongoing over SE France ) will move towards the east, crossing N-Italy from the west.

During the next few hours, rapid thunderstorm evolution also takes place along the front over Switzerland, SW - west central - NW Germany . Latest thinking is that best dynamics and strengthening thetae gradient over SW Germany will support widespread thunderstorm development with a cluster of storms moving towards the E / NE.
Those storms encounter an environment with better instability and strengthening is forecast.

GFS still has signs of a squalline over S / SE Germany as a consequence of a rapidly intensifying barrier jet north of the Alpes. We do not trust the model's intense winds at 850hPa ( 40-50kt ) as the model's parameterization seems to fail ( developing a non existent, tropical-like mid-level circulation as a consequence of extreme latent heat release ), but weather pattern and strong front hint on a possibly organized line of storms, racing towards the east over S / SE Germany during the afternoon hours. Momentum could support a swath of strong to severe wind gusts and hence an upgrade became necessary for those regions.
In addition....more discrete cells ahead of the line could produce an isolated large hail report as SRH3 values increase during the day.

A somewhat strengthening surface depression over eastern Germany / the Czech Republich was calculated stronger during the past few runs with a nice backing wind field over NE Germany and NW Poland.
Intense diabatic heating and dewpoints around 15°C will be enough for at least moderate instability release in a well mixed BL. The main inhibiting factor for a more robust thunderstorm event will be the decreasing wind speed in all levels.
Severe weather threat over N / E Germany should be bimodal with an enhanced large hail / strong downburst risk over E-Germany and a locally enhanced tornado risk over N-Germany, where T-Td spreads are lower.

Tornado risk over NE Germany / NW Poland could increase during the early evening hours, too as surface temperatures will cool and the surface depression intensifies.
A level-1 seems to be enough as weather pattern favors rapid storm clustering with a lower chance for a long-lived, discrete storm mode.

Again...impressive UL dynamics, weak steering flow and high PWAT values all point to a excessive rainfall risk, especially under slow moving storm clusters. A significant flash flood risk will be the result !