Storm Forecast
Valid: Fri 04 May 2007 06:00 to Sat 05 May 2007 06:00 UTC
Issued: Thu 03 May 2007 19:57
Forecaster: TUSCHY

SYNOPSIS

Geopotential heights over western Europe constantly declined as a broad upper-level trough slowly evolved eastwards. Upper-level jet configuration of in-/outcoming streaks supports a neutral inclination for at least the next 24 hours over western Europe.
At lower levels, low-level depression will slowly move towards the NE, covering the alpine region and surrounding areas.
Downstream of this feature, very warm air will advect northward, affecting parts of extreme SE Europe, while cool and mostly stable conditions prevail over northern Europe.
However, a few short-lived thunderstorms could evolve over N-Europe, but coverage will be very limited.


DISCUSSION

The dimension of the upper-level trough and of the cyclonic swirl at lower levels results in a wide area of enhanced thunderstorm activity . The following regions will be affected:

.... N / CNTRL Italy, Slovenia,Croatia and Austria....

A somewhat complex weather scenario unfolds for those regions regarding the area with the most robust thunderstorm threat.
Latest replication of satellite images clearly shows the center of the vertically stacked depression along the border of Spain - France.
Comparatively dry air on its downstream side covers the western Mediterranean and indicates a strong jet-max placed in the depression's southeastern quadrant.
This dry belt combined with nice lift already helped to trigger storms just south of France ( yesterday 17Z onwards ) and indicates a fine environment for storm evolution ( also refer to yesterday's 12Z sounding of Palma de Mallorca ).
This airmass should advance towards the east and enough potential instability release for scattered thunderstorm development can be expected with the most robust threat over N-CNTRL Italy and over NW Italy.
An inhibiting factor will be a pretty humid airmass and attendant weak lapse rates but this could be offset by some diabatic heating, when dry air works northeastward.
Instability should be on the lower-end side, but deep moist convection looks increasingly likely mainly near / under the dry slot over N-CNTRL Italy ( EL values of 8-10km ).
Main threat should be an enhanced tornado threat ( most concentrated around and north of Roma ), given nice veering and strong SRH 1km values, but also isolated large hail.

The same for extreme NE-Italy, Slovenia and W / SW Croatia, where models show nice instability values during the afternoon / evening hours and in combination with the slowly weakening dry slot, a few storms should manage to form. Shear will be supportive for storm organisation with an isolated severe wind gust / tornado risk.

Areas of southern Austria were included in the general thunderstorm area, because of isolated storm, embedded in a general stratiform rain shield along the southern Alpes. Right now there are no signals that more than a sporadic thunderstorm during the later afternoon/ evening hours could develop over N / E - Austria.

...Germany, France, Switzerland...

A few thunderstorms can be expected as upper-level low will approach S-Germany during the afternoon hours. Overall mode of storms should be a pulsating one with gusty winds and marginal hail...the latter one especially just north of the Alps, where models indicate somewhat steeper mid-level lapse rates and up to 15m/s DLS.
A moisture convergence zone somewhere over central Germany could be another focus for a local thunderstorm threat.
Updrafts, which manage to develop in this low-end instability environment would undergo enhanced SRH 3km shear and hail could attend those storms. Missing instability should keep hail size well below severe criteria and hence no categorical level will be issued.

The opposite for France, where better instability but nearly nonexisting shear will be present. Northernmost storms will have the best chance to produce isolated large hail , otherwise storms should generate gusty winds and subsevere hail.

The same for W - Switzerland, where an isolated stronger thunderstorm could produce marginal hail and subsevere wind gusts.