Storm Forecast
Valid: Tue 24 Apr 2007 06:00 to Wed 25 Apr 2007 06:00 UTC
Issued: Tue 24 Apr 2007 10:08
Forecaster: VAN DER VELDE

SYNOPSIS

A somewhat tricky episode. A high pressure axis from Spain to Scandinavia develops into a blocking omega over the next few days, with low pressure systems affecting the west and east of the map. The transition starts today with a midlevel trough coming down from Poland over the western Balkan, while GFS indicates instability buildup over Spain and France in advance of an Atlantic front... there is low confidence this would do anything today. 00Z soundings indicate by far not the low-level instability visible in GFS... whose precipitation signals are also weak.
Common isolated convective cells seem again likely over higher grounds in France, Italy and the western Balkan, though GFS has fairly warm EL temperatures and mid level lapse rates are decreasing somewhat.
A ridge of warm air in the mid levels with associated cloudiness stretches from the Netherlands into the western Balkan.

DISCUSSION

...Germany, Czech Republic, Austria...
While low levels are too dry to initiate convection, levels around 800 hPa are moist with mid level lapse rates steep enough to generate elevated convection. It is thought this will not occur at large scale. There seems to be a central zone over Germany where profiles are saturated and stable (soundings Bergen, Meiningen 00Z). To the east, elevated instability is present (soundings Lindenberg, Praha-Libus 00Z) while to the west, there is marginal surface-based instability (Stuttgart 00Z).
There is some chance of strong gusts with elevated convection. If heating over Czech Republic is sufficient, storms may become rooted in the boundary layer but low level wind field does not appear to have much shear.

...southern Germany, northeastern Italy, western Austria and Slovenia...

A strong jet leads to decent mid-level shear. GFS 00Z displays moderate to fairly strong (15-20 m/s) 0-6 km vectors and somewhat enhanced 0-3 km SREH, but this would only affect BL-based convection. Cells may develop rotation and hence, large hail seems slightly possible in this marginal instability setup. Dry boundary layers may augment chances of isolated severe gusts.
Elevated convection may still profit from strong upper jet and lead to organized convective clusters, possibly with a chance of isolated strong gusts.

...Poland into western Romania, Hungary, Serbia...

The shortwave trough slipping southward at the east side of the large ridge is forecast to affect this area with thunderstorm activity from afternoon to beyond the end of the period. Mostly elevated convection during the night and it seems the moderate shear band is not crossing the area of slight instability.