Storm Forecast
Valid: Fri 10 Aug 2007 06:00 to Sat 11 Aug 2007 06:00 UTC
Issued: Fri 10 Aug 2007 10:14
Forecaster: VAN DER VELDE

SYNOPSIS

A large upper cut-off low covers much of central Europe, the Balkan, and Italy where the core is. Weak low pressure is present over a large area at the surface, and synoptic/mesoscale dynamics are mostly confined to the region of Italy and western Balkan.

Low level convergence zones are present along old frontal boundaries: from northeastern France to Czech Republic, over the mountains near the Czech-Polish border into northeastern Germany, bending into Denmark and Sweden (a larger scale cyclonic flow pattern), and more complex convergence regions over the western Balkan, by a combination of orography and ascent/descent patterns due to dynamic forcing.

Instability is maximized along these zones, generally highest forecast over the Balkan and northeastern Germany, 500-1500 J/kg.
Vertical shear is weak, and today much confined to the Balkan and a shortwave mid level trough near western Belarus. Slow storm motion vectors over a large area suggest enhanced threat of local flash floods and landspouts.


DISCUSSION

...Balkan, Hungary...

With moderate instability and ample quasi-geostrophic forcing east of the upper low, a large coverage of storms is expected. The presence of at least 10 m/s deep layer shear (and 100-200 m2/s2 SREH) and persistent convergence zones suggests that long-lived multicells are likely, which will have a reasonable chance of large hail given the amount of instability.

...Adriatic and Tyrrhenean Sea...

Waterspouts seem possible given steep low level lapse rates and buoycancy, especially during night and early morning when nocturnal land breezes have converged into a line (especially Adriatic), providing vertical vorticity.

...Moldova, Ukraine...

00Z Odesa sounding showed 1600 J/kg MLCAPE50 starting very low to the ground from a saturated parcel, with very weak winds through a deep layer. GFS 00Z predicts large amount of precipitation during morning hours (at time of writing indeed some storms). If a source of vertical vorticity is present, strong stretching may result, triggering land/waterspouts, and storms may also grow large hail with such amounts of instability, as well as flash floods with slow storm motion.

...NW Ukraine, W Belarus...

Low-mid layer shear is increased somewhat along the mid level shortwave trough, 0-1 km shear even to values of 10-15 m/s. Despite low instability predicted, the shortwave may trigger enhanced convection, which under these conditions may produce strong gusts. The situation seems a bit marginal, though, and cloudiness may inhibit build-up of instability too much.