Storm Forecast
Valid: Tue 23 Sep 2008 06:00 to Wed 24 Sep 2008 06:00 UTC
Issued: Tue 23 Sep 2008 01:43
Forecaster: VAN DER VELDE

SYNOPSIS

A blocked pattern is present with Atlantic high pressure stretching out over the British Isles and Scandinavia, low pressure at Mediterranean latitudes, and an upper cold pool over central Europe. A zone of warm midlevel theta-e is advected westward over Poland, Germany and Benelux and in combination with a shortwave trough creates a rather rainy episode.

Low level lapse rates are steep over the warm Mediterranean Sea, giving rise to shallow moist convection over northern parts, and deep moist convection along waves in a frontal zone moving east along the 38 degrees latitude from the Balearic Islands/Sardegna to southern Italy/Sicily.

Another focus for storms is the area east of Greece, northern Turkey and Black Sea.
A small area of steep mid level lapse rates could allow some thundery convection over northern Italy and Adriatic Sea.


DISCUSSION

...northern Mediterranean Sea, northern Aegean Sea, Black Sea...

Chances exist for waterspouts to occur west and east of Italy, especially along convergence lines. The Adriatic nocturnal landbreeze convergence zone could be a focus. However, in these areas instability as forecast by GFS is really marginal, and winds could be more favorable (weaker), but very steep lapse rates near the water surface is in favor.
More significant instability, low level buoyancy and similar lapse rates, as well as weak low level winds are forecast in the Aegean/Black Sea area, for which confidence in several spout occurrences is high enough to issue a level 1.

...Tunesia, southern Mediterranean Sea...

Just off the map, high instability, 25 m/s DLS and >200 m2/s2 SREH can create potent multi/supercell storms with large hail and severe winds at the passage of a trough. Level 2 appears adequate for this.
To the north, around the low pressure core between Sardegna and southern Italy, winds from the east and north create more than 300 m2/s2 SREH and >20 m/s 0-6 km shear. This can generate rotating storms with large hail and perhaps severe winds.

...SW Mediterranean...

DLS of >20 m/s and some >100 m2/s2 SREH are sufficient for well-organized multicells with potentially rotating updrafts capable of delivering large hail. The threat is limited however by a lack of organizing factors and weak GFS predicted mid level lapse rates.