Storm Forecast
Valid: Wed 05 Nov 2008 06:00 to Thu 06 Nov 2008 06:00 UTC
Issued: Wed 05 Nov 2008 00:10
Forecaster: PUCIK

SYNOPSIS

An omega type circulation prevails over Europe and no significant change is expected during the day. A cold-core low with its center over France and the Mediterranean will affect the weather over SW Europe. To the east, large ridge of high pressure will stretch from Greece to Scandinavia. On the surface, an intensifying high pressure system over Norway/Sweden is forecast to spread southward. This feature will stabilize the conditions for most of the Northern, Eastern and Central Europe. On the other hand, low pressure over France is expected to decay, although, persisting mid-level low will bring further spells of rain and thunderstorms from Algeria to the Adriatic sea.

DISCUSSION

...Tunisia to the N Adriatic sea....

Ahead of the weak cold front, warm and unstable airmass is advected over the region. GFS keeps simulating plume of high mid-level lapse rate airmass, which in combination with warm SSTs should result in MLCAPEs above 1500 J/kg, especially over the southern parts. To the north, degree of instability will quickly decrease. With the decaying surface low, kinematic fields in the lowest 2-3 kms are expected to weaken. Mid-level flow will however, stay strong and a belt of 15 - 25 m/s DLS will stretch from Tunisia to N Italy. Shear in the lowest 3 km layer should stay around 10 - 15 m/s. Slight veering of the winds is expected and locally, SREH values will exceed 100 J/kg, which combined with 20 m/s DLS will increase the chances for rotating updrafts.

Nevertheless, storms should exist mostly in the form of multicells but isolated supercell is not ruled out either. Several MCS are possible, where GFS shows strong surface convergence - one might form on the SW coast of Italy and another one in the N Adriatics. Large hail seems to be the primary threat, especially if some supercells manage to form. With considerable LL instability release and isolated spots of 10 m/s low level shear, one or two tornado / waterspout reports are anticipated. The overall risk will slowly shift eastward with time.

Over the SW part of Italy and Sicily, best shear ( DLS over 20 m/s) / instability (MLCAPEs up to 1500 J/kg) overlap will exist. Moreover, GFS is simulating high ICAPE values along with significant moisture inflow and surface convergence. Very large hail will be possible in stronger and more isolated cells. One or two tornadoes and an isolated severe wind gust ( especially by the early afternoon) can also occur. As widespread storm initiation is expected, severe weather chances will be quite high and a Level 2 is introduced.

With copious rainfall recorded the previous day, further rounds of thunderstorms will probably worsen the flash flood threat over the Italy, especially over the areas, where MCS will track.