Storm Forecast
Valid: Fri 26 Oct 2007 06:00 to Sat 27 Oct 2007 06:00 UTC
Issued: Thu 25 Oct 2007 19:16
Forecaster: TUSCHY

SYNOPSIS

A weakening but with up to 165kt still very strong upper-level jet continues to speed eastwards during the early period, slowing down somewhere northwest of Ireland while re-intensifying back to over 180kt. This very dynamic environment is the result of a cold and well developed vortex over Greenland with attendant intense surface depression of 970-975hPa north of Iceland.
This constellation results in a re-strengthening ridge just west of Europe with rapidly rising geopotential heights. The consequence is a slow eastward drift of a positive tilted upper-level trough which is composed of three separate upper-level low pressure areas ( which can be seen mainly in the temperature maps ), although two of them will finally dominate ( one over the western Mediterranean and the other one over Germany ). The eastward motion will likely not yet be felt during the current forecast period.

The rest of Europe ( east / southeast / northeast ) has quiet and stable conditions...at least convective-wise.


DISCUSSION

... Western and central Mediterranean...

...Italy, Sicily, the Tyrrhenian Sea and the Adriatic Sea...

The main focus for thunderstorm activity will be a weakening upper-level trough and attendant surface depression. The positive tilt of this upper-level feature and an increasing geopotential gradient / attendant strengthening jet streak on its upstream side, supporting no change of this tilt during the forecast period, create not the best environment for better instability release. Temperatures in nearly all levels ( despite the 500hPa level ) start to warm-up slightly over Italy during the next 24 hours . This evolution supports at least moderate mid-level lapse rates although we see no reason to expect those impressive lapse rates, calculated by GFS ( e.g. Sicily during the night hours ).
A short wave is forecast to cross the Tyrrhenian Sea and Italy during the late morning and early afternoon hours, reaching the Ionian Sea during the evening hours. Attendant intense PVA will serve as a focus for isolated to scattered thunderstorm development.

The thermodynamic environment north of Corsica, over northern Italy and the northern Adriatic Sea is more favorable for a more robust instability release with an attendant higher thunderstorm coverage.

Despite the warming, there is another point which should lower the severe weather risk. As surface depression weakens during the day and the upper-level low becomes more diffuse over the western Mediterranean, the overall shear starts to relax. Only the subtropical jet over Tunisia and eastwards enhances mainly the deep layer shear and also storm relative helicity, as a result of strong veering over this area . But right now this region stays too warm for any convective activity.

We issued a level-1 mainly for the coastal areas, where better instability and stronger LL shear overlap and an isolated tornadic cell looks possible. The level-1 was also expanded well inland over parts of northern Italy, as better boundary layer moisture will be present, enhancing the chance of at least low-end instability release. The same for southern Italy.
Each stronger cell around the Adriatic and Ionian Sea could also produce isolated large hail as veering is soemwhat enhanced.

The same story for parts of Sicily.


...Strait of Gibraltar and northeast / eastwards...

As upper-level trough continues to develop south-southwestwards, lapse rates steepen and some modest instability release looks increasingly likely. This area experiences weak shear and no severe thunderstorms are expected.

Somewhat more interesting will be the region around the Balearic Islands and westwards. A pool of cool temperatures at 850hPa ( about 4°C ) over SSTs of 20-21°C will steepen LL lapse rates significantly and waterspouts will be possible as background shear stays weak.
The main focus for a few spouts looks like to be along a shear zone just west of the Balearic Islands as dry and cool air flows offshore from NE Spain. As it is hard to forecast the final position of such small features that far out, no higher probabilities will be issued.