Storm Forecast
Valid: Sat 30 Nov 2013 06:00 to Sun 01 Dec 2013 06:00 UTC
Issued: Fri 29 Nov 2013 23:02
Forecaster: PISTOTNIK

A level 1 was issued for S Italy, Malta and the Ionian Sea for excessive precipitation, severe convective wind gusts and to a lesser degree for large hail and tornadoes.

A level 1 was issued for parts of the Norwegian Sea for severe convective wind gusts.

SYNOPSIS

Between a blocking high over the Atlantic Ocean and an amplified long-wave trough over Western Russia, cold and mostly dry air has spread out over the European continent and even into Spain and the Western Mediterranean. A small upper-level low is situated over Central Europe on Saturday morning before it splits up. Its Eastern part will reconnect to the Russian trough, while its Western part continues to slip to the Balearic Islands.
In the second half of the forecast period, the Northwesterly flow tightens over Scandinavia, and a short-wave trough and a well-developed cyclone will move onshore from the Norwegian Sea.

DISCUSSION

... Western Mediterranean and Tyrrhenian Sea ...

With a southward pushing jet streak over France, a strong vorticity lobe overspreads the Western Mediterranean region ahead of the upper-level low, though its QG lift will partly be compensated by ongoing cold air advection. Extensive pressure fall is forecast again with a possible evolution of one or several shallow low pressure centers scattered between Italy and Spain.
A lot of shower activity will go on but it is not until the second half of the forecast period that its chances to produce some thunder will rise, especially between the Balearic Islands and Algeria, where the strongest QG lift is going to head. However, moisture is scarce in the Northerly flow regime, and both CAPE and vertical wind shear are too limited to enhance the severe weather threat. If anything, one or two waterspouts and localized heavy rain can be expected towards the end of the forecast period near the Algerian coast.

... Southern Italy, Malta, Ionian Sea ...

The most interesting setup for convection develops further downstream, where a broad warm air advection regime establishes over the Central Mediterranean region. Steepening lapse rates will overspread a moist maritime boundary layer and will create low to moderate CAPE. A lot of rain with increasing chances of embedded thunderstorms can be expected across the entire Ionian Sea. Especially the Southeastern coasts of Sicily and mainland Italy face an enhanced risk of excessive precipitation with a threat of flash floods, as the 850 hPa onshore flow increases to 20 m/s.
Shear signals are highly heterogeneous, but at least the fringes of the warm air body will be overlapped by patches of enhanced shear and helicity near the Greek coastlines (possibly too close to the warm front to be consumed by convection, though) and around Malta and Sicily. Especially the latter region will also see more and more lift support in the left exit region of a small jet streak that will arrive from the South in the evening hours. The chances for better organized storms with severe wind gusts, moderate hail and possibly a few tornadoes increase in the evening and overnight.
It needs to be mentioned that some WRF runs show a very aggressive solution, which does not only result in CAPE in excess of 1000 J/kg but also in the development of a subtropical cyclone off the Libyan coast that is forecast to make a landfall in Sicily by Sun 06 UTC. In this case, severe wind gusts and excessive rain could become more widespread. However, this scenario is probably based on overly optimistic-looking 2m dewpoint forecasts with values around 17°C. A level 2 would be required if confidence was higher about the right track, intensity and timely arrival of such a feature.

... Central Norway and Central Sweden ...

Another powerful North Atlantic cyclone will bring widespread severe wind gusts in coastal areas and at least isolated ones further inland between Sat 18 UTC and Sun 06 UTC. A level 1 was issued for parts of the Norwegian Sea, where they will be convectively enhanced by postfrontal showers.
At the cold front itself, the development of a shallow convective line is not ruled out but does not seem too probable, given that the powerful short-wave trough aloft will not fully catch up with the front before 21 UTC, when it already moves inland and loses its access to maritime moisture. This should preclude an involvement of deep convection, which would be required to include severe wind gusts into the ESTOFEX threat level scheme.