Storm Forecast
Valid: Sun 13 Dec 2009 06:00 to Mon 14 Dec 2009 06:00 UTC
Issued: Sat 12 Dec 2009 20:59
Forecaster: SCHLENCZEK

No threat levels have been issued.

SYNOPSIS

Ahead of an upper ridge over the British Isles, very cold air overspreads most parts of continental Europe. There are only two regions where deep moist convection will be possible: the Mediterranean and the Bay of Biscay. Quite high SSTs over the Bay of Biscay (around 14 - 15°C) and an upper cold core with temperatures below -30°C at the 500hPa layer will lead to low-end instability. Showers and thunderstorms may develop but severe weather as well as significant lightning activity are unlikely.

The area of interest on Sunday is the western / northern Mediterranean ahead of an upper trough which stretches from Russia all the way down to southern Iberia. In the vicinity of a 35 m/s westerly jet streak at 500hPa, upper level cooling leads to 300 - 500 J/kg CAPE. Another area with some hundred J/kg CAPE is the central Mediterranean, including the Tyrrhenian Sea and Sicily.


DISCUSSION

...N Algeria, S Balearic Islands, SW Mediterranean...

In the warm airmass ahead of the upper trough, quite marginal instability will allow some thunderstorms, mostly over the water. As the LL lapse rates steepen during the day when the trough overspreads the region, an isolated waterspout may occur but a level 1 seems not warranted.

...S France...

Ahead of an upper vort-max, warm and moist air near the 850hPa layer is advected towards S France. QG forcing should be strong enough for elevated convection which could be maintained over this area for several hours. It is not clear whether the amount of precipitation will match our level 1 criterion for excessive rainfall and therefore no threat level is issued. SRH1 / SRH3 will reach values around 300 / 500 m²/s² but tornadoes are not expected as surface-based convection will probably not occur due to very strong CAA in the lowest levels.

...C Mediterranean, Tyrrhenian Sea...

Some thunderstorms are forecast to develop in an environment with 15 - 20 m/s deep layer shear and some hundred J/kg CAPE. As the surface low approaches N Italy, low level winds will strengthen to 15 - 20 m/s at 850hPa. Strong to marginally severe wind gusts are expected with the strongest storms but overall threat should not exceed the level 1 threshold.