Storm Forecast
Valid: Sun 29 Nov 2009 06:00 to Mon 30 Nov 2009 06:00 UTC
Issued: Sun 29 Nov 2009 05:58
Forecaster: SCHLENCZEK
A level 1 was issued for N France and S UK mainly for severe wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for the W Mediterranean, including Corsica and Sardinia, mainly for severe wind gusts and tornadoes and to a lesser extent for large hail.
A level 1 was issued for N / NE Italy mainly for excessive rainfall and to a lesser extent for severe wind gusts.
SYNOPSIS
On Sunday 06 UTC, a quasi-stationary upper trough over the Irish Sea is forecast to dig southward. A 75 m/s northerly jet streak in the wake of the trough will have its left exit region over N Iberia. Ahead of the trough, an occluding 980 hPa surface low is present over SW UK with its cold front stretching from the British Channel via N France and N-central Iberia towards SW Portugal. East of the cold front, moderately warm air overspreads large portions of the Mediterranean Sea. Two upper short-waves over the Mediterranean will move eastward during the period, affecting parts of Italy and Greece / W Turkey.
DISCUSSION
...SE Spain, Balearic Islands, W Mediterranean region, N Italy...
During the afternoon, the cold front of the UK low pressure system will affect eastern Spain and the SW Mediterranean. Some hundred J/kg CAPE ahead of the front and 20 - 25 m/s deep layer shear are forecast and convection may develop in the vicinity of an upper vort-max which overspreads the W Mediterranean in the afternoon / evening hours. GFS has some tendencies that cyclogenesis may occur north of the Balearic Islands which should provide backing LL winds ahead of the trough. SRH values between 150 mē/sē and 300 mē/sē are in place with the unstable air and this setup may allow organised multicells and / or a few supercells which are capable of producing severe wind gusts and isolated large hail. Near the coastlines, LL shear is slightly emphasized and an isolated tornado is not ruled out with the supercellular storms. Convection will probably develop offshore of the Spanish coast, affecting the Balearic Islands and Corsica / Sardinia.
In the late afternoon / evening hours, the cold front will also affect the northern / central parts of Italy. 20 - 25 m/s wind speed at 850 hPa are expected and severe gusts should be possible. The main threat in northern / northeastern Italy will be torrential rainfall with a high risk of flash floods as strong moisture advection at lower levels will sustain convective systems for several hours. As the upper vort-max moves across Iberia, a second maximum of convective activity is expected in the SW Mediterranean on early Monday morning.
...N France, British Channel, S UK...
Near the centre of the low pressure system, moderate to strong deep layer shear is in place with a few hundred J/kg CAPE. Most of the showers / thunderstorms tend to stay discrete but a narrow line of thunderstorms may form in this region as well. Severe wind gusts will be the main threat with the strongest storms but an isolated tornado cannot be discounted. The tornado / severe wind gust threat will diminish during the afternoon / evening hours as deep layer shear and forcing will decrease.
In the wake of the trough, deep layer shear decreases to less than 10 m/s in most places. Numerous showers and thunderstorms are forecast in this region but severe weather should be unlikely.
