Storm Forecast
Valid: Sat 01 Nov 2008 06:00 to Sun 02 Nov 2008 06:00 UTC
Issued: Sat 01 Nov 2008 02:14
Forecaster: SCHLENCZEK

SYNOPSIS

A large upper trough over southwestern Europe will be the main focus for convective activity on Saturday. Ahead of this trough, very moist and unstable air is advected into parts of Italy and the Aegean / Ionian Sea ahead of a well-defined cold front. Surface winds are forecast to weaken as the pressure gradient decreases due to an occlusion process. A very strong southwesterly upper flow with wind speeds in order of 30 - 40 m/s at 500hPa will persist over the whole western / central Mediterranean. Severe weather should become more likely in the western Mediterranean on early Sunday morning as the upper trough overspreads a region with SSTs around 20°C.

DISCUSSION

...Central Mediterranean, Balkans, Aegean Sea...

The cold front over the central Mediterranean, associated with a small surface low over Italy, will translate northeastward towards the southern Balkans. Recent soundings show an EML in the warm sector airmass which leads to about 1 kJ/kg MLCAPE. Mid level cooling will reduce CIN but CAPE also decreases to the west. LCL heights should stay relatively low as surface dewpoints around 20°C are forecast over the Aegean Sea.

The kinematic environment should be much weaker than yesterday but 25 m/s deep layer shear and 15 m/s low level shear should be in place. Highest values of SRH1 and SRH3 (200 - 400 J/kg) are forecast for northern Greece and Albania. Remnants of present convective systems will probably reduce the potential for organized severe convection on Saturday but strong multicells as well as a few supercells will likely evolve, capable of producing severe / damaging gusts and / or an isolated tornado. Isolated large hail is not discounted either but severe gusts will be the main threat. A LVL2 is not warranted at the moment but if forcing is stronger and an organized convective line develops, an update will be issued.

...E Spain, Balearic Islands, W Mediterranean...

A well-defined cold front that stretches from the N Maghreb States towards central France will move northeastward. As the upper trough overspreads the western Mediterranean and parts of Spain, some hundred J/kg of CAPE will be created in an environment with 30 m/s deep layer shear and up to 20 m/s low level shear. Especially in the north of the Balearic Islands, southeasterly winds near the center of the surface depression will lead to SRH3 in order of 400 J/kg. Organized multicells and supercells will develop during the afternoon which will tend to cluster into one or two large MCSes. Those storms will likely be accompanied by isolated large hail, severe gusts and possibly an isolated tornado. The tornado / severe gust threat will decrease in the late evening but torrential rainfall may lead to flash floods, especially along the Spanish coast.

...East-central France...

Low-end instability is forecast by GFS in an environment with very strong LL winds (30 m/s at 850hPa). Strong QG forcing will allow some organized multicells / linear systems that may produce severe / damaging gusts. Locally enhanced SRH1 may also allow an isolated tornado. A small LVL1 was issued for the region where instability and shear / SRH should be best.