Storm Forecast
Valid: Sun 07 Mar 2010 06:00 to Mon 08 Mar 2010 06:00 UTC
Issued: Sat 06 Mar 2010 22:38
Forecaster: SCHLENCZEK
A level 2 is issued for most parts of E Algeria and Tunisia for large hail, severe wind gusts, tornadoes and excessive rainfall.
A level 1 is issued for Sicily mainly for excessive rainfall and severe wind gusts.
A level 1 is issued for the C Mediterranean and S Greece mainly for severe wind gusts and tornadoes.
SYNOPSIS
A large upper trough, filled with cold air, is present over east-central Europe. At its southern flank, a strong westerly upper jet stretches from SW France to the Black Sea. CAA will be strongest over SE Europe in the wake of a low pressure system over the eastern Ukraine which will move to the northeast. Warm air with 850 hPa temperatures around 10°C is located over Tunisia / Libya and will be advected towards S Greece and Crete during the forecast period.
DISCUSSION
...S Iberia...
Near the southern tip of an upper trough over SW Iberia, steepening lapse rates will lead to a few hundred J/kg CAPE. 0-6 km deep layer shear should stay below 15 m/s and some briefly organised multicells may develop. Severe weather is not likely but a few waterspouts are not ruled out.
...NE Algeria, Tunisia, C Mediterranean...
Ahead of a low pressure system which is centered over S Algeria, hot and moist air is advected into parts of NE Algeria and N Tunisia. GFS predicts moderate instability with MLCAPE in order of 500 J/kg in the vicinity of an upper vort-max associated with an upper shortwave trough. Low level winds will back to the east / northeast ahead of the cold front and even though the Mediterranean Sea is rather cool, this will lead to a mixing ratio in order of 10 g/kg. As convection developed also on Saturday evening in the warm sector of the low, forcing is probably sufficient for convective initiation. From 850 to 600 hPa, the Saturday 12 UTC DAAG sounding shows an elevated mixed layer in the warm sector of the low which will overspread the discussed area in the morning / afternoon hours. A westerly 25 m/s upper jet near 500 hPa leads to very strong deep layer shear (20 - 30 m/s) and very high values of 0-3 km SRH (close to 600 m2/s2 over E Tunisia). Any storm in this environment may become supercellular, posing a threat of (very) large hail, damaging wind gusts and tornadoes. During the afternoon, storms close to the centre of the low pressure system will likely merge into a MCS with severe wind gusts and heavy convective rainfall being the main threat. Later in the period, storms will tend to weaken as instability and forcing decrease towards the central Mediterranean and W Ionian Sea but an isolated excessive rainfall / severe wind gust event over Sicily is not ruled out.
...Ionian Sea, SW / S Greece...
In the late evening / night hours, remnants of the convective systems over Tunisia / C Mediterranean will cross the Aegean Sea and parts of Greece. Even though instability and forcing are slightly reduced, very strong winds at mid levels (around 30 m/s at 700 hPa) will lead to an enhanced risk of severe wind gusts. 0-1 km SRH forecast by GFS should be in order of 600 - 800 m2/s2 which is favourable for strong low level rotation. If convective cells stay quite discrete, they will likely become tornadic supercells with strong tornadoes (F2 - F3) possible. At this moment, confidence is too low that discrete storms indeed develop in this region and only a level 1 is issued. An upgrade may follow in the afternoon / evening.
