Storm Forecast
Valid: Sat 07 Nov 2009 06:00 to Sun 08 Nov 2009 06:00 UTC
Issued: Fri 06 Nov 2009 22:19
Forecaster: KOROSEC
A level 2 was issued for southern Albania, western Greece and Ionian sea mainly for tornadoes, large hail, excessive rainfalls and severe wind gusts.
A level 1 was issued for the rest of Greece, Aegean sea and western Turkey mainly for large hail, excessive rainfalls, severe wind gusts and lesser extent for tornadoes.
A level 1 was issued for west-central Italy towards Sardinia mainly for large hail, excessive rainfalls, severe wind gusts and tornadoes.
A level 1 was issued for SW UK, western France and Bay of Biscay with coastal areas mainly for severe wind gusts and lesser extent for tornadoes/waterspouts.
SYNOPSIS
The large trough over central Mediterranean becomes negatively tilted while moving eastwards across SE Europe. A weak surface depression forms ahead of the trough axis over Greece. During the day, another trough digs into western Mediterranean from NW with a rapidly forming surface low over Ligurian sea and Bay of Genoa. Within the main deep trough over western Europe, a weakening deep cyclone crosses British Isles while rather polar airmass continues to spread southwards.
DISCUSSION
... southern Albania, Greece, Ionian sea ...
Starting already in the early morning hours, a strong cold front serves as a focus for initiation of deep convection. A widespread ascent ahead of the trough should allow storms easily to form and organize in highly sheared and unstable environment. Moderate instability, very strong deep-layer shear near 30m/s and impressive SREH values 500-1000m^2/s^2 just ahead of the front will be favorable for robust convective activity. Numerous multicells and supercells, as well as few large clusters/MCSes, with threat for large hail, severe winds and tornadoes will be possible. Given the enhanced LL shear and helicity in the area where intense backed flow is pronounced, strong mesocyclones and tornadoes are possible as well. For this reason, a level 2 was issued for Ionian sea, western Greece and Crete island.
Strong SSW flow ahead of the trough/front and high moisture content should be favorable for excessive rainfalls as well. Especially where orographic effects will combine with large convective clusters, more likely over western Greece.
... Aegean sea and western Turkey later in the period ...
In the second half of the forecast period, strong warm air advection ahead of the front overspreads eastern Greece and Aegean sea. As a result, rather strong instability with MLCAPE values exceeding 1500 J/kg will become available beneath the steepening mid-level lapse rates. Therefore large hail and strong winds will be the main threat. Less intense shear, helicity and coverage ahead of the front should limit the higher threat and do not warrant an extend of level 2 more to the east at the moment.
... west-central Italy towards Sardinia ...
With an approaching deepening trough into western Mediterranean, a rapidly forming surface depression takes place over Ligurian sea. Accompanied by this low, a cold front rapidly moves SE-wards across Sardinia towards west-central Italy and Tyrrhenian sea. These areas will be right under the left-exit jet region with strong divergence in the upper levels. Rather warm air advection will be ongoing ahead of the front northwards. As the dry intrusion with steepening lapse rates overspreads it, several hundreds of MLCAPE should result. Moderate LL and deep-layer shear should allow some organized convection, including a few multicells and supercells. Tornado threat seems limited mostly to southern Sardinia and coastal areas of west-central Italy where LL shear/helicity will be the most enhanced. There will also be enhanced orographic lifting along the coast of Italy and therefore excessive rainfalls could locally occur.
... SW UK, western France and Bay of Biscay with coastal areas ...
Another voticity maxima crosses British Isles in the afternoon/evening hours with a weakening low and an occluding front moving SSE-wards overnight. At upper levels, very intense NW-erly jet streak up to 85m/s at 300hPa level is placed on its western side. The occluded front moving beneath the left-exit jet region should serve as a focus for some shallow convective activity. Therefore some convective storms with a threat for severe wind gusts and tornadoes or waterspouts will be possible. Elsewhere within the polar airmass characterized by very steep lapse rates, marginal instability will result and numerous convective cells should form.
