Storm Forecast
Valid: Wed 16 Sep 2009 06:00 to Thu 17 Sep 2009 06:00 UTC
Issued: Tue 15 Sep 2009 22:40
Forecaster: PUCIK

A level 1 was issued for Spain mainly for large hail.

A level 2 was issued for Sicily, Corsica, Tyrrhenian Sea and Central Italy mainly for excessive precipitation, tornadoes and to lesser extent for large hail, wind gusts.

A level 1 was issued for a broad region surrounding a level 2 mainly for excessive precipitation, large hail, tornadoes and to lesser extent wind gusts.

SYNOPSIS

A broad cyclonic vortex will influence the weather over most of the Mediterranean and Southwestern Europe. By the end of forecast period, its center will have settled over Iberia. Several smaller scale disturbances will rotate around the low. A strong shortwave trough over Scandinavia will amplify during its progress to the East, connecting with a shallow but extensive trough over Eastern Europe. Small ridge will extend from the Eastern Mediterranean to Central Europe. At lower levels, a shallow low pressure system will stall over the Mediterranean and a large high with center over the British Isles will move slowly eastwards. Another high will stagnate over Russia.

DISCUSSION

... Eastern Spain ...

A belt of enhanced mid-level flow will strenghten DLS towards the afternoon hours, with expected values between 20-25 m/s. The degree of instability remains questionable but both GFS and ECMWF produce sufficient signals of the deep-moist convection formation. Provided storms form, they will attain good organisation, with supercells not ruled out. Large hail might be a threat and as numerous storms are expected in the region, also thanks to the synoptic scale forcing from the low a Level 1 is warranted.

... Northern Tunisia, Sicily, Corsica, Tyrrhenian Sea and Central Italy ...

WAA regime at the forward flank of the low will enable an advection of high Theta-E airmass over most of the region. This factor, combined with very warm SSTs, moist layer above the sea surface and steep mid-level lapse rates will allow for significant destabilization. All models agree on a pool of very high CAPEs in a belt from eastern coast of Tunisia towards Sicily and Tyrrhenian Sea with values over 3000 J/kg.

As of Tuesday 20 UTC, explosive thunderstorm development is already underway across this area with MCS forming east of Sardinia and Corsica. MCS is expected to affect coast of Italy during the Wednesday morning and another storm systems will follow during the forecast period. Their course is variable in different model runs, but it seems quite probable that Italy, especially the Central part, has the highest chances of being affected, as well as Sardinia and Corsica.

Moderate to Strong wind shear (15-20 m/s in 0-6 km layer) and enhanced values of SREH with the backing flow should enable a good level of storm organisation. As mentioned above, the formation of one or more MCS is very probable (as suggested by strong convergence fields and typical "precipitation bomb" patterns by GFS). However, if some storms manage to stay isolated, supercells might form as well.

The highest threat should be the excessive precipitation as very moist airmass, low LCLs and storm movements parallel to the boundary are suggestive of high precipitation falling from the storms. Especially the regions in paths of MCS might see very high amounts of precipitation. Strong wind shear in lower levels, possibly enhanced locally by MCS circulations, might result in tornadoes with the threat being most prominent for Sicily and western coastal areas of Italy. Due to the very high amounts of CAPE and steep mid-level lapse rates, large hail might occur in the stronger storms in the southern part of Level 2 around Sicily and Tunisia. Severe wind gusts are not ruled out either, again mostly in southern part of the level, where high values of Delta Theta-E are simulated and steep lapse rates present.

... Region surrounding Level 2...

Conditions will be less favorable to the north and east of Level 2 for significant thunderstorms. Nevertheless, excessive precipitation threat from MCS affecting the region will be present and a Level 1 is warranted.

Strong wind shear with DLS values climbing over 20 m/s are forecast over eastern Algeria along with a plume of unstable airmass especially along the coastal areas. Well organised storms, including supercells might form with threats of large hail or severe wind gusts. As the storm coverage should not very high, only Level 1 is warranted for this region.