Storm Forecast
Valid: Tue 15 Sep 2009 06:00 to Wed 16 Sep 2009 06:00 UTC
Issued: Mon 14 Sep 2009 21:28
Forecaster: TUSCHY

A level 2 was issued for Sardinia, Corsica and the N-Tyrrhenian Sea mainly for large hail, severe wind gusts, tornadoes and excessive rainfall.

A level 1 was issued for the western Mediterranean mainly for excessive rain and tornadoes.

SYNOPSIS

A SW-ward moving upper low crosses France and will be situated over extreme northern Spain during the morning hours. Numerous disturbances rotate around this feature and cause unsettled conditions over the western Mediterranean. Thunderstorms are also forecast over SE-Europe. NE-Europe remains stable.

DISCUSSION

... Balearic Sea, Ligurian Sea, Sardinia, Corsica and the Balearic Islands ...

The latest AVHRR SST analysis of those areas has roughly 23-25°C over the Ligurian and northern Balearic Sea, increasing to 24-27°C further south. Positive SST anomalies are widespread but mostly in the 0-5°C to 1.5°C range (OISST).

A positive tilted trough channel is situated over far western Europe and gets reinforced by the approaching cold-core low from Germany/France. A consequence of this pattern is a strong SW-erly flow over those areas during the forecast along the eastern fringe of the low. Numerous vorticity lobes are embedded in this flow and assist in re-intensification of thunderstorms with WAA resulting in persistent lift. At the surface, the exact surface pressure evolution is not clear with global models like GFS, ECMWF, GEM and WRF all show a developing depression over the Balearic Islands during the morning hours, moving constantly to the northeast with gradual intensification (WRF the most aggressive one with a sub-1000hPa central pressure west of Corsica at the end of the forecast). The local MM5/WAM however hint on numerous cyclonic vortices to evolve over the western Mediterranean with a more easterly track compared to the global models. For now, we stick with the global models and therefore expect a strengthening surface depression just south of France during the morning hours.

A slight warm-up in the mid-levels and rapidly weakening lapse rates at mid-levels cause CAPE to be less than during the past 48h. Nevertheless, deep convection is forecast with overall weak shear in the range of 10m/s in the lowest 6km.

The slowly consolidating surface depression, the passage of the mid/upper disturbances from the SW and a moist air mass throughout the troposphere display a good environment for repeated, slow moving thunderstorms with a flash flood risk especially over the Balearic Islands. Another risk arises with increasing 0-3km CAPE release, LCL height of at or below 500m and outflow boundaries from decaying storms, which point to a tornado/waterspout risk.

Thunderstorms continue until the morning hours over the complete area, probably maximized over the Balearic Islands with two active periods, one during the afternoon hours and another one after midnight, as a cold front approaches from eastern Spain.

... Sardinia, Corsica and the Tyrrhenian Sea during the evening and night hours ...

Latest AVHRR SST has a tongue of very warm waters east of Sardinia and Tunisia with SST values of 25-28°C and even higher values south of Sicily. This is in line with an impressive positive SST anomaly west of Italy of 3°C to more than 4°C (OISST).

The main focus for initiation will be the approach of a strong mid-/upper impulse from N-Algeria, resulting in increasingly diffluent conditions over the area of interest. GFS also forecast a potential coupled jet configuration between a polar front jet streak over S-France and a subtropical streak over extreme N-Africa. The environment for intense lift will overspread the region from the SW during the night hours.

The airmass remains capped during the day with WAA (850hPa 20°C-isotherm approaches Sardinia during the night) but rapidly steepening lapse rates and high BL moisture content above the very warm waters cause a rapid CAPE build-up. Even the more reluctant ECMWF develops a tongue of more than 3000 J/kg MLCAPE with ICAPE approaching 4000 kJ/m^2 west of Sicily and east of Sardinia during the morning hours.

0-6km bulk shear of 15-20m/s and the same magnitude in the lowest 3km overlap with 200-300 m^2/s^2 SRH3. Models like GFS reveal a typical convective feedback problem with unrealistic wind field adjustments, so regional extreme shear values are discarded. The air mass remains capped offshore well into the night, so explosive thunderstorm development is forecast first over Sardinia and Corsica, moving rapidly towards the NE. Later on, thunderstorm initiation becomes also increasingly likely over the Tyrrhenian Sea as cap erodes. Any developing thunderstorm gains organization rapidly with a very large hail, severe wind gusts and tornado threat first over Sardinia and Corsica, spreading rapidly northeastward and affecting the west-central coast of Italy during the morning hours.

A constantly strengthening LLJ helps to advect the very moist air to the north, feeding developing thunderstorms with moisture, so excessive rainfall is a major threat, especially if backbuilding into the moisture source region occurs. Just at the end of the forecast, WRF has an eastward moving convergence zone over the Tyrrhenian Sea with an extremely unstable prefrontal air mass, so the risk constantly shifts southeastwards and will continue after 06Z (next forecast period).

The level-2 was issued for the organized thunderstorm risk but also for excessive rain over Sardinia and Corsica. Later model and remote sensing data will be studied for a southward expansion of the level 2 area as models differ significantly when initiation ought to take place west and over of Sicily.

... Rest of the general thunderstorm areas ...

Weak shear and moderate instability preclude any organized thunderstorms. There is an isolated large hail risk beneath the cold-core low over SW-France but otherwise, marginal hail and strong wind gusts will be the main risk.