Storm Forecast
Valid: Fri 02 Sep 2011 06:00 to Sat 03 Sep 2011 06:00 UTC
Issued: Thu 01 Sep 2011 19:41
Forecaster: TUSCHY

A level 1 was issued for SE/E/NE-Spain and the extreme W-Mediterranean mainly for large to isolated very large hail and a few strong to severe wind gusts.

A level 1 was issued for SW-France mainly for excessive rainfall.

A level 1 was issued for parts of N-Italy and the coast (adjacent areas) of the NE Adriatic Sea mainly for large hail and severe wind gusts.

SYNOPSIS

An active upper low enters the Iberian Peninsula from the west but starts to weaken during the forecast, as large-scale WAA of an active Iceland vortex affects that feature. Strong ridging is present over the central Mediterranean with hot/stable conditions and cool/unsettled weather continues over NE-Europe, as large-scale upper low remains in place.

DISCUSSION

... Portugal, Spain and the western Mediterranean ...

Gradually weakening upper low slowly moves eastwards during the forecast and affects the extreme W-Mediterranean during the late night hours. Part of an extensive Saharan Air Layer to the south becomes included into the system and hence allows quite dry air to spread northwards over Spain and the W-Mediterranean during the day. However, strengthening LL baroclinic zone over Morocco and the E-Strait of Gibraltar, moistening atop the warm SSTs of the Mediterranean and deep isentropic lift ahead of the eastward pushing trough allow overall moisture recovery during the evening and night hours over a broad/deep atmospheric column.

During the day, thunderstorms and showers keep going all day long over Portugal and Spain as some diabatic heating, forcing and cold mid-levels assist in instability and initiation. Placed beneath center of upper low, kinematics remain weak, so pulsating storms with marginal hail/strong wind gusts will be the predominant risk.

Later on /afternoon hours onwards/ overall conditions become more supportive for initiation along the SE/E/NE coast of Spain and also offshore during the late night hours, as thermal ridge shifts eastwards, reducing CIN. Enhanced dynamic lift, 1000 J/kg MLCAPE and 15-20 m/s 0-6 km bulk shear allow well organized multicells to form with large hail and strong/severe wind gusts. An isolated very large hail event can't be ruled out, although displacement of best shear/CAPE axis lowers confidence somewhat (also below the level 2 threshold). Thunderstorm coverage and intensity increases offshore during the end of the forecast.

Further east, over the west-central Mediterranean (including Corsica and Sardinia), forecast soundings indicate an increase in CIN during the day as thermal ridge moves in from the west. Nevertheless, weak vorticity maxima eject out of the base of the Iberian Peninsula trough and may allow an isolated, elevated thunderstorm to evolve. Model discrepancies and uncertainty of placement and timing of any disturbance preclude a broad thunderstorm area for now. However can't rule out a very isolated thunderstorm all the way to the Tyrrhenian Sea.

... Coast/adjacent areas of the NE-Adriatic Sea and N-Italy ...

Probabilistic output of CAPE magnitude (GFS) hints on maximized values over N-Italy and the NE Adriatic Sea. Very rich BL moisture gradually advects northwards beneath steepening lapse rates, so robust, elevated CAPE is expected. A level 1 for large hail and isolated severe wind gusts was added, as DLS shear remains in the order of 15 m/s....enough for well organized multicell events. Despite weak UVV signals, no real focus for forced initiation can be detected, so main activity will be bound to the rough topography ... probably expanding more to the south over N-Italy due to a very moist/unstable inflow.

... SW France ...

Placed in the diffluent cone end of the Iberian trough, wind field relaxes with DLS around or less 10 m/s forecast. Constant influx of moist Mediterranean air and ongoing diabatic heating allow modest instability build-up in the order of 500 - 1000 J/kg MLCAPE. Scattered to widespread thunderstorms are forecast to develop during the afternoon, evening and early night hours. A missing LLJ may decrease the training risk of thunderstorms, but slow storm motion and rapid clustering may enhance the flash flood risk over the level 1 area. An isolated large hail event can't be ruled out mainly during the initiation stage. Thunderstorms keep going all night long.

... Highlighted thunderstorm areas over NE/E-Europe...

Either low-end shear or modest CAPE or both preclude any more organized severe risk. Pulsating thunderstorms with marginal hail/strong wind gusts will be the primary hazard. Isolated large hail may accompany better organized storms over N-Romania. Thunderstorms diminish after sunset.