Storm Forecast
Valid: Fri 06 May 2011 06:00 to Sat 07 May 2011 06:00 UTC
Issued: Thu 05 May 2011 12:31
Forecaster: TUSCHY

A level 1 was issued for Portugal and Spain mainly for large hail and severe wind gusts. An isolated significant hail event can't be ruled out over south-central Spain, e.g. Madrid southwards.

A level 1 was issued for NW-France mainly for large hail and an isolated severe wind gust/tornado risk.

A level 1 was issued for S-Turkey mainly for flash flood producing thunderstorms.

SYNOPSIS

CAA continues over east-central/east Europe with a quasi-stationary longwave trough still in place. Strengthening ridging over central Europe further assists in the advection of a well mixed, dry and cool air mass far to the south, even affecting parts of the central/eastern Mediterranean with negative surface dewpoints. A north-south aligned air mass boundary has established to the east of that feature and attendant surface high pressure are with a sharpening moisture gradient present over far eastern Ukraine and parts of Russia.
Over the far northeastern Atlantic, an extensive deep cyclonic vortex is anchored with little eastward movement forecast. However, a significant impulse along its southwestern fringe is about to assist in the development of a sharp mid-/upper trough with a much shorter wavelength. This feature circles around the southern part of the main vortex and approaches Portugal as a significant vorticity lobe from the west during the night hours. Brisk SW-erly flow covers regions from the Iberian Peninsula all the way to UK, atop gradually moistening LL. Hence, a more widespread thunderstorm risk is in store for the upcoming forecast period.
Stable conditions persist beneath increasingly tilted high pressure/ridge features over central Europe.

Models agree reasonably well with the main pattern, revealing some discrepancies within the BL qualitiy...especially over NW-France.

DISCUSSION

... Portugal and Spain ...

Brisk southwesterly flow affects all of the Iberian Peninsula all day long. However, as aforementioned vorticity lobe approaches from the west, wind field at mid-levels ramps up significantly during the early afternoon hours onwards with shear magnitude increasing by roughly 10 m/s at lowest 3 and 6 km to 20 and 25 m/s respectively. According to a deep WAA pattern, veering is expected within the mid tropospheric layer and widespread 200 m^2/s^2 SRH-3 occur, increasing betimes over E-Spain. Also, a 35-40 m/s diffluent high-level streamline pattern (~300 hPa) covers the forecast area, also persisting as sharpening trough approaches from the west.
BL quality is still the main concern with yesterday's soundings revealing high T-Td depressions and inverted-V typed profiles. However, persistent influx of better moisture from the Mediterranean assist in further BL recovery and better LL moisture to work with. The pattern is also not supportive for widespread, thick cirrus, so expect again another round of good diabatic heating and orographically bound initiation. CIN remains weak with rapid mix out of any nocturnal stabilization, so scattered thunderstorms evolve around noon, increasing during the afternoon hours in coverage and strength. Large hail is well possible with any multicell/supercell thunderstorm. During the later afternoon, an isolated significant hail risk evolves over central/south-central Spain with any discrete supercell, given favorable parameter overlap ( ~800 J/kg MLCAPE, 30 m/s DLS and increasing SRH). Strong to severe wind gusts are also forecast as BL remains deeply mixed and 0-3 km shear increases betimes.

During the night, thunderstorms move slowly towards the east, as upper trough approaches. However, BL stabilization results in rapidly decreasing CAPE, although steep lapse rates may offset that effect with modest MUCAPE present all night long. Anyhow, expect severe risk to diminish rapidly until 00Z.

An upgrade to a level 2 may become necessary, if CAPE/shear overlap remains in place during upcoming model outputs....mainly over central/south-central Spain. The main risk would be signficant hail.

... W-France ...

Models still diverge regarding the final BL moisture content. ECMWF and GFS all indicate an ongoing moisture advection regime during the forecast with dewpoints increasing to 10°C and more. GFS remains very bullish, developing sfc. dewpoints in the upper tens all the way to the W-English Channel with ECMWF showing a modest solution. Nevertheless, all models indicate modest MLCAPE of at least 500 J/kg, probably maximized over NW-France during the afternoon. Shear remains weaker compared to Spain, but roughly 15 m/s DLS and somewhat enhanced SRH-3 indicate a large hail risk with stronger thunderstorms, next to isolated severe wind gusts. Also, LCLs around 1000 m and somewhat enhanced LL shear may also cause an isolated tornado event. As thunderstorms spread to the N/NE during the night, decoupling from the BL results in a mainly elevated thunderstorm risk over SW-UK. Despite an isolated marginal hail risk, no severe is forecast.

... S-Turkey ...

Daytime driven thunderstorms evolve over S-Turkey. Slow storm motion and a moist inflow from the south allow flash flood prducing thunderstorms. A level 1 was hence introduced. Thunderstorms gradually decay after sunset.