Storm Forecast
Valid: Sun 01 Apr 2012 06:00 to Mon 02 Apr 2012 06:00 UTC
Issued: Sat 31 Mar 2012 18:07
Forecaster: TUSCHY

A level 1 was issued for parts of Portugal and Spain mainly for isolated large hail.

A level 1 was issued for the Tell Atlas region (N-Algeria) mainly for isolated large hail.

SYNOPSIS

Extensive branch of the polar vortex with cool and dry air covers most of N/E Europe. Only sporadic thunderstorm activity is forecast due to slim BL moisture despite favorable vertical temperature gradient.
Ill defined upper low affects the Iberian Peninsula and results in scattered showers and thunderstorms.

DISCUSSION

... Portugal and Spain ...

At 16 Z (31st March) the surface 10°C isodrosotherm moved well ashore and this onshore flow continues during the night before moist inflow relaxes as cyclonic vortex moves more to the east. Confidence is high that best BL moisture will be found from the Serra da Estrela (Portugal) to Sierra de Gredos and all the way to the Sierra Nevada (Spain) with dewpoints in the lower tens. Forecast soundings show drier air atop that moist surface layer, so daytime mixing may bring dewpoints down a few degrees. Further east, moisture becomes worse with highest values confined to the mountainous areas.

With 500 hPa temperature dropping to less than -20 °C, lapse rates become steep enough for modest MLCAPE build-up, probably maximized over Portugal and W-CNTRL Spain. CAPE peaks at 500-800 J/kg with a rapid decrease to the east. In the LL CAPE field, signals of abundant CAPE appear with weak moisture pooling along the upslope side of the complex topography...up to 150 J/kg LL CAPE is forecast.

Shear will be very weak, so pulsating storms with a gradual tendency for messy clustering are expected. Locally stronger updrafts and a combination of a lowered WBZ and a dry subcloud layer all assist in the chance of an isolated large hail event and strong wind gusts. Given aforementioned CAPE distribution, an isolated funnel/short-lived tornado event can't be excluded due to a messy dispersal of various storm induced outflow boundaries, which locally increase shear on the meso-gamma scale. The thunderstorm activity slowly decreases after sunset as the BL cools down, but the activity may last well into the night with cold-core vortex atop rough topography.

... N-Morocco and N-Algeria ...

Weak onshore flow beneath plume of steep mid-level lapse rates results in a concentrated swath of enhanced MLCAPE. The main activity will be found along the Tell Atlas, where up to 800 J/kg MLCAPE are locally possible. DLS of 15 m/s and expected thermodynamic profile indicate a risk for large hail and strong wind gusts with any thunderstorm. During the night, thunderstorms gradually decay while moving offshore.

...Rest of Europe...

Sporadic thunderstorm activity is possible over parts of E-CNTRL/NE Europe. Sparse BL moisture however keeps SBCAPE values limited. Only a few concentrated 15-% thunderstorm areas were added.

Somewhat better moisture is present over far NW-Turkey/NE Greece, where a few thunderstorms are forecast during the afternoon hours. Marginal hail and strong wind gusts accompany those storms as better DLS will be present. The activity diminishes after sunset.