Storm Forecast
Valid: Sat 09 Mar 2013 06:00 to Sun 10 Mar 2013 06:00 UTC
Issued: Fri 08 Mar 2013 18:46
Forecaster: PISTOTNIK

No threat levels were issued.

SYNOPSIS

While the only dominant cyclone on the forecast maps stays well offshore to the Northwest of Spain, a broad and diffuse area of low pressure extends all the way East via the Alpine region into the Ukraine. Along its Southern flank, a zonal subtropical jet runs smoothly across the Mediterranean Sea. To the North, a late outbreak of arctic air continues to affect Russia, the Baltic States and Scandinavia while it slowly makes its way further South and West.

DISCUSSION

... Southern Portugal, Southern Spain, entire Mediterranean Sea ...

A succession of three subtle short-wave troughs is embedded in the Westerly upper-level flow. In the absence of pronounced cyclonic features, their lift support is the only noteworthy instrument for regionally bringing together moderate lapse rates and wafts of increased low-level moisture in order to allow the build-up of a few hundred J/kg of CAPE. Isolated to scattered thunderstorms are expected over maritime areas and along west-facing coasts mainly in the path of these troughs, which will shift as follows during the forecast period:
(1) From Albania and Western Greece (Sat 06 UTC) via the Turkish West Coast (Sat 18 UTC) to Central Turkey (Sun 06 UTC),
(2) From the Pyrenees (Sat 06 UTC) via Northern Italy (Sat 18 UTC) to Serbia (Sun 06 UTC), and
(3) From Portugal (Sat 18 UTC) to the Spanish East coast (Sun 06 UTC).
The majority of thunderstorms will occur in a weakly sheared environment, which - along with the limited CAPE supply - will make severe weather unlikely. However, vertical wind shear generally increases from North to South until it finally reaches up to 25 m/s underneath the jet axis, where lift support is weaker and the air mass probably more strongly capped than at its cyclonic flank. Hence it seems questionable if convective initiation can be achieved far enough South to benefit from the better kinematic conditions. If indeed a few storms manage to form near the Southern fringes of the various highlighted areas (or by accident even beyond them), severe wind gusts are not ruled out.

... Northern Portugal, Northern Spain, Bay of Biscay ...

Maritime convection with isolated to scattered thunderstorms is expected in an environment of weak shear and low instability. Two foci become apparent near the center of the Atlantic low and in Northern Portugal, where a marginal rain risk exists with repeated rounds of thunderstorms under sustained onshore flow.

... France ...

The air mass in the range of the zonal surface trough is rather mild and moderately moist. Patchy insolation and "moisture piling" with convergent low-level winds may be sufficient to create first springtime thundery showers in the afternoon hours.