Storm Forecast
Valid: Fri 08 Mar 2013 06:00 to Sat 09 Mar 2013 06:00 UTC
Issued: Thu 07 Mar 2013 20:39
Forecaster: TUSCHY

A level 1 was issued for extreme NW Spain and parts of Portugal mainly for isolated large hail and an isolated waterspout (offshore) or funnel/tornado event.

SYNOPSIS

An extensive low pressure area sits over the E-Atlantic with no real net-motion to the east expected. A phasing of the jets occurs along the southern fringe of that depression with the resulting deep/broad jet spreading east and affecting Portugal, Spain and NW Africa. Further north, weak height gradients and a cool and dry air mass keep thunderstorm probabilities limited. Beside that, an active branch of the disturbed polar vortex approaches Norway/Sweden with bitterly/unseasonably cold air especially at 500 hPa, where temperatures drop well below -45°C. Attending arctic surface high however limits deeper convection despite steep mid-level lapse rates as surface moisture remains scarce to say at least.

Therefore most robust thunderstorm activity remains confined to the W/C Mediterranean with sporadic activity as far N as NW France.

DISCUSSION

... Portugal, Spain and the W/C-Mediterranean ...

Embedded in this brisk W-erly flow, numerous more or less active impulses race east and support temporarily enhanced thunderstorm periods over a broad area with the most widespread activity probably confined to the westward facing shorelines of Portugal, Italy and Greece/SW Albania.

The activity over the W-C Mediterranean remains mostly sub-severe due to limited CAPE/shear overlap and low magnitude of both parameters. However, an isolated waterspout event is possible mainly within the 50-% thunder areas.

Along the W-coast of Portugal however, DLS remains at or above 20 m/s throughout most the forecast period, before decreasing during the end of the day. An isolated large hail event will be the main hazard although a few stronger gusts could also accompany deeper updrafts, whose downdrafts might bring some stronger winds down to the surface. Once again, with an onshore advected moist maritime air mass and cooling mid-levels atop, some more robust LL CAPE is forecast over C/N-Portugal which could assist in an isolated funnel/tornado event also well inland. A marginal level 1 was issued to cover that mixed risk.

... France, Italy into the Balkans ...

As stated above, models like GFS hint at some spotty electrified convection due to the ongoing weak WAA regime and slightly cooling mid-levels. Despite ongoing BL moisture bias in the American model, we saw worse performance of that layer in the past transitional periods and the overall environmental set-up could squeeze out a few short-lived thunderstorms. However, we're a bit concerned that the cool-down in the mid-layers may occur a bit too late and beyond the maximized heating period (which in fact is also limited by cloudy skies) but also about the not predictable influence of mesoscale phenomena (e.g. locally enhanced BL moisture) and hence no broad lightning area was issued.
An exception will be the N-Balkans, where moisture from the Adriatic Sea might help to spark a few short-lived storms. Any thunderstorm will decay rapidly, especially after sunset.