Storm Forecast
Valid: Mon 25 Mar 2013 06:00 to Tue 26 Mar 2013 06:00 UTC
Issued: Sun 24 Mar 2013 21:07
Forecaster: TUSCHY

A level 1 was issued for parts of S-C Italy mainly for an isolated tornado/waterspout event.

A level 1 was issued for the S-Adriatic Sea, including Montenegro and parts of Albania mainly for heavy rainfall amounts. Along the coasts, an isolated tornado event is possible.

A level 1 was issued for most of the Aegean Sea mainly for an isolated tornado and large hail event.

SYNOPSIS

20Z data (24th) paints a major impulse atop the W-C Mediterranean with some slow eastward motion noticed. Models do a good job regarding strength and position of that feature. With no significant spread in thickness fields and jet positions of ensemble forecasts seen, confidence is high for that stable wave to traverse east throughout the forecast. Somewhat stronger winds at jet level enter the base and force that trough into a more negatively amplified position during the forecast period with ongoing southward digging expected. This should keep the strongest vorticity lobe along the SE/E fringe of that trough with adequate lift for widespread CI.

Further west, a transient zonal-like pattern evolves over Spain/S-France, which also marks the inactive frontal zone, still separating cold and dry continental air to its north from more humid/warmer air to its south. With increasing relative vorticity along its cyclonically sheared northern fringe, support for a few storms may exist over SW France.
A significant impulse, embedded in the subtropical jet, approaches the Iberian Peninsula during the end of the forecast. Its main influence will be to place Spain and Portugal beneath weak ridging with strengthening NVA. An influx of increasingly moist air from the subtropics causes widespread shower activity, but DMC seems unlikely.

Persistent arctic high north of the frontal zone keeps most places stable convective-wise, including NW/N-C and NE Europe.


DISCUSSION

... SW France ...

Some moisture convergence occurs north of the Pyrenees with a temporarily strengthening frontal zone atop. The air mass will be weakly capped beyond 12Z , so a few thunderstorms probably evolve, especially as the upwards pointing part of the direct thermal circulation of the 300 hPa jet's entrance region approaches from the west. Given marginal BL moisture/dewpoints and faint warming at mid-levels as the ridge over Spain grazes the area to the south, only modest CAPE build-up of 200-500 J/kg is forecast. Shear at low and mid-levels remains weak, but stronger flow at jet level may result in a few stronger pulsating storms with marginal hail and strong wind gusts. CAPE rapidly vanishes around sunset as BL stabilizes and so does the thunderstorm activity.

... C Mediterranean and SE Europe ...

A messy set-up is forecast for those areas. With a 995 hPa depression gradually filling during its eastward drift, the main focus for CI becomes the surface occlusion, which crosses the Adriatic Sea around noon from west to east. Prefrontal 25-30 m/s LLJ assures robust moisture advection towards the C-Adriatic Sea with most widespread thunderstorm development forecast from S-C Italy towards S-Bosnia and Herzegovina, Montenegro and Albania...in fact all areas, which see a clash of the eastbound occlusion with the northward pointing moisture tongue. Areas to the east, e.g. N-Greece and Macedonia, may see a downswing of storm activity as inflow trajectories have to cross Greece from S to N, which limits the moisture content of the air mass. Nevertheless, increasing moisture influx from the Aegean Sea also keeps BL moisture at an adequate level for scattered CI for those areas. During the overnight hours, the thunderstorm activity diminishes in aforementioned areas, but remains high along the W-coast of Greece (incoming occlusion/cold front) and atop the Aegean Sea.

Shear/CAPE fields remain displaced for most of the times, so despite active pulsating storms, which may grow upscale into disorganized clusters of showers/thunderstorms, no severe risk is anticipated. One exception will be the S-Adriatic Sea (including S-Bosnia and Herzegovina, Montenegro and NW-Albania), where transient back-building of prefrontal thunderstorm activity is possible around noon and during the early afternoon hours. Heavy rain will be the main risk, although tail-end storms along the coastal areas may also pose a tornado risk, with increasing LL shear. I kept this event in a level 1 due to the transient nature of the moisture tongue and the progressive boundary.

All coastal and offshore areas over the central Mediterranean see a low-end waterspout risk with the cold-core trough atop warm SSTs and with weak background shear in place. S-C-Italy may be the primary focus for that risk with ongoing onshore flow due to a faint tilt of the trough axis with height. Enhanced LLCAPE may be realized and a level 1 was added for that risk....even well onshore.

During the overnight hours, an overlap of 15-20 m/s DLS with up to 800 J/kg SBCAPE atop the Aegean Sea assists in a few better organized multicell storms, which may produce an isolated waterspout/tornado event and large hail, as mid-level lapse rates become steeper. Strongest low-level southerly winds remain well ahead of the incoming occlusion, so we'll keep the heavy rainfall risk on the lower end side for now.