Storm Forecast
Valid: Fri 01 Mar 2013 06:00 to Sat 02 Mar 2013 06:00 UTC
Issued: Thu 28 Feb 2013 22:08
Forecaster: TUSCHY

A level 1 was issued for the W-Balearic Islands and E/NE Spain mainly for a few waterspouts and locally heavy rain.

A level 1 was issued for parts of Sardinia, Sicily and N-Tunisia mainly for heavy to excessive rainfall amounts. For N-Tunisia and Sicily, there also exists an isolated tornado/large hail risk.

SYNOPSIS

High over low pattern over W-Europe breaks apart as the upper low litterally splits into two pieces - one moving straight west along the southern fringe of the UK-high and the other one moving east under the influence of an amplifying trough over NE Europe. This eastward shifting part will offer the main show for some thunderstorm activity atop the W/S-C Mediterranean.
Further north, cold and dry air prohibits any electrified DMC with one exception being a rapidly deepening depression just west of C-Norway. Some short-lived thunderstorm activity is possible during the overnight hours with strong wind gusts the main risk. However, the overall confidence in more than sporadic thunderstorm activity kept us from issuing any lightning areas for now.

DISCUSSION

... W/S-C Mediterranean ...

The synoptic story is told with a few words: A structuring surface low near the Balearic Islands translates east during the forecast with a moderate strengthening phase expected during the overnight hours, as it approaches the S-Tyrrhenian Sea. By then this depression will have two centers during the overnight hours - a decaying one SE of the Balearic Islands and a strengthening one entering the Tyrrhenian Sea with favorable mid/upper dynamics. With that said, three areas of interest evolve:

a) Latest IR images already show a consolidating deformation zone which extends from C to NE Spain and towards the NW Mediterranean (28th, 20Z). This zone shifts east and it will affect a region from NE Spain to the Balearic Islands and north during the daytime hours. Although thermal conditions within that wrap-around occlusion become somewhat more stable betimes due to slight LL WAA, the interest shifts to the southern fringe of that occlusion, where conditions seem supportive of a few waterspouts, given enhanced LL CAPE/LL vorticity and weak shear (compared to 30 m/s at 850 hPa further north in the occlusion). The southern part of the level 1 marks the waterspout area (E-Spain to the Balearic Islands). Another risk will be heavy rain due to slow moving / repeatedly onshore moving storms. That's why the level 1 was extended far north and also covers NE Spain. The overall risk subsides after sunset, as the depression weakens/shifts east.

b) A few thunderstorms evolve south/southeast of the Balearic Islands all the way to N-Algeria. This will be weakly organized convection with marginal CAPE/shear support. An isolated waterspout event is possible along the coastal areas of N-Algeria but the overall risk will be too marginal for a level area. The risk vanishes during the night from west to east.

c) Probably the most active thunderstorm development will be seen along a leisurely eastward shifting cold front, which affects Sicily during the end of the forecast period. In fact there are some signals that the area Sardinia-Tunisia-Sicily-Sardinia may see a SE-ward back-building cluster of thunderstorms with an excessive rainfall risk.
Around midnight, the exit region of a 40 m/s mid-level streak, strong PVA and upper divergence overlap with a slowly eastward moving surface cold front entering the scene from the west. Increasing prefrontal moisture convergence with strengthening inflow from the SE should help to increase thunderstorm activity along the cold front, as it passes south of Sardinia during the afternoon hours onwards. During the night, constant re-development along the southern part of the NW-SE aligned MCS occurs and also affects Sicily during the end of the forecast period. Enhanced LL speed and directional shear overlap with some MLCAPE over Sicily and therefore an isolated tornado and large hail event is possible....although the rainfall risk will be of most concern.

A broad level 1 was issued to highlight the extent of the rainfall risk although parts of this level 1 (especially the southeastern one) may see an upgrade later-on (where also the tornado risk will be maximized). The northern part of the level area was extended beyond the 50-% thunderstorm area as decaying convection likely won't be so active lightning-wise but still be capable of producing heavy rain. Despite some models showing rain sums in excess of 100 l/24h over SE Sardinia, a high-end level 1 seems enough for now (given the more stratiform nature of the precipiation). Nevertheless there could be a pretty significant flash flood risk evolving over SE Sardinia during the night and an upgrade may become necessary later during the day.