Storm Forecast
Valid: Mon 01 Apr 2013 06:00 to Tue 02 Apr 2013 06:00 UTC
Issued: Sun 31 Mar 2013 22:34
Forecaster: TUSCHY

A level 1 was issued for S-Portugal mainly for an isolated large hail event.

A level 1 was issued for NE-Algeria and N-Tunisia mainly for a few large hail and severe wind gust events.

A level 1 was issued for W-Turkey mainly for a few large hail events.

A level 1 was issued for the far NW-Mediterranean mainly for heavy rain and an isolated tornado/marine waterspout event.

SYNOPSIS

The northern hemisphere polar vortex remains disturbed with AO/NAO reflecting that on an impressive manner. Despite the hints of those indices for some longe-range improvement, this surely won't have any influence on the current outlook.

The polar front jet remains displaced far south, now placed from S-Portugal and Spain to N-Africa with a sharp bend to the N over E-Europe. Once again, numerous disturbances in this flow affect the Mediterranean and any thunderstorm activity will be confined to the Mediterranean and adjacent areas. There are no DMC prospects north of the frontal zone with ongoing influx of unseasonably dry and cold air.

The main show for electrified convection will be fostered by a eastward moving mid-level impulse, which amplifies and strengthens somewhat during its eastward motion. It exists NE Spain during the start of the forecast and approaches N/C Italy during the morning hours of the following day.

DISCUSSION

... W-Mediterranean towards Italy ...

The aforementioned mid-level impulse enters the NW Mediterranean during the start of the forecast with a rapid shift to the east. With the mid-level vortex gradually strengthening and expanding downwards atop a sharpening E-W aligned baroclinic zone over the NW Mediterranean, a broad LL vortex evolves with only modest deepening forecast until the end of the forecast. With a vertically tilted vortex from the SW to the NE, rich moisture from the south becomes advected beneath the base of the flat upper wave/evolving mid-level vortex which boosts MLCAPE to 400-800 J/kg over the far NW Mediterranean and even higher but capped values to the south of the Balearic Islands. With the NW-ward advected moisture and gradually structuring surface fronts, a broad area with strong LL convergence signals is expected over the NW Mediterranean, where probably the most active thunderstorm activity of the W-Mediterranean is forecast.

Regarding the thunderstorm mode, we expect numerous clusters of showers and thunderstorms between NE Spain and the Balearic Island during the daytime hours with some of those probaly revealing back-building tendencies towards the SE into the inflow of moist/warm air. No pronounced LLJ points towards those clusters and the depression becomes progressive betimes, so only a temporal training activity is forecast, which is still enough for heavy rain. Dependent on how much moisture advection towards NE Spain will verify, a short period between 12-15Z might offer enough LL CAPE support for an isolated tornado or marine waterspout event, especially if the pronounced convergence zone of HIRLAM over E/NE Catalonia verifies. The risk gradually decays after sunset as the depression moves east.
Then, the sharpening cold and warm front both become the foci for isolated DMC. The cold front moves into the well capped air mass south of the Balearic Islands but will be able to spark an isolated storm or two. 10-15 m/s DLS, somewhat enhanced SRH-3 and MLCAPE in excess of 500-800 J/kg result in strong pulsating storms with isolated large hail and strong wind gusts. Regarding spout potential, there are no real signals present, given warm low levels (weak LL CAPE) and also weak LL convergent signals next to 15-20 m/s NW-erly flow in the boundary layer. The coverage seems to be too sparse for a level 1.
The warm front affects N-C Italy during the night with a few storms, but this activity likely remains sub-severe given weak shear and only modest CAPE. Corsica may see some heavy rain with slow moving showers/isolated thunderstorms as the center of the depression moves atop.

... NE Algeria, Tunisia and east ...

Another area with enhanced thunderstorm probabilities evolves from NE Algeria, N-Tunisia to the east with the development of another weak surface low east of Tunisia and the incoming cold front from the aforementioned disturbance from the W.

CI likely awaits the appraoching cold front from the NW next to the grazing mid-level impulse to the north, which will occur at 12Z onwards. Overalp of steep mid-level lapse rates and improving BL moisture along the coast/just onshore assist in a concentrated area with 500-800 J/kg MLCAPE along the coast. DLS increases to 20-25 m/s, mainly unidirectional in nature. Numerous large clusters of showers and thunderstorm evolve over NE Algeria and N-Tunisia with a rapid translation to the east. Given steep lapse rates and strong shear, a few large hail and severe wind gust events are forecast with that activity. With no significant cooling at mid-levels and NE-ward advection of a dry African air mass towards NE Tunisia expected, storm clusters likely lose organization/strength betimes when moving offshore, likely becoming elevated betimes (some MUCAPE still present). With 30 m/s DLS present, I do not want to rule out a few marginal hail events over Sicily and Malta in case adequate MUCAPE will be present, but current model data does not support that idea.

... Portugal and Spain ...

In the wake of the aforementioned impulse over the NW Mediterranean, a more zonal flow pattern becomes established over the Iberian Peninsula. Forcing will be weak, but the combination of a well mixed maritime onshore flow beneath cooling mid-levels results in a weakly capped air mass, which features 400-800 J/kg MLCAPE mainly over Portugal and decreasing values to the east. Scattered and pulsating thunderstorms are expected. DLS of 15-20 m/s over Portugal may support a few stronger storm events with isolated large hail and strong wind gusts. A marginal level 1 was issued where the CAPE/shear overalp will be the best. After sunset, rapidly decaying CAPE lowers the thunderstorm risk significantly with still some ongoing activity along the W/S coast of Portugal.

... C-Ukraine ...

A cold front over the W-Ukraine moves east during the daytime hours, although its forward motion becomes slower betimes due to the increasing along-front flow component of the background flow. Nevertheless, prefrontal moisture advection places mixing ratios up to 7 g/kg over C-Ukraine. Favorable timing of the front during strongest daytime heating, coupled with the passage of a weakening mid-level vorticity maximum should result in a few thunderstorms to devlop along the cold front. DLS in excess of 20 m/s would point to organized convection, but limited CAPE and updraft depth probably keep the overall severe risk on the lower end. Nevertheless, a few strong pulsating storms are forecast, capable of producing isolated large hail and strong wind gusts. The activity peaks already during the early afternoon hours when CAPE diminishes. No level was issued due to the low coverage of storms and expected short period of storms to become strong during their lifetimes.

... W-Turkey and E-Bulgaria ...

A NE-ward moving trough at mid-levels and a pronounced surface trough steer a moist air mass from the Aegean Sea to the NE. Coupled with 7 K/km mid-level lapse rates, adequate MLCAPE for strong updrafts is expected...500-800 J/kg over W-Turkey with decreasing values to the E/NE. A leisurely eastward moving cold front will spark scattered thunderstorms from W-Turkey to E-Blugaria. Time my be too limited to draw enough moisture as far north as E-Bulgaria for realizing up to 400 J/kg MLCAPE, but 15-20 m/s DLS may still support a few strong thunderstorm until noon with marginal hail and gusty winds. The level 1 however was kept over W-Turkey, where strongest CAPE signals exist. A few severe hail events are likely with this set-up especially until 15 Z, before lapse rates start to weaken from S to N....the same with CAPE fields. Thunderstorms likely cluster betimes and move offshore atop the SW Black Sea before weakening due to decreasing CAPE around sunset. Storm development also stops from W to E over W-Turkey as cold front moves inland and prefronal CAPE diminishes after sunset.