Storm Forecast
Valid: Fri 18 Dec 2009 06:00 to Sat 19 Dec 2009 06:00 UTC
Issued: Thu 17 Dec 2009 11:14
Forecaster: TUSCHY

A level 2 was issued for W-Turkey mainly for tornadoes, large hail, severe wind gusts and excessive rainfall.

A level 1 was issued for parts of the W-Black Sea mainly for isolated, severe wind gusts.

A level 1 was issued for the Strait of Gibraltar and NW-Morocco mainly for excessive rainfall.

A level 1 was issued for SW-Greece mainly for excessive rainfall and to a lesser extent for tornadoes.

A level 1 was issued for a small part of the NE Adriatic Sea mainly for excessive rainfall.

A level 1 was issued for a small part of W-CNTRL Italy mainly for an isolated tornado risk.

SYNOPSIS

An active branch of the disturbed polar vortex has established over most parts of Europe. A strong arctic front along its southern edge pushes southwards and affects the Mediterranean during the forecast. North of this front, the main convective focus is bound to isolated, electrified convection over the North Sea and the English Channel. However for onshore regions, very scarce moisture content (locally less than 1-2 g/kg 500m mean-layer mixed) precludes deep convection. There will be one exception, as thunderstorm probabilities over the north-central Mediterranean and central Italy increase again during the night, as another disturbance approaches from the west. For the rest, thunderstorms occur along and south of the arctic front.

DISCUSSION

... W-Turkey ...

Far southward dropping arctic front is the focus for numerous depressions, one affecting W-Turkey during the forecast. Regarding dynamics, favorable placement beneath intense mid-upper jet favors rapid strengthening of this depression, although global models are all over the place in respect of how deep the surface depression will be. We stick with the GFS/ECMWF and NOGAPS solution, showing an extensive depression, entering the Black Sea around noon. Bound to the eastward moving trough axis, the surface depression outruns favorable wind field further south while entering a highly diffluent upper streamline pattern. Hence, weakening over the Black Sea betimes is a likely scenario.

Broad warm sector evolves over W-Turkey, however, quality of the air mass in this sector is marginal at best due to limited advection over the rough terrain, flooded with cold air. Highest moisture content is likely seen along the eastward racing cold front due to moisture pooling (affecting the W-coast of Turkey between 12-18 UTC from NW to SE) and next to the occlusion point over the W-Black Sea.

Instability of well above 500J/kg MLCAPE is probable along the W/SW-coast of Turkey, diminishing betimes as cold front pushes higher theta-e air mass southeastwards. Instability also decreases rapidly more inland, as ascent over the cold airmass/rough topography keeps going. Mainly MUCAPE of a few 100 J/kg are forecast well inland with 2 exceptions: First the NW-part of Turkey, where strong dynamics and attendant wind field erodes the cold air mass with even SBCAPE possible around noon/afternoon. The other focus is the cold front itself with unstable surface parcels possible as far east as north-central Turkey (north of the mountains).

Shear is quite intense, especially during the first part of the forecast over W-Turkey with impressive LL shear magnitudes along the coast. Otherwise, the warm sector is moderately sheared with 15-20m/s DLS.

This is a multi-severe risk event with multicells/supercells possible along the W/SW-coast, where large hail, tornadoes, severe wind gusts and excessive rain all raise the probabilities well into the level-2 criterion. Further inland, the severe thunderstorm risk gradually modifies to a regional flash flood risk (mainly along the upslope side of W-Anatolia) although no sign is present for an organized and training MCS event. Hence a level 1 covers the risk well.

The level 2 was expanded well eastwards over NW-Turkey due to the overlap of some instability and strong shear. Tornadoes and severe wind gusts will be the main risk. An isolated severe wind gust event is also possible over the Black Sea, as cold front pushes northeastwards.

... SW-Greece ...

The level 1 was issued until noon, as ongoing storms from yesterday start to exit the area. Conditions for regional strong rainfall and isolated tornadoes are still adequate for a level 1until noon.

... Central Italy and N-Adriatic Sea ...

Gradually strengthening depression enters the W-Mediterranean during the late afternoon hours and the central Mediterranean thereafter. Surface pressure drop is quite significant due to favorable dynamics. This feature, well north of the arctic boundary, draws some moisture northwards. Despite the limited moisture advection, very cold mid-/upper levels cause some weak instability over central Italy with moderate values up to 300-500 J/kg SBCAPE offshore. As the cold front over Italy moves southwards, a tornado risk arises in the level 1, as the uncapped and moist prefrontal air mass is nicely sheared at lowest 1km. However, thunderstorm activity will be marginal at best.

Higher probabilities are needed over the NE-Adriatic Sea during the morning hours, as a small cluster of storms probably evolves beneath the left exit of a 45m/s mid-level streak. Isolated severe wind gusts and locally excessive rainfall increased probabilities to a level 1.

... Strait of Gibraltar, S-Portugal, extreme SW-Spain and N-Morocco ...

The ingredients seem to come together for a major freezing rain/snow event over south/central Spain! In respect of convection, elevated thunderstorms can't be ruled out well inland of S-Spain, which would locally increase the heavy (freezing) rain and snow risk significantly.

During the daytime hours, the active arctic front pushes southwards, reaching S Portugal/Spain during the late night hours. At the same time, a vigorous depression off the Azores keeps moving eastwards while filling. Due to the low latitude of this depression, the warm sector is filled with true tropical air mass and persistent influx of this very moist air continues during the forecast. Isolated to scattered thunderstorms are possible in the warm sector mainly west of the Strait of Gibraltar during the daytime hours due to the weakly capped airmass and very moist BL, but overall thunderstorm coverage is on the increase during the night, as the arctic front at the surface approaches the strait from the north. Shear is not that strong and multicells are the primary storm mode. Clustering of those storms is likely with some upper divergence overspreading the area from the west with an excessive rain risk. An isolated tornado/waterspout risk can't be excluded mainly west of the Strait of Gibraltar, as LL CAPE increases. Despite the signals of high rain amounts, the coverage of storms in this (sub) tropical airmass could be not as widespread as expected by models due to the clustering of storms and the evolution of a more stratiform rain field with embedded, convective elements. We therefore went with a high-end level 1 for excessive rain.

A low probability thunderstorm area was issued well inland, as some MUCAPE atop of the surface cold front may yield an adequate environment for isolated thunderstorms. These storms may pose an increased risk for sleet, marginal hail and freezing rain, as WBZ lowers.