Storm Forecast
Valid: Fri 13 Feb 2009 06:00 to Sat 14 Feb 2009 06:00 UTC
Issued: Thu 12 Feb 2009 17:23
Forecaster: TUSCHY

SYNOPSIS/DISCUSSION

High amplitude upper trough, filled with a cold continental airmass, is centered over Europe and the evolution of deep,organized and long-lived convection onshore can be neglected. However, CAA atop the warm waters of the Mediterranean generate broad areas of some low-end instability. Better LL convergence is confined to the Tyrrhenian Sea mainly during the night hours as coldest air at 500hPa arrives from the north,too. We decided to give the dry/cold continental airmass some fetch offshore to humidify before enough instability can be released for electrified activity. Thunderstorm chances are on the increase mainly over the southern part of the Ionian Sea, where stronger forcing, better LL airmass quality and somewhat enhanced LL convergence overlap. A more active day for widespread thunderstorms will be imminent around Crete and the Aegean Sea as a surface depression consolidates and intensifies during the day. Deep layer shear around 20m/s and low EL temperatures hint on a few deep and organized thunderstorms and hence a marginal level-1 was issued due to a local large hail/strong to severe wind gust event during the daytime hours. However a very compact and intense UVV field enters the area from the SW after sunset and a cluster of showers/thunderstorms evolves somewhere around/north of Crete, moving northeastwards, reaching SW-Turkey around midnight and this cluster then builds eastwards (see area forecast discussion below). Strong wind gusts and torrential rain will be the main risk.

SW-Europe remains under strong ridging with stable conditions.

... SW/S-coast of Turkey ...

Aforementioned surface depression comes ashore over SW - Turkey around midnight and apart from UKMO, the models agree well in intensity (~1000hPa) and track of the feature. Hence confidence in the development of this depression is high enough to increase probabilities.

Strong SW-flow aloft backs to southerly directions below 700hPa with parcel trajectories emanating from Egypt. Aside from warm mid-levels, near surface airmass needs some time for mixing, so we think that there will be a sharp line where thunderstorms manage to evolve and where airmass remains capped. Conditions are supportive for repeated shower/thunderstorm development along the SW/S-coast of Turkey during the daytime hours in an environment with 25-30m/s DLS. Strongest instability field remains out of our area of responsibility, but a few large hail/severe wind gust events are already possible until 15Z.

Thereafter, instability axis builds westwards as LL depression over Crete intensifies and surface flow responses with backing wind fields. CAA at mid-levels continues, so up to 1000 J/kg MLCAPE and even higher ICAPE are likely all the way to Rhodes. Synoptic lift is weaker compared to the Aegean Sea and mid-levels remain a tad warmer, so the possibility for more discrete thunderstorm development is augmented. Moderate mid-level lapse rates but very strong directional shear indicate the chance of large hail. Hence the level-1 was expanded northwards despite the very rough terrain, as plume of elevated instability fans out well inland. The near parallel/steady track of the surface depression to the mid-/upper level flow and the constant influx of moist air from the south should yield a favorable environment for a constantly eastward propagating MCS, most active along the SW/S-coast of Turkey, turning into a more stratiform-like heavy rain threat over central Turkey. Some LL CAPE is present all day long along the coast and just offshore but increases rapidly during the evening and night hours. LCLs remain low and LL shear increases along the coast, so there will be an isolated tornado risk, especially if storms remain discrete for some time.