Storm Forecast
Valid: Fri 15 Jan 2010 06:00 to Sat 16 Jan 2010 06:00 UTC
Issued: Thu 14 Jan 2010 20:52
Forecaster: TUSCHY

A level 1 was issued for parts of the S-Mediterranean mainly for excessive rainfall and to a lesser extent for severe wind gusts.

SYNOPSIS

Europe is flooded by cool/cold and dry air, so convective chances are negligible for most parts. However, a deep trough affects the south-central Mediterranean and attendant surface depression causes enhanced chances for thunderstorms.

DISCUSSION

... Gulf of Tunis down to the Gulf of Gabes eastwards, covering most parts of the Ionian Sea ...

There is not much in the way for surprises as the forecast is straightforward. Models agree well in dropping a strong trough into the south-central Mediterranean area with LL pressure decrease over the highlighted region being the consequence. However - what a surprise - minor discrepancies exist in the placement of a LL depression, which evolves somewhere over and east of Malta. Due to the modest dynamics and moisture content especially in the BL, overall organized thunderstorm risk is marginal and therefore not much time was spent for that issue. It may be of interest that this depression also batters an unusal way in slowing down its eastward progression south of Sicily, starting a retrograde motion, before dropping southward towards Libya. This motion causes a slow-down of the cold front as it outruns the better dynamics over the SE Ionian Sea.

The main activity will be confined to the cold front and a loosely organized line of storms probably accompanies the front, but weak shear and gradually abating dynamics keep the severe probabilities limited - an isolated severe wind gust is the main risk.

A more serious risk is present along the occlusion/warm front and during the latter part of the forecast also along the nearly stalling cold front over the SE Ionian Sea in terms of excessive rainfall risk. Upper streamline pattern reveals a favorable divergent pattern with a vorticity lobe crossing the surface baroclinic zones from west to east. Due to persistent isentropic lift and moderate to good moisture, a shield of stratiform rain probably evolves along the north side of the surface depression with embedded convection mainly along the south side of the baroclinic zone (warm front / occlusion). A level 1 will cover the risk, but Sicily could get some good rain from the elevated, stratiform northern part of that boundary. After midnight, the focus for heavy precipitation shifts to extreme SW-Greece due to the favorable placement in the exit of the mid/upper divergent jet configuration and the trailing cold front. We have to monitor the position of this boundary and higher probabilities may be needed for SW-Greece. LL lapse rates increase somewhat but the parameters are not that impressive, so only an isolated waterspout event is forecast. Low-end thunderstorm probabilities were expanded eastwards to reflect some uncertainties in respect of how far eastwards the cold front will move.

Finally, another front enters the area of interest from the NW around midnight (fed by some wrap-around moisture from the surface depression), so thunderstorm activity is again on the increase, but shear is too weak for anything organized.

... NE-coast of Algeria until 18 UTC ....

The air mass has enough time to recover over the west-central Mediterranean with favorable north-south trajectories. In combination with better LL moisture, the coastal convergence also increases chances for deep convection. LL CAPE is supportive for strong LL updrafts and a few waterspout events are possible. The risk decreases during the early evening hours.