Storm Forecast
Valid: Fri 11 Sep 2009 06:00 to Sat 12 Sep 2009 06:00 UTC
Issued: Thu 10 Sep 2009 20:57
Forecaster: TUSCHY

A level 1 was issued for N-Morocco and N-Algeria mainly for excessive rainfall.

A level 1 was issued for Crete, the Aegean Sea and surrounding areas mainly for excessive rainfall.


SYNOPSIS

Weak geopotential height gradients persist over southern Europe, where widespread showers and thunderstorms are forecast. A strong high pressure over NW-Europe keeps most parts of NW and central Europe stable and dry. The progressive pattern over far northern Europe persists. An intense depression crosses Iceland during the forecast and races eastwards. The attendant cold front could become quite active regarding showers and isolated thunderstorms. Damaging wind gusts accompany the cold front passage, but everything remains out of our forecast area.

DISCUSSION

... Crete, Aegean Sea, W-Turkey and the east coast of Greece ...

Latest SST data of the Mediterranean reveals temperatures of 25-30°C west of Crete, decreasing to roughly 25°C around Crete and even lower values over the central and eastern Aegean Sea but then increase again over the far eastern Mediterranean. Those values are on track with the 20-year SST anomaly and only a marginal positive bias west of Crete is found. However, an impressive 3-4°C positive SST anomaly is present around Italy.

A flat upper trough covers the south-central and eastern Mediterranean and causes unsettled conditions. The warm sea surface beneath cooling low-/mid-levels causes unstable conditions and widespread showers and thunderstorms evolve in a weakly capped and moist airmass. Excessive rainfall is the main risk with those thunderstorms with short 0-6km bulk shear vectors and moist lower/mid troposphere. NOGAPS, GFS and ECMWF are in remarkably agreement, showing a convective precipitation maximum west and over Crete. A cyclonic vortex below 3km moves along a weak baroclinic zone towards the NE, crossing Crete and it decays to the east later-on. Crete could see serious flash flooding, if this scenario verifies.
Slow moving storms are possible also over NW and W-Turkey with a flash flood risk.


... Cyprus and the far eastern Mediterranean ...

Despite the fact that this area is east of our forecast area, a short remark is done. WAA keeps going over that region with very steep lapse rates and a moist BL airmass. Capped conditions persist probably well into the night, but global models still indicate isolated thunderstorms around Cyprus and northeastwards after midnight. Shear and instability all favor supercells with large hail, severe wind gusts and intense rain.

... N-Morocco and N-Algeria ...

Conditions for heavy, convectively enhanced precipitation remain favorable. TRMM shows a broad field of 50-100 mm during the past 7 days over NW-Algeria, most of that measured in a short time. Numerous impulses cross N-Africa from the WSW during the forecast, along the southern fringe of a weak upper trough, which is west-east aligned. The majority of the models support the idea that widespread showers and thunderstorms again evolve with a serious flash flood risk, especially over far NW-Algeria. Thunderstorms keep going well into the night, mainly over N-Algeria.