Storm Forecast
Valid: Mon 11 Nov 2013 06:00 to Tue 12 Nov 2013 06:00 UTC
Issued: Sun 10 Nov 2013 22:52
Forecaster: PISTOTNIK

A level 2 was issued for S Albania and W Greece mainly for excessive precipitation and to a lesser degree for severe wind gusts.

A level 1 was issued for the Ionian and S Adriatic Sea, the rest of Albania, coastal Montenegro, Bosnia and Croatia for excessive precipitation and severe wind gusts.

A level 1 was issued for S Italy and the Tyrrhenian Sea for excessive precipitation and to a lesser degree for waterspouts.

A level 1 was issued for NE Algeria and N Tunisia mainly for severe wind gusts and to a lesser degree for excessive precipitation.

A level 1 was issued for the rest of Greece mainly for excessive precipitation.

SYNOPSIS

The pronounced upper-level low that has cut off over Italy and the Tyrrhenian Sea on Sunday is the most striking feature in the new forecast period, while it slowly wobbles southward. On the other hand, a strong and meandering zonal flow still has the Northern half of Europe under control: the remainder of the first trough proceeds into Northeastern Europe, and a new trough plus the frontal system of a mighty Atlantic cyclone start to affect the British Isles and Scandinavia. In-between, a positively tilted upper-level ridge builds from Portugal towards the Baltic Sea, and a belt of high surface pressure forms from the Bay of Biscay via Central Europe to Belarus.

DISCUSSION

... Central Mediterranean ...

Convection on Monday will mostly be tied to the Italian cyclone and will be twofold:
(1) In the moisture tongue along its Southeastern flank, some CAPE overlaps with enhanced deep-layer shear, though the noisy structures and the strong gradients in both fields make it difficult to assess the final risk for organized storms. The main trigger mechanism is provided by a well-marked wind shift line near the surface and a pronounced tropopause anomaly aloft, both of which move slowly eastward over the Ionian Sea. Scattered to widespread storms in this zone can organize into strong multicells with a backbuilding tendency towards the warm and moist Southerly inflow. Excessive precipitation and severe wind gusts are the dominant risk with this activity. Especially the level 2 area along the coastlines of Southern Albania and Western Greece is very exposed to repeated storms capable of producing flash floods. Further North in the Southern Adriatic region, the risk decreases somewhat due to less moisture access, but still suffices for a level 1.
(2) In closer vicinity to the cyclone, cold mid-levels on top of the warm sea surface continue to create limited CAPE in a weakly sheared environment. Revolving thunderstorm clusters can produce localized excessive precipitation over the Tyrrhenian Sea and the Southern half of Italy throughout the forecast period. Waterspouts are possible in a confined area near the center of the circulation, which slowly moves from the Tyrrhenian Sea towards the strait between Sicily and Tunisia.

Besides, non-convective hazards at the Northwestern flank of the cyclone are also worth mentioning as a side note: The strong Northeasterly flow will result in heavy rainfalls in inland Bosnia and Croatia and especially along the Italian East coast, where they will possibly exceed 100 mm. The North Adriatic Sea (very strong Bora!) as well as Corsica, Sardegna and parts of the Western Mediterranean Sea (Mistral) will experience wind gusts in excess of 25 m/s. Remember that these risks are not covered by the ESTOFEX threat level scheme.

Near the coastline of N Tunisia and NE Albania, the Mistral winds may catch enough moisture again to produce precipitating convection. With limited CAPE and limited shear, these showers will stay unorganized, rather shallow and weakly electrified, but may locally bring severe wind gusts (850 hPa winds around 25 m/s) and heavy precipitation (strong upslope flow).

... Aegean Sea ...

Rich moisture is advected northward and allows the buildup of CAPE up to 1000 J/kg. In the absence of synoptic forcing mechanisms, onshore and upslope flow act as the main trigger. Scattered thunderstorms are expected, the more the closer to the coastline of mainland Greece. A few heavy precipitation events are possible with this activity. Otherwise, severe weather risk does not seem to be particularly enhanced, given the lack of vertical wind shear (10-15 m/s between 0 and 6 km).