Storm Forecast
Valid: Fri 22 Nov 2013 06:00 to Sat 23 Nov 2013 06:00 UTC
Issued: Thu 21 Nov 2013 23:40
Forecaster: GATZEN

A level 1 was issued for the Tyrrhenain Sea, southern Italy, and Adriatic Sea region mainly for excessive precipitation.

A level 1 was issued for south-western Turkey and the south Aegean Sea region mainly for excessive precipitation.

SYNOPSIS

The intense cut-off low across France digs southward into the west Mediterranean Sea. Its main vort-max will rotate around the troughs base and affects the central Mediterranean as an negatively-tilted short-wave trough at the end of the period. Before that happens, warm air advection and rising geopotential is forecast ahead of the trough from Italy to the Ionian and Aegean Sea region. Given that low-level moisture is weak across the west and central Mediterranean, the thunderstorm potential is limited. Better moisture can be found across the Ionian and Aegean Sea region.

DISCUSSION

Tyrrhenian into Adriatic Sea region, southern Italy

Dry and cold air masses continue to flood the west Mediterranean Sea with Mistral winds across southern France. The south-eastern flank of the strong cold air advection will become parallel to the low- to mid-level flow, allowing for a quasi-stationary cold-frontal area from the Sea north of Tunisia to southern Italy and the Adriatic Sea. The westerly to south-westerly low-level flow across the warm Mediterranean Sea allows for some moisture recovery on Friday.

Lapse rates will be moist adiabatic along the cold front with steeper lapse rates (near dry adiabatic) at low-levels given the warm sea surface. Latest soundings indicate the weak moisture does allow only for weak CAPE at most places that is limited to low levels. However, with the approach of the main vort-max, steeper mid-level lapse rates are forecast and CAPE will become more substantial.

Current thinking is that widespread precipitation will go on along the frontal boundary, affecting parts of the Tyrrhenian Sea, southern Italy and the Adriatic Sea for most of the period. Embedded storms are possible that will enhance the threat of excessive precipitation. Late in the period, increasing lapse rates will allow for increasing convective activity spreading from west to east. Slightly increasing vertical wind shear will support parallel stratiform convection with a further enhanced possibility of excessive rain. Therefore, a level 1 threat is introduced for most of the affected area. The potential of severe convection will decrease from the west with the passage of the cold front during the evening and night hours.

South Aegean to southern Turkey

Ahead of the intense low across the west Mediterranean, a mid-level trough moves north-east across Turkey. It will be associated with DCVA spreading north. At lower levels, models indicate a low-level jet pointing towards south-west Turkey in the morning hours. Rich low-level moisture and steep lapse rates will overlap over a broad area initially that allows for widespread thunderstorms going on in the first forecast hours.

This convection will be able to organize given strong vertical wind shear (about 15 m/s in the lowest 3 km). Multicells are forecast to be the dominant mode over the Sea but the stronger low-level shear near the coasts may also support embedded mesocyclones. Large hail and a tornado are not ruled out with these mesocyclones, but overall threat seems to be low except for excessive precipitation. With the passage of the trough, convection will weaken during the day. Decreasing lapse rates and weakening of the low-level jet will also lead to weaker potential of organized and severe convection.