Storm Forecast
Valid: Thu 14 Nov 2013 06:00 to Fri 15 Nov 2013 06:00 UTC
Issued: Thu 14 Nov 2013 03:20
Forecaster: GATZEN

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

SYNOPSIS

The east Mediterranean cut-off low will weaken and moves east during the day. It will be associated with widespread thunderstorms initially. Further north, a zone of rather high geopotential across Europe is crossed by a strong trough over western Europe that forms a closed low over southern France on Friday. It is associated with strong cold air advection.

DISCUSSION

Southern Greece to southern Turkey and surroundings

The Mediterranean cut-off low weakens and moves east on Thursday. It will increasingly affect Turkey during the period. Ahead of this trough, a westerly jet streak will spread east to the south of Crete, providing strong DCVA. Correspondingly, latest GFS indicates steep lapse rates above the boundary-layer. In the boundary-layer, a southerly flow is expected that continues to advect rich moisture with a mixing ratio in excess of 10 g/kg. Weakly capped CAPE could peak near 1000 J/kg across the warm Mediterranean Sea to the south of Greece and Turkey. However, these values are expected to decrease when the trough axis of the accelerating trough approaches. Westerly winds and cold air advection will spread east leading to weakening low-level convergence along the cold front. Additionally, models indicate that low-level moisture will be mixed out by cold air advection during the day.

However, rather widespread thunderstorms are forecast in the morning hours along the broad convergence zone ahead of the cold front. Given strong vertical wind shear, an MCS has formed to the west of Greece and will move east across the southern Aegean Sea. Storms will spread east into Turkey during the day. Severe threat is forecast to be slightly weaker compared to Wednesday given that the flow gradually shifts to the west reducing the chance of upslope flow across southern Turkey. Highest chance for excessive precipitation may be across south-western Turkey. Additionally, vertical wind shear around 15 m/s in the lowest 3 km may support embedded mesocyclones initially, capable of producing large hail or a tornado. Overall threat is too weak for a level 1 threat over most places, though. Later in the day, decreasing CAPE, low-level convergence, and vertical wind shear are forecast to reduce the severe threat further.

Ionian Sea and surroundings

Below the base of the cut-off low, morning showers and thunderstorms may be capable of producing waterspouts, especially along land/sea-breeze convergences. Chance will decrease until noon when the trough moves east.

France, western Mediterranean

An intense trough will rapidly move south across western Europe. It is associated with very strong DCVA especially at the cyclonically sheared flank of a strong mid-level jet streak that will spread from north-western France to southern France during the day. The trough will start to form another closed low centred over southern France at the end of the period.

At lower levels, the approaching trough is associated with strong cold air advection. The cold front will rapidly move across France today. Due to the formation of a north-Mediterranean low pressure system in the wake of the Alps, northerly winds will evolve even to the south of the cold front and low-level convergence will be weak. Additionally, latest observations indicate that the cold front moves into a stable, cold and dry boundary layer that is not supportive for thunderstorms. Best chances are expected near the North Sea, where low-level moisture is better. Further south, some convection may develop above the boundary layer due to steep lapse rates forming near the base of the trough, but thunderstorms are too unlikely to issue a 15-percent line. Nevertheless, the cold front will be associated with strong and gusty winds that may exceed severe limits locally.

Over southern France, strong Mistral winds will develop during the day that will spread across the Mediterranean Sea. As a consequence, moisture will decrease and current convective activity will decease. New storms are forecast in the evening and night hours due to the approach of the mid-level trough. Severe storms are not expected as vertical wind shear will be rather weak near the base of the trough.