Storm Forecast
Valid: Thu 10 Sep 2015 06:00 to Fri 11 Sep 2015 06:00 UTC
Issued: Wed 09 Sep 2015 22:43
Forecaster: GATZEN

A level 2 was issued for the Ionian Sea and western Greece mainly for excessive precipitation and to a lesser extent large or very large hail, severe wind gusts, and tornadoes.

A level 1 was issued for Greece, the southern Balkans, Aegean Sea, and northern Turkey mainly for large hail and to a lesser extent excessive rain, severe wind gusts, and tornadoes.

A level 1 was issued for eastern Spain and the west Mediterranean mainly for large hail and to a lesser extent severe wind gusts, excessive rain, and tornadoes.

A level 1 was issued for the central Alpine region mainly for large hail.

SYNOPSIS

High-over-low blocking across northern and central Europe affects the forecast period. Ahead of the closed low centred north of the Alps, a south-westerly jet streak spreads into Greece, the Aegean, and Black Sea area. Another vort-max rotating around the low will move over the Pyrenees into the west Mediterranean. At lower levels, rich moisture is left from the Bay of Biscay into the southern Mediterranean as well as from the Ionian Sea into the Aegean and Black Sea. Moisture can partly overlap with steep lapse rates expected across eastern Spain and from the Ionian Sea to the Aegean and Turkey.

DISCUSSION

Ionian Sea into Greece, Aegean, and the Black Sea region

Severe thunderstorms have formed to the west of Greece and will continue on Thursday ahead of a second vort-max that follows from southern Italy. Rich low-level moisture has developed ahead of a cold front moving into Greece on Thursday. An elevated mixed layer originating from Africa will overlap with this moisture and CAPE in the order of 3 KJ is expected. Across Greece and Turkey, strong capping inversion is present that will erode from the west as the cold front approaches.

Current thinking is that widespread storms will form across the Ionian Sea ahead and along the cold front in the morning hours. As they spread onshore, weakening is expected across Greece and the southern Balkans. Later in the day, some storms may become stronger across the west Aegean as well given the weaker CIN due to frontogenetical lift. Additional storms may form along the northern coasts of Turkey given diurnal heating and upslope flow of moist air.

Storms will organize into MCSs given strong forcing and vertical wind shear. Additionally, supercells are not ruled out. Large or very large hail is forecast. Given strong low-level vertical wind shear, supercells are forecast to produce tornadoes as well, although the rather stable boundary layer across Greece and further east can limit the potential. Severe wind gusts may occur. Main threat will be excessive precipitation along the western coasts of Greece, though. Severe threat spreads east during the day.

Eastern Spain to west Mediterranean

Another jet streak and associated vort-max travel across Spain into the west Mediterranean. During the day, upslope flow of Mediterranean moisture is expected across eastern Spain together with steep lapse rates spreading east from the Iberian Plateau. CAPE of 500 J/kg is expected across the west Mediterranean, together with strong deep layer vertical wind shear around 15 m/s in the lowest 6 km.

Storms are forecast to develop across eastern Spain during the day and spread into the west Mediterranean. Main threat will be supercells capable of producing large hail. Severe wind gusts, tornadoes, and excessive rain are not ruled out. Storms will gradually decrease given weak forcing across the west Mediterranean and increasing CIN to the south.

Alps

South-westerly flow at the flank of the closed low will allow for some warm air advection over the Alps, together with advection of steep lapse rates. Low-level moisture is limited but a few storms may develop in the afternoon. These can become well-organized given strong deep layer vertical wind shear, and large hail may be possible.