Storm Forecast
Valid: Fri 01 Dec 2017 06:00 to Sat 02 Dec 2017 06:00 UTC
Issued: Thu 30 Nov 2017 16:12
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for W Greece and S Albania mainly for excessive convective precipitation and to a lesser degree for severe convective wind gusts.

A level 1 is issued for the Ionian Sea for severe convective wind gusts, tornadoes and large hail.

SYNOPSIS

A deep mid-level trough affects most of Europe and cuts off into a separate mid-level low, which moves from France into the Western Mediterranean region. Dry and cold polar air brings a spell of early winter over most of the continent.
Near the surface, a strong cyclone is placed over the Baltic States at Fri 06 UTC and will move into Finland while filling up. Mild subtropic air is advected far northward ahead of it, followed by a long and distinct cold front from the central Ukraine across the southern Balkans to the Ionian and Southern Mediterranean Sea, which makes slow SE-ward progress.
Another surface cyclone will form in the second half of the forecast period over the western Mediterranean region.

DISCUSSION

... Greece, Ionian Sea ...

Strong SW-erly mid-level flow advects an elevated mixed layer (EML) from Libya towards the Ionian Sea and Greece. Its fringes were sampled by the Thu 12 UTC Brindisi sounding, which is thought to characterize the convective environment pretty well. With a few hundred J/kg of CAPE and strong lift under ~20 m/s of 0-3 km bulk shear, it is conducive to organized storms.
The most likely scenario for Friday is a persistent MCS which slowly works its way SE-ward along the Greek west coast, where all sources of lift (QG, frontal and orographic) coincide best. Strong onshore and upslope flow raises the risk of backbuilding or a periodic regeneration of the MCS, and fine-meshed models predict precipitation peaks far in excess of 100 mm over NW Greece. Another round of possibly life-threatening flash floods is expected!
Over the open sea, the EML places a stronger cap on top of the moist near-surface air. Despite ongoing QG lift, convective initiation may await the arrival of the cold front. The farther south, the more likely storms along the cold front will stay discrete and may turn supercellular under the influence of enhanced vertical wind shear and helicity across the lowest 3 km. Severe wind gusts, large hail and a few tornadoes are possible.

The strengthening S-erly flow over the Aegean Sea will also bring plentiful orographic precipitation to NE Greece. However, first embedded thunderstorms are not expected before nightfall, when increasing lift and instability enter the area from the SW. These storms will likely stay elevated until the end of the forecast period and do not pose a particular severe weather risk.

... W Mediterranean region ...

The polar airmass creates very steep lapse rates on top of the mild sea surface. Despite only a few hundred J/kg of CAPE due to limited low-level moisture, thunderstorms will become scattered to widespread in the course of the forecast period due to lift ahead of a sharp vorticity lobe and the absence of any capping inversion. The highest activity is expected around Sardegna and Corsica in the afternoon to evening, followed by a second round further west overnight.
Isolated severe wind gusts are not ruled out due to the rather strong background wind field. Otherwise, severe weather is unlikely due to weak vertical wind shear and limited CAPE.

More widespread severe wind gusts are possible at the northern and western flank of the deepening low, but deep convection is not expected there due to offshore flow of cold and dry air.