Storm Forecast
Valid: Mon 19 Nov 2018 08:00 to Tue 20 Nov 2018 06:00 UTC
Issued: Mon 19 Nov 2018 09:01
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts the central Mediterranean region for flash floods (especially in the northern half) and for large hail, tornadoes and severe wind gusts (especially in the southern half).

SYNOPSIS

A strong blocking pattern is in full swing. The blocking high is centered over the Norwegian Sea. At its southern flank, an elongated zone of low 500 hPa geopotential stretches from the Bay of Biscay all the way to the Ukraine and beyond. Its dominant center is placed over France and maintains a slow westbound track.
A broad stream of continental cold air from the northeast into continental Europe and fairly warm, moist air over the Mediterranean Sea enhance a baroclinic zone, along which several surface cyclone spin up, namely over the W Mediterranean Sea, coastal Ukraine and northern Portugal. The former one is of highest interest, as a robust overlap of CAPE and vertical wind shear develops in its warm sector over the central Mediterranean region.

DISCUSSION

... central Mediterranean region ...

The surface cyclone is centered near Sardegna. Its warm front lifts northward across central Italy and the Balkans. In its warm sector, strong southwesterly mid-level flow advects an elevated mixed layer from the Sahara onto the Mediterranean Sea, and plentiful low-level moisture is contracted beneath it. By Monday night, CAPE south and east of Sicily will likely exceed 1000 J/kg under a strongly sheared and helical flow (0-3 km shear and storm-relative helicity around 15-20 m/s and 100-200 m^2/s^2, respectively).
Forecast models do not show convective initiation in the area of the best CAPE-and-shear overlap until the end of the current forecast period. Despite increasing synoptic lift overnight, the capping inversion appears to be too strong. These areas are therefore only included into a level 1 and a low probability lightning area.
Convective initiation becomes more likely further north, where still some hundred J/kg of CAPE are in place and the capping inversion is eroded by stronger synoptic lift. Several storms will probably initiate west of Sardegna, over the Tyrrhenian and (later) Ionian Sea, and they will organize and possibly grow upscale into side- or backbuilding MCSs. Especially near the warm front, where CAPE fades and becomes elevated, and in general along west-facing coasts of southern Italy and (later) the western Balkans, heavy rainfall with flash floods becomes the main risk. Tail-end storms further south will likely include supercells and may bring the whole spectrum of severe weather, including very large hail and a few tornadoes.
Timing and location of the "convective nuclei" over the open sea that could later grow upscale and carry concentrated swaths of severe weather into coastal areas are uncertain, hence a broad level 1 seems to be the best solution. Limited-area models (WRF and MOLOCH) point to an enhanced probability that a powerful MCS will develop over or just east of Calabria in the evening (possibly stimulated by upslope flow) and will move onshore in southern portions of Albania overnight. This corridor is upgraded to a level 2.

... coastal areas of Portugal, Morocco and Algeria ...

While little convective activity is foreseen in the well-mixed, maritime postfrontal air mass, where a few hundred J/kg of CAPE are present. Some heavy showers and gusty winds can be expected, but should mostly stay below or severe weather thresholds.

... central Black Sea ...

The cold front of the Ukrainian cyclone accelerates eastward in the afternoon and evening. Supported by a vorticity maximum aloft, it is not ruled out that a convective line with a risk of strong to marginally severe wind gusts forms. Since only open waters will be affected and the convection may not grow deep enough to produce lightning, no level 1 is issued.