Storm Forecast
Valid: Fri 18 Sep 2020 06:00 to Sat 19 Sep 2020 06:00 UTC
Issued: Thu 17 Sep 2020 22:48
Forecaster: PISTOTNIK

A level 3 is issued for a confined area in W Greece for severe convective wind gusts.

A level 1 and level 2 are issued for larger areas of Greece for severe convective wind gusts and excessive convective precipitation.

A level 1 and level 2 are issued for parts of Morocco, Portugal, Spain, S France for excessive convective precipitation, severe convective wind gusts and large hail. In coastal areas, tornadoes are not ruled out.

SYNOPSIS

A blocking anticyclone stretches from the North Atlantic Ocean to Romania. It is flanked to the SW by a cut-off low near Portugal, to the N by a strong zonal flow over Scandinavia, to the NE by another cut-off low in W Russia and to the SE by a suptropical cyclone over the Ionian Sea. Overall, the configuration somewhat resembles an "omega" pattern.
While a surge of polar air advances to the Black Sea in the wake of the eastern cut-off low, very warm air creates still summer-like conditions to the southwest of the axis of highest pressure / geopotential.

DISCUSSION

... Greece and surrounding sea waters ...

The subtropical storm 2020M01 over the Ionian Sea has possibly reached hurricane strength by Thursday evening while it maintains a slow eastward track towards the central west coast of Greece. The Thursday 12 UTC model runs havy largely converged to a track passing between the islands of Zakynthos and Kefalonia shortly before Friday 06 UTC (the beginning of this forecast period). WRF is the only remaining forecast model that still shows a southerly track over the open sea staying slightly southwest of Zakynthos.
Widespread hurricane-force wind gusts (>32 m/s) and extremely intense rainfall are very likely within a few tens of kilometers around the cyclone's center, and this risk extends for at least some hours into the current forecast period. It is not certain whether the innermost rain bands will also be accompanied by thunderstorms - there were only occasional flare-ups of electric activity on Thursday -, but with a few hundred J/kg CAPE in place the chances are high enough to meet our level 3 criteria in a confined area around the expected track of the cyclone's center. In inland areas, the wind risk diminishes betimes, but extraordinary rainfall accumulations, probably exceeding 200 mm in some regions with either prolonged upslope flow or beneath almost stationary and backbuilding rain bands, can easily cause catastrophic and life-threatening flash floods, mudslides and debris flows. Apart from the immediate vicinity of the cyclone center, another hot spot for abundant precipitation with embedded convection is expected to emerge on the northeastern side of the Greek mainland, where very moist and slightly unstable air from the Aegean Sea is persistenly advected against the mountains at the northern periphery of the circulation.
The further fate of the subtropical storm after Fri 12 UTC - either a quick filling over the Gulf of Patras, "disturbed" by the surrounding land mass, or a loop back southward into open sea waters with a slower filling or even partial recovery - will govern how quickly or slowly the wind risk decreases from Friday afternoon onwards, and how large the affected regions will be. The flooding risk is expected to continue into Saturday. For more details and updated information on this subtropical storm, please refer to our experimental 12-hourly Subtropical Cyclone Track and Intensity Forecasts and to the official warnings from the Hellenic National Meteorological Service (HNMS).

... central to S Italy and surrounding sea waters ...

Where not overworked by the subtropical storm, the very warm airmass in the central Mediterranean region features fairly steep lapse rates on top of particularly rich low-level moisture (2m dewpoints often between 20 and 24C). The Thursday 12 UTC soundings from Pratica di Mare, Brindisi and Trapani indicated CAPE between 1500 and 3000 J/kg, rising from north to south.
This CAPE reservoir will largely remain in place on Friday (only gently pushed south by northerly surface winds), but compensating subsidence from the subtropic storm creates a rather strong cap and should mostly suppress convective initiation. At best, only a few short-lived and struggling storms are imaginable over mountains that are high enough to penetrate the cap.

... Iberian Peninsula, France ...

Ahead of the western cut-off low, a fairly strong southerly flow advects steep lapse rates from NW Africa and the Spanish highlands on top of rich coastal moisture. The resulting CAPE will probably reach 1000-2000 J/kg along the Spanish east coast, while some hundred or locally up to 1000 J/kg may also develop further inland, depending on how much moisture can be advected inland and how effective the counteracting mixing processes will be. Increasing mid-level flow drives 0-6 km shear to 15-25 m/s.
The most likely focus for storm initiation are a diffuse cold front and a superimposed mid-level vorticity maximum that cross the eastern half of Spain from SW to NE. Considering a considerable cap, first storms are expected to form either over the mountains or to root down from Altocumulus levels around noon near the western edge of the drawn level 2 area. The CAPE-and-shear overlap allows well-organized storms, including upscale growth into one or several MCSs with a main risk of severe wind gusts at their leading edge(s) and excessive rain. Tail-end storms will most likely move along the east coast or stay just offshore, they can turn supercellular and produce large to very large hail plus one or two tornadoes (considerung low cloud bases and enhanced 0-1 km shear in the sea breeze regime).
Convection will gradually weaken after sunset while it spreads into France. If storms stay still surface-based, some severe weather events are also still possible in the French Pyrenees forelands.
The most impressive mid-level wind maximum overspreads SE Spain in the afternoon to evening and creates 0-6 km shear even up to 30 m/s with strongly veering wind profiles. Convection forming in this environment could produce a few extreme hail and wind events. However, the onset of synoptic subsidence makes convective initiation increasingly unlikely by then. Nonetheless, a monitoring of this area until the evening is recommended.

Behind the cold front, lapse rates are weaker, but moisture from the Atlantic is advected far inland and daytime heating creates a few hundred J/kg CAPE. Under still enhanced vertical wind shear, scattered and rather low-topped, daytime-driven convection is expected. Ths strongest storms may organize into multi- and perhaps a few supercells and produce heavy rain, isolated severe wind gusts and marginally large hail. Again, a tornado is not ruled out in coastal areas, where cloud bases are lowest.