Storm Forecast
Valid: Thu 28 Oct 2021 06:00 to Fri 29 Oct 2021 06:00 UTC
Issued: Wed 27 Oct 2021 18:47
Forecaster: GATZEN

A level 2 was issued across southern Sicily, southern Ionian Sea, Tyrrhenian Sea, and Sardinia for extreme rainfall.

A level 1 was issued across the Ionian and Tyrrhenian Sea region for extreme rainfall and to a lesser extent tornadoes.

A level 1 was issued across western Potugal and north-western Spain for extreme rainfall and to a lesser extent tornadoes.

DISCUSSION

Low geopotential extends across the Mediterranean, with mid-level vorticity maxima across the Balearic Islands and Turkey. South of Italy, a warm-core subtropical storm has developed that is expected to move northward during the period, likely reaching southern Italy at Friday morning. Central Europe is affected by mid-level ridging, whereas a lifting trough enters western Europe.

At lower levels, rich moisture is present across the Mediterranean and along a frontal boundary that extends from west Iberia towards Wales and England. CAPE is expected to be rather low due to weak lapse rates.

DISCUSSION

Ionian Sea, Tyrrhenian Sea, southern Italy and Sardinia and Corsica

Heavy rain is expected during the period due to ongoing thunderstorm activity in a moist, almost saturated air mass. One focus will be a convergence zone across the Tyrrhenian Sea, with a moist easterly low-level flow to the east that partly spreads upslope across Sardinia and Corsica. Thunderstorm activity is forecast to be centered in the southern parts of the area, where steeper mid-level lapse rates and lower equilibrium level temperature are forecast.

South of Italy, the subtropical storm is expected to move northward. Models indicate that MLCAPE will increase to the east and south of this storm and clusters of thunderstorms are forecast, whereas weak lightning activity is likely close to the center, where CAPE is low. Main threat is extreme rainfall, also due to deep warm clouds within the south-easterly low-level flow that enters the storm. Additionally, tornadoes are not ruled out due to strong low-level vertical wind shear. The potential threat will likely continue on Friday.

Western Portugal and Spain

A slow moving frontal boundary enters western Europe. Moist air will spread northward along the boundary, while steep lapse rates spread east in its wake. An overlap is not likely, and MLCAPE is expected to be low. Main threat will be excessive rain along the frontal rain band that enters western Iberia late in the peroid. Additionally, low-level vertical wind shear around 10 m/s is expected along the coasts that will overlap with weak CAPE. A tornado is not excluded.

Balearic Islands, Cyprus

Below the centers of mid-level troughs, steep lapse rates affect these areas, and some deeper storms can develop due to rich low-level moisture and low-level convergence. Weak vertical wind shear is expected, and waterspouts are forecast along the land-sea-breeze convergence zones.