Storm Forecast
Valid: Wed 23 Sep 2020 06:00 to Thu 24 Sep 2020 06:00 UTC
Issued: Tue 22 Sep 2020 19:47
Forecaster: PUCIK

A level 1 was issued across Sardegna, Corsica, central Italy and western Balkans coastline mainly for excessive rainfall.

A level 2 was issued across the southern Adriatic and northern Ionian Sea and adjacent coastlines mainly for excessive rainfall and to the lesser degree for large to very large hail, severe wind gusts and tornadoes.

A level 1 was issued across Tunisia, southern Italy and Sicily mainly for excessive rainfall and large hail.


SYNOPSIS

An active weather pattern is forecast across western and southern Europe. Synoptic-scale pattern will be dictated by a trough over the Atlantic and a ridge over Russia with southwesterly flow in between of these features, resulting in seasonably warm conditions across much of central, eastern and southeastern Europe. Towards the west, a pronounced short-wave trough is forecast to cross British Isles towards the Northern Sea, followed by another trough digging towards the south in the nose of a powerful upper-tropospheric jet-streak. Isolated thunderstorms are forecast across and behind the advancing cold front, as well as in the plume of lapse rates exceeding 6.5 K/km that will stretch from Iberia into Norway.

More widespread storm activity is forecast across the warm and moist central Meditteranean, underneath a subtle trough. Here, environment will also support an enhanced probability of severe convective storms.

DISCUSSION

... Central Mediterranean ...

At the time of forecast writing (Tuesday 19 - 20 UTC), scattered storms are ongoing over the Adriatic and Ionian Sea. Environment over the Meditteranean features moist and unstable airmass with dewpoints / CAPE and also vertical wind shear increasing towards the south. On Wednesday, a short-wave trough will approach the region from the Balearic Sea, arriving over the Ionian Sea by Thursday morning hours. Ahead of the short-wave, increase in synoptic-scale lift is forecast, as well as the strengthening of mid-tropospheric wind field. 0-6 km vertical wind shear will reach 20 m/s over the southern Tyrrhenian and the Ionian Sea. At the same time, plume of steep mid-tropospheric lapse rates advected from northern Africa will contribute to 2 to 3 kJ/kg MLCAPE values over the Ionian Sea.

A large level 1 is issued for the central Mediterranean to take into the account high likelihood of heavy convective precipitation with any of the storms that form over the area, given high moisture content, low LCLs and also high RH in the mid-troposphere. Level 2 is introduced for the southern Adriatic / northern Ionian Sea, where numerous waves of thunderstorms are forecast during the whole forecast period. Convective coverage may increase during Wednesday night as the short-wave trough approaches, further exccerbating the flash flood threat near the coastline. With 0-6 km between 15 and 25 m/s and rather steep mid-tropospheric lapse rates, stronger updrafts may also produce large to very large hail. This threat may be mitigated somewhat by the presence of deep warm cloud layer depth. Severe wind gust and tornado risk will be also lower compared to the heavy rain risk.