Storm Forecast
Valid: Tue 22 Sep 2020 06:00 to Wed 23 Sep 2020 06:00 UTC
Issued: Mon 21 Sep 2020 21:29
Forecaster: DAFIS

A level 1 was issued for parts of Spain, SE France, Italy, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, and Albania mainly for excessive precipitation and large hail.

A level 1 was issued for NE Algeria and N Tunisia mainly for severe convective wind gusts and large hail.

SYNOPSIS

A strong zonal flow has been established in Northern Europe with unseasonably high temperatures at the surface, but it will be interrupted on Wednesday 23/09 as a Rossby wave is expected from the Atlantic. Western and Eastern Europe are affected by a cut-off low and a ridge respectively. The cut-off low, now placed over France, will gradually be deformed but it is surrounded by several short-wave troughs. One of them will cross the Western Mediterranean providing lift to unstable air masses and resulting in DMC events. Another one is about to cross South Italy and will end up on the western coasts of the Balkans. As the long-wave trough from the Atlantic will be approaching continental Europe, baroclinicity over West Mediterranean will increase and as a consequence, a jet-stream is expected to develop extending from N Morocco towards South Italy, increasing significantly the DLS.

DISCUSSION

.... Spain, SE France, Italy, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, and Albania ....

Abundant low-level moisture and warm air masses create a few hundreds of CAPE, mostly over the Mediterranean. The highest values of both instability and PWAT will be found close to the coasts of Central and Western Mediterranean where in most places, the weak mid-level flow increases the threat for excessive precipitation events during slow-moving storms. More to the south, over the Balearic Islands, Sardinia and South Italy, DLS is forecast to range between 15-20 m/s, overlapping with up to 1000 J/kg MLCAPE and along with steep lapse rates, all these ingredients suggest that storms will be able to produce large hail. Waterspouts are also expected, mostly in the Gulf of Genoa and the Adriatic Sea.
In the Balkans, in the early afternoon, the main threat will be large hail from isolated storms and during the night excessive precipitation from training convection.

.... Algeria and Tunisia ....

Even though CI is questionable in this area given the dry air masses above the PBL and the high LFC heights, if storms succeed in developing due to diurnal heating (no QG-forcing is forecast) will get the benefit of strong DLS and high values of SREH0-3km to build supercells. These storms may become dangerous, producing severe wind gusts and large hail. In other circumstances, if CI was more certain, a level-2 threat could be justified for this weather set-up.