Storm Forecast
Valid: Mon 21 Sep 2020 06:00 to Tue 22 Sep 2020 06:00 UTC
Issued: Mon 21 Sep 2020 01:38
Forecaster: VAN DER VELDE

A level 1 was issued across SW France and NE Spain for excessive convective rainfall.

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

DISCUSSION

Low pressure from France to the Balkan with humid and warm low level air produces unstable weather. A few upper shortwave troughs move eastward between N Spain, C Italy and Balkan. A weak surface low pressure center over SW France with weakly capped air is likely to produce slow-moving convective cells in a very moist environment conducive to excessive local rainfall. The same applies to the south side of the Pyrenees with upslope winds. A shortwave trough in marked in GFS by a PV anomaly arrives at the Catalonia coast by 15-18Z and then may trigger storms across the NW Mediterranean and Corsica. Across C Italy (mainly) the activity should develop mainly in the early afternoon, and activate after nightfall over the Adriatic Sea.

High CAPE (>2500 J/kg) over Tunisia will be weakly capped but lacking a clear synoptic trigger. Sea breeze may help initiation. In this region, models predict 250-400 m2/s2 of SREH which is clearly supportive of supercell mode. These storms then produce large hail and severe wind gusts and perhaps a tornado. A level 1 is issued as development may be localized in lack of a clear trigger and only during the afternoon.