Storm Forecast
Valid: Thu 19 Oct 2017 06:00 to Fri 20 Oct 2017 06:00 UTC
Issued: Wed 18 Oct 2017 21:42
Forecaster: GATZEN

A level 1 was issued for southern France mainly for excessive precipitation.

A level 1 was issued for the south Mediterranean mainly for large or very large hail.

SYNOPSIS / DISCUSSION

Ridging across Europe weakens as troughs approach over western Europe. An Iberian cut-off will slowly move south-east as will a follow-up trough over the British Isles and France. Ahead of these two troughs, a southerly flow affects western Europe. Rich low-level moisture is advected into southern France from the Mediterranean. Relatively moist air masses are also advected into northern France and the British Isles from the Bay of Biscay. However, lapse rates are very limited and CAPE is most likely across the Mediterranean and the southern British Isles close to the trough axis of the approaching trough. Vertical wind shear is strong especially in the lowest 3 km over the Mediterranean, where multicells are forecast. These will move along the frontal boundary and may be capable of producing excessive rain. Over the British Isles, locally stronger 0-1 km vertical wind shear overlaps with low cloud bases and some low-level buoyancy especially near the southern shores. Severe wind gusts and a tornado is not ruled out here but potential is too weak for a level 1.

Across the southern Mediterranean, increasing deep layer vertical wind shear evolves late in the period ahead of the approaching cut-off trough. At the same time, an elevated mixed layer overlaps with rich low-level moisture. CAPE may exceed 2000 J/kg with DLS around 15 m/s. Given limited CIN, thunderstorms are forecast. Multicells may be the main mode given limited DLS, but also a few supercells are forecast especially in the evening hours, when 0-3 km storm-relative helicity is 150 m²/s². Large hail is the main threat, with supercells also very large hail. Tornadoes are not ruled out.