Storm Forecast
Valid: Thu 03 Oct 2013 06:00 to Fri 04 Oct 2013 06:00 UTC
Issued: Wed 02 Oct 2013 22:35
Forecaster: GATZEN

A level 2 was issued for south-western France mainly for large hail and severe wind gusts.

A level 1 was issued for western and central France, central and northern Spain, northern Portugal, southern British Isles, western Netherlands, and Belgium mainly for severe wind gusts, large hail, tornadoes, and excessive precipitation.

SYNOPSIS


A cold and dry air mass has spread across most of Europe and low-level mixing ratio is below 9 g/kg from the North Sea to Italy and southern Turkey. Given weak lapse rates, CAPE is not expected over most places. Across western Europe, much better moisture is present that has been advected northward from the Atlantic Ocean and westward from the Mediterranean Sea. A tongue of rather well-mixed warm air will spread northward into France and surroundings ahead of an approaching Atlantic trough. QG ascend is expected as a short-wave trough moves north-east across Iberia, France, and the southern British Isles. Given the strong vertical wind shear, potential of severe thundestorms is rather high.

DISCUSSION

Spain, France, Benelux, southern British Isles

Rich low-level moisture is present across France, sampled by latest Bordeaux sounding. This moist air mass is expected to spread into northern France and the southern British Isles on Thursday. Ample moisture is also present across the Atlantic west of Portugal and the western Mediterranean Sea. The flow will continue to advect the moist air mass onshore across Iberia and southern France. Although lapse rates are not too large, CAPE is present and weak capping has allowed for thunderstorms across Portugal along a cold front that spread into the Bay of Biscay during the night. Further east, steeper lapse rates and a stronger capping inversion are present.

On Thursday, warm air advection will continue across France and the British Isles ahead of the Atlantic trough. The first thunderstorms will likely spread northward into north-western France and further into the British Isles until noon, where they will become gradually elevated. New storms are expected to initiate across Spain in the moist upslope flow. Some QG ascend can also assist in initiation as a short-wave impulse travels north-eastward in the noon hours. Later in the day, this short-wave trough is expected to increase the storm potential across eastern Spain, western and central France and the Benelux countries.

In the strongly-sheared warm air advection regime with 0-6 km bulk shear in excess of 15 to 20 m/s and 0-3 km SRH around 100-200 m²/s², organized storms are forecast. Multicells and supercells capable of producing large hail are forecast. Especially in the afternoon and evening hours, large low-level hodographs and adequate low-level buoyancy are present, and tornadoes are forecast especially with isolated supercells. Strong tornadoes are not ruled out. The highest risk of severe storms is expected across south-western France, where capping will be rather weak and low-level wind shear will be strong due to backing winds north of the Pyrenee mountains. Severe wind gusts may also occur, especially with bow echoes forming from merging storms. Larger storm clusters and mesoscale convective systems can also produce excessive rain. The threat will decrease after sunset due to the boundary-layer stabilization.