Storm Forecast
Valid: Sun 20 Oct 2013 06:00 to Mon 21 Oct 2013 06:00 UTC
Issued: Sun 20 Oct 2013 04:25
Forecaster: VAN DER VELDE

A level 1 was issued for northern France, Benelux, southern UK and western Germany mainly for a chance of tornadoes.

A level 1 was issued for southern France and northwestern Italy mainly for excessive convective precipitation.

SYNOPSIS

A low pressure area just west of northern Ireland created a large warm sector with southerly winds mainly over France. A mid level cold front moves into the unstable airmass (some 500 J/kg CAPE) from the southwest. The strong flow is already apparent in 00Z soundings with 45 kts at 850 hPa in western France where thunderstorms occur in the early morning. A mid level shortwave trough will move through the English Channel region, northern France and Benelux.
Advection of moist unstable air into the Massif Central and later the Apennine Mountains combined with the passage of the cold front should trigger long-lasting thunderstorms with excessive rainfall.

DISCUSSION

...northern France, Benelux, southern UK and western Germany...

A shortwave trough is expected to provide lift which will continue to keep the airmass unstable. Deep layer shear over the European mainland is 20-30 m/s while low-level shear is greater than 10 m/s most of the time across the entire region. GFS hodographs look quite circular during the afternoon in the eastern half of the level 1 area. Some of the storms may exhibit rotating updrafts and with also the low LCL heights (600-1000m), some chance of tornadoes should exist. Deep layer/mid level shear is only strong in the southern half, but the left exit of the jet should pass over the Netherlands. Bow echo structures can also form and severe wind gusts may result.

...southern France and northwestern Italy...

A stationary band of storms will likely form roughly between Montpellier and Switzerland. GFS hodographs become nicely circular in the 0-3 km levels (tornado chances) at the south coast where 1000 J/kg CAPE is advected to (2000 J/kg near central Italy). During the evening the focus turns to Italy. Tens of millimeters will be common in the regions but some places could receive over 100mm.
Supercells may be found near or embedded in the multicell systems and could produce additional phenomena as large hail or a tornado. Relatively modest deep layer shear (15 m/s) and low level shear keep the threat level at 1 although a very slow motion of the predicted complex over Italy may require an update to level 2 for excessive precipitation.