Storm Forecast
Valid: Thu 15 Sep 2016 06:00 to Fri 16 Sep 2016 06:00 UTC
Issued: Thu 15 Sep 2016 00:13
Forecaster: GATZEN

A level 1 was issued for southern Switzerland, western Italy, Corsica, Sardinia, the western Tyrrhenian Sea mainy for excessive convective precipitation and large hail.

A level 1 was issued for the western British Isles mainly for excessive rain and to a lesser extent large hail.

SYNOPSIS

A cut-off low centred across the Bay of Biscay slowly moves east. Warm-core depression connected to the trough makes landfall during the period and weakens. Strong winds and high precipitation are likely but not addressed in this convective outlook as thunderstorms embedded in the storm will not significantly contribute to the threat. Downstream of the cut-off low, an amplified ridge will still affect central and northern Europe. A plume of steep lapse rates advects northward across the UK and the North Sea, whereas lapse rates decrease further south-east due to low-level cooling. At the same time, dry air is advected westward ahead of a cold front that enters Germany from the west. This leads to reduced CAPE compared to the last days what limits the chance of severe thunderstorms.

Across the central Mediterranean, latest soundings indicate rich low-level moisture and steep lapse rates. However, mid-level ridging ahead of the approaching cut-off low will also lead to weaker convective activity. Main focus will be a frontal boundary that affects southern France and western Italy especially at the start of the period.

DISCUSSION

UK

Within the plume of warm air, CAPE will again build during the day although steepest lapse rates will be pushed northward during the day. Rich low-level moisture and a well-developed convergence zone oriented mostly parallel to the storm motion vector can again pose a threat of excessive precipitation. Large hail is not ruled out. Storms are expected to move to the North Sea in the evening hours where they become elevated.

Bay of Biscay

Strong low-level vertical wind shear and some low-level buoyancy can be expected during the landfall of the warm-core system. Therefore, tornadoes are not ruled out. Main severe weather threat will be not caused by organized deep moist convection and a threat level is not issued.

Sardinia, Corsica, central and northern Italy, southern Alps

Along the frontal boundary, rich low-level moisture overlaps with steep lapse rates. Weakly capped instability is expected and thunderstorms will form along the frontal boundary as well as along land-sea-breeze convergences. Strong deep layer vertical wind shear over the Mediterranean supports the development of multicells that can grew into convective systems, capable of producing large hail and excessive rain. Veering profiles can also result in supercells, especially in the eastern portions of the risk area, where large hail is more likely. Over the northern portions of the risk area, weaker vertical wind shear will limit the threat of large hail, but excessive rain is forecast. Overall threat is not too high as mid-level sinking leads to some inversions especially over the southern portions of the level 1.