Storm Forecast
Valid: Thu 13 Aug 2020 06:00 to Fri 14 Aug 2020 06:00 UTC
Issued: Wed 12 Aug 2020 23:15
Forecaster: GATZEN

A level 2 was issued across southern England and Wales, northern Belgium, Netherlands, western and central Germany and Alpine region mainly for heavy precipitation and to a lesser extent large hail and severe wind gusts.

A level 2 was issued across north-western Spain mainly for large hail and severe wind gusts.

A level 1 was issued across southern British Isles into France and Germany towards the northern Balkans mainly for heavy precipitation and locally large hail and severe wind gusts.

SYNOPSIS

Weak mid-level flow remains across most of Europe with a ridge axis from the northern British Isles to southern Scandinavia and into the central Mediterranean and a weakening, almost quasi-stationary trough across France. A plume of warm and moist air is wrapped around the French trough whereas dry air continues to advect into eastern Europe. Main focus for convective storms is a convergence zone ahead of the warmest air mass from England to central Germany with some warm air advection taking place on Thursday.

DISCUSSION

England to central Germany and northern France

Rich moisture is present along the convergence zone that overlaps with steep lapse rates to create MLCAPE in the order of 1000 to 2000 J/kg, with highest CAPE expected across the Netherlands and north-western Germany. Storms are forecast to develop during the afternoon and evening when warm air advection increases and frontal lift is forecast to become stronger. With weak vertical wind shear, clusters of disorganized cells are expected. With skinny CAPE profiles, slow storm motion, and rich moisture, heavy precipitation is forecast. Additionally, well-developed storms can produce locally large hail and severe wind gusts. Along the convergence zone, storms can go on through-out the night.

Across the western and northern parts of the level 1, surface-based CAPE is weaker, and storms are expected to be partly elevated, limiting the risk of severe weather.

Alps

Diurnally driven storms is a very humid air mass with skinny CAPE profiles and slow storm motion supports heavy precipitation. Large hail is also forecast due to high CAPE values in particular on the southern flanks of the Alps. Additionally, severe wind gusts are not ruled out. Storms are forecast to weaken after sunset.

North-western Spain into southern France

A mid-level short-wave trough is forecast to spread over the area late on Thursday. Ahead of this short-wave, models suggest Mediterranean moisture to spread into north-western Spain that overlaps with steep lapse rates spreading east from central Spain. CAPE will develop and models suggest convection initiation over the mountains. Ahead of the trough, long strainght-line hodographs are present, with DLS in the order of 15 m/s. Multicells and supercells are forecast to form, with large hail and severe wind gusts the main threat.