Storm Forecast
Valid: Thu 19 Aug 2021 06:00 to Fri 20 Aug 2021 06:00 UTC
Issued: Wed 18 Aug 2021 22:11
Forecaster: GATZEN

A level 1 and 2 were issued across north-western Russia for heavy rain, severe wind gusts, tornadoes, and large hail.

A level 1 was issued across western Russia, eastern Ukraine, and Black Sea region for heavy rain.

A level 1 was issued across north-eastern Spain for heavy rain.

A level 1 was issued across eastern Spain for large hail and severe wind gusts.

A level 1 was issued across northern Tunisia and north-eastern Algeria for severe wind gusts.

SYNOPSIS

Low geopotential is centred across Scandinavia and a mid-level jet curves from the North and Baltic Seas across north-western Russia. A frontal boundary has moved towards higher geopotential across Russia, and cooler air is advected into most of Europe. Mid-level cut-off troughs slowly progress across the Black Sea, east Iberia, and the British Isles.

DISCUSSION

North-western Russia

A well-developed frontal boundary has outrun the European trough that has split into the Scandinavian trough and a cut-off trough that diggs into the Black Sea region. Increasing mid-level geopotential across the Ukraine supports a right entry region of a jet across north-western Russia that affects the frontal boundary on Thursday. Some warm air advection ahead of the cut-off trough is also expected to maintain the frontal gradient.

Ample low-level moisture has build on the cool side of the boundary, partly overlapping with steep lapse rates advected north-east ahead of the trough. Models agree on weakly-capped MLCAPE in the order of 1000 to 2000 J/kg to build during the day. Frontal lift will be sufficient for widespread convection initiation.

Numerous rounds of storms are expected to move along the slow-moving frontal boundary in a strongly-sheared environment. Mostly straight-line hodographs and strong 0-3 km shear support multicells and transient supercells, capable of producing heavy rain and locally large hail. Severe wind gusts are most likely along the southern flank of the level 2 area where deeper boundary layer mixing and associated cold pool formation can result in some cold-pool driven mesoscale convective systems including bow echoes.

Finally, strong low-level vertical wind shear in the order of 10 m/s in the lowest km, also assiciated with some low-level warm air advection and curved low-level hodographs point out some tornado potential with supercells across the eastern part of the level 2.

Western Russia, eastern Ukraine, Black Sea region

Rich low-level moisture and modest lapse rates result in rather skinny and deep CAPE profiles in the weakly sheared environment. Storms are expeced to develop during the day ahead of the approaching and digging cut-off trough. These pose a threat of heavy rain. Storms are forecast to continue through-out the night across the central Black Sea.

Eastern Spain

Ahead of a slowly progressing mid-level trough, some warm air advection affects eastern Spain and the west Mediterranean. A moist boundary layer across the west Mediterranean advects into Spain where upslope flow is present. Moderate MLCAPE is forecast to develop in response to diurnal heating and increasing mid-level lapse rates advected from the higher terrain over central Spain.

Storms are forecast to develop over the mountains in the afternoon. Weak Shear across the north-eastern parts will limit storm organization, but still, heavy rain is expected and some marginally severe hail is also not ruled out. Farther south, stronger deep-layer shear supports better storm organization with some supercells possible, capable of large hail and severe wind gusts. Storms are forecast to spread into the Mediterranean with a decreasing severe potential due to the stable boundary layer, but some large hail is not ruled out.

Northern Africa

Some diurnally driven storms are forecast over the Atlas mountains. Deep boundary-layer mixing results in large downdraft-CAPE and some severe wind gusts are forecast.

Wales, west England, Baltic States and Finland

Mid-level fronts are affecting these areas on Thursday, associated with low-level stretching and improving lapse rates. Low-topped showers and thunderstorms are expected to develop. Vertical wind shear is rather weak at low-levels. While overall severe threat is low, some non-mesocyclonic tornadoes are not ruled out due to low LCLs, weak shear, and some low-level CAPE.