Storm Forecast
Valid: Wed 27 Sep 2023 06:00 to Thu 28 Sep 2023 06:00 UTC
Issued: Tue 26 Sep 2023 22:53
Forecaster: GATZEN

A level 2 has been issued across Ireland, Wales, and W England for severe wind gusts and tornadoes.

A level 1 was been issued across the British Islands and NW France for severe wind gusts and tornadoes.

A level 1 was been issued across S Norway, Sweden, Finland for severe wind gusts and tornadoes.

A level 1 has been issued across N Greece for heavy rain and to a lesser extent large hail.

A level 1 has been issued across W Turkey for large hail, severe wind gusts, and heavy rain.

SYNOPSIS

A closed upper vortex still sits across the Aegean and will only move slowly east. It is flanked by a belt of high geopotential that extends from SW Europe towards Russia. A strong mid-level south-westerly flow curves around the European ridge, with two intense vort-maxima rapidly traveling across southern Scandinavia and the British Isles. At lower levels, warm air spreads into the southern parts of Scandinavia ahead of a frontal wave that travels east during the day. Downstream, an intense low pressure system will affect the British Isles. Ahead of the trough across the Aegean Sea, a cold front moves into western Turkey.

DISCUSSION

British Isles

Rapid cyclogenesis is underway across the E Atlantic and will result in an intense low-pressure system approaching the British Isles at the start of the forecast period. A moist maritime air mass will be advected north behind a well-developed warm front. Within the warm air advection regime, models indicate some marginal elevated instability in a strongly sheared environment. Instability is expected to become surface-based just south of the cold front with about 100 J/kg CAPE overlapping with very strong low-level vertical wins shear. Impressive low-level hodographs with 20+ m/s low-level shear and 400+ J/kg 0-1 SRH are indicated by latest models.

Ahead and along the approaching ill-defined cold front, some showers are forecast to develop given weak CIN and low cloud bases. Although instability is marginal, the impressive shear profile suggests that showers have a high probability to be accompanied by severe wind gusts. Additionally, low-level updraft rotation is likely to develop with any sustained storm that occurs. The combination of high low-level streamwise vorticity and low cloud bases points to the risk of tornadoes, including strong tornadoes. Main uncertainty is that the shallow convection could be too weak and short-lived to form longer-lived rotation.

As the cold front pushes east-northeast, instability and shear over the UK gets weaker, with showers and isolated thunderstorms becoming gradually less severe. Still, some severe wind gusts and tornadoes are forecast until the afternoon.

Southern Scandinavia

A frontal wave travels rapidly east in the morning. Warm and moist air masses spread into southern Norway and Sweden that will be affected by some dynamic stretching what results in weak CAPE. Frontal lift will support convection initiation and some low-topped thunderstorms are forecast to develop that will rapidly move east. In a highly-sheared environment with curved hodographs, these storms are forecast to produce severe wind gusts, in particular when small-scale bow echoes manage to form. Additionally, low-level streamwise vorticity is rather high in some places, especially across southern Norway, where tornadoes and (marginally) large hail are forecast as well. Storms are expected to spread into Finland late in the period, where the potential for wind gusts and to a lesser extent tornadoes still exist.

Greece, western Turkey

A cold front pushes into western Turkey, adding lift to an air mass characterized by steep lapse rates and rather limited low-level moisture. High-based thunderstorms are forecast to develop along and ahead of the cold front during the day. Deep-layer shear around 15 m/s indicates the potential of multicells and transient supercells. Main threat will be some large hail and severe wind gusts. Farther west across Greece, potential of large hail is less due to weaker CAPE and vertical wind shear. However, clusters of slow-moving storms still pose a threat of heavy rain and flooding.