Storm Forecast
Valid: Thu 29 Sep 2016 06:00 to Fri 30 Sep 2016 06:00 UTC
Issued: Wed 28 Sep 2016 21:50
Forecaster: GATZEN

A level 1 was issued for parts of central Scandinavia mainly for tornadoes and severe wind gusts.

A level 1 was issued for the south-west Mediterranean mainly for excessive rain.

A level 1 was issued for the south Mediterranean mainly for large or very large hail.

SYNOPSIS

A powerful mid-level jet spreads east into the British Isles, North, and Baltic Sea region on Thursday. Stable warm air masses are advected into central and eastern Europe at its anticyclonically sheared flank. To the north, a well-developed mid-level front crosses the northern British Isles, North Sea, and central Scandinavia.

DISCUSSION

British Isles to Baltic Sea region

A plume of steep lapse rates evolves with the mid-level front, however, it only briefly overlaps with better moisture spreading north-east along the surface cold front. The best chances for thunderstorms exist over south-western Norway due to supporting upslope flow. Further storms can form over the central / northern Baltic Sea region given a local moisture maximum over the warm waters. The storms that will be embedded in frontal precipitation form with very strong low-level vertical wind shear in excess of 15 m/s in the lowest kilometre. Shallow mesocyclones capable of producing tornadoes are the main threat. Severe wind gusts are also possible, especially when storms manage to organize into a cold-pool driven system. The system will cross Finland and reaches the White Sea late in the period.

Upstream, another vort-max enters the UK later in the day, supporting scattered showers and thunderstorms. Weakening vertical wind shear and marginal CAPE / lift will limit severe potential.

South-western Mediterranean

Influenced by broad low geopotential centred over Morocco, rich Mediterranean moisture overlaps with relatively steep mid-level lapse rates advected from the south. CAPE around 500-1000 J/kg is expected. Storms will form along the sea-breeze convergence with offshore storms in the night hours and daytime storms over the Atlas mountains. Given the slow storm movement and quite substantial moisture, excessive rain is expected especially along the shores of Algeria and Tunisia, although main activity will stay over the Sea. Late in the period, deep layer vertical wind shear increases to the east of Tunisia, allowing better storm organization. Large hail will be the main threat due to possible night time supercells.