Severe storms to be expected in Sweden, Finland and other thunderstorm areas south of 50N

BarboraSamkova

Valid: Fri 23 Aug 2019 06:00 to Sat 24 Aug 2019 06:00 UTC
Issued: Thu 22 Aug 2019 23:27
Forecaster: PISTOTNIK

A level 1 is issued for S Finland mainly for severe convective wind gusts, tornadoes and to a lesser degree for large hail.
Level 1 areas are issued for parts of NE Algeria, NW Tunisia, mainland Italy, Slovakia and NE Turkey mainly for excessive convective precipitation and large hail.
Level 1 areas are issued for the islands of Sardegna and Corsica and for NE Turkey mainly for excessive convective precipitation, large hail and non-mesocyclonic tornadoes (waterspouts).

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Synopsis

A strong zonal flow is present across N Europe with an axis near 60N. Two embedded short-wave troughs and their attendant surface cyclones cross Finland and approach the Faroe Islands, respectively.
Further south, very warm to hot air covers most of Europe under weak pressure and temperature gradients. An extensive surface anticyclone is centered over Belarus, while two faint upper-level lows are placed over Corsica and over E Turkey.

Discussion

Sweden, Finland

The cold front of the travelling cyclone attains a "split front" character, as mid-levels cool down under strong synoptic lift whereas S-erly to SW-erly winds still transport moist and not too cool air from the Baltic Sea inland. A few hundred J/kg CAPE will build, and scattered, low-topped showers and thunderstorms are possible mostly from noon to evening. Towards the south, enhanced vertical wind shear and veering profiles (0-3 km shear and storm-relative helicity on the order of 10-15 m/s and 100-200 m^2/s^2, respectively) create favorable conditions for multicells and possibly a few supercells or bowing line segments.

CAPE and enhanced vertical wind shear overlap best in S Finland during peak heating hours. Severe wind gusts, one or two tornadoes (over land as well as over water) and marginally large hail are possible, while these storms travel eastward towards the Russian border.

other thunderstorm areas south of 50N

Isolated to scattered, mostly daytime-driven thunderstorms will form especially over orographic features. With low to moderate CAPE (up to 1000 J/kg) and weak vertical wind shear (around or below 10 m/s), single- and multicells will be the primary storm mode. A few flash floods and moderately large (2-4 cm) hail events are possible. Level 1 areas are issued for some regions at the forward flanks of the mentioned cut-off lows, where the highest storm coverage and the relatively strongest vertical wind shear are expected.

Storms that manage to propagate to coastal and offshore areas along land nocturnal breeze fronts can produce a few waterspouts. This is a distinct scenario over the SE Black Sea (early and late in the forecast period) and around Corsica and Sardegna early in the forecast period), where models agree on convective precipitation over the SE Black Sea early and late in the forecast period. Otherwise, surface-based convection over the sea is questionable due to a capping inversion and a lack of synoptic lift support.

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