Storm Forecast
Valid: Thu 13 Jul 2017 06:00 to Fri 14 Jul 2017 06:00 UTC
Issued: Wed 12 Jul 2017 22:08
Forecaster: GATZEN

A level 1 was issued for eastern Romania, Moldova, and north-eastern Bulgaria mainly for large hail, excessive rain and tornadoes and to a lesser extent severe wind gusts.

A level 1 was issued for western Russia mainly for large hail, excessive rain and tornadoes and to a lesser extent severe wind gusts.

A level 1 was issued for the Baltic States, Belarus, and Ukraine mainly for excessive precipitation and severe wind gusts and to a lesser extent large hail.

A level 1 was issued for northern Italy mainly for large hail and severe wind gusts.

A level 1 was issued for northern Sweden and Finland / north-western Russia mainly for large hail and severe wind gusts.

A level 1 was issued for the Pyrenees mainly for large hail and severe wind gusts.

SYNOPSIS

At the southern flank of intense low geopotential across Scandinavia, polar jet stream extents from the British Isles to the Ukraine. In the wake of an intense short-wave trough moving over eastern Europe, Atlantic air masses spread into Central Europe. Limited moisture and weak lapse rates are associated with this air mass. Better moisture is left along the cold front from the Baltic States / western Russia towards the Black Sea and Aegean. However, poor lapse rates will overlap with the good moisture over most places. Over southern Europe, mid-level ridging will limit the potential for initiation.

DISCUSSION

Western Russia and Baltic States to Belarus, Ukraine, Moldova, Romania, and northern Bulgaria

Ahead of the approaching cold front, a warm air mass associated with limited lapse rates but rich low-level moisture is expected to allow for around 1000 J/kg CAPE on Thursday. Thunderstorms are likely along the cold front. Front-parallel flow will support clustering of storms. Vertical wind shear is weak with around 10 m/s deep layer bulk shear in the warm air mass. Main threat with the storm clusters will be therefore excessive rain. Stronger storms can also produce large hail. Severe wind gusts are not ruled out as well. Along the sea-breeze convergence across eastern Romania, about 10 m/s low-level vertical wind shear point out a chance of mesocyclones capable of producing also tornadoes. Tornadoes may also occur across western Russia where rich low-level moisture overlaps with stronger low-level vertical wind shear in the afternoon and evening as well.

Northern Italy

In a north-westerly flow, steeper lapse rates that develop over the Alps are advected into northern Italy where they overlap with rich low-level moisture. CAPE in the order of 500-1000 J/kg is expected. A passing short-wave trough in the afternoon will support initiation of storms, likely along the Föhn convergence. These storms can quickly become well-organized given strong deep-layer vertical wind shear around 20 m/s, and supercells are forecast, capable of producing large hail and severe wind gusts. Limited CAPE reduces the severe threat.

Northern Sweden and Finland / north-western Russia

A strong mid-level jet streak travels northward ahead of the trough axis of the Scandinavian trough. Intense QG forcing is expected. Current models indicate steepening lapse rates that yield CAPE where it overlaps with better moisture of an occlusion. Main uncertainty is low-level buoyancy due to clouds and rain in the morning hours. However, if surface based storms can form, strong deep-layer bulk shear indicates that supercells can form, capable of producing large hail and severe wind gusts.

Pyrenees

An isolated storm over the Pyrenees is not ruled out on Thursday. Strong vertical wind shear will support organization, and large hail and severe wind gusts are possible.