Storm Forecast
Valid: Thu 31 Jul 2014 06:00 to Fri 01 Aug 2014 06:00 UTC
Issued: Thu 31 Jul 2014 00:30
Forecaster: GATZEN

A level 2 was issued for parts of Finland mainly for severe wind gusts, tornadoes, and large hail.

A level 1 was issued for Finland and north-western Russia mainly for severe wind gusts, tornadoes, and large hail.

A level 1 was issued for Greece to Bulgaria and southern Romania mainly for large hail and excessive precipitation.

A level 1 was issued for the Baltic States, eastern Poland into Czech Rep, Slovakia, eastern Austria, Hungary, and the northern Balkans as well as the Alpine region mainly for excessive precipitation and to a lower extend large hail.

SYNOPSIS

A well-developed cut-off low moves slowly from the Adriatic Sea to the Balkan Peninsula. To its north, an intensifying ridge stretches from south-western Europe across central Europe to western Russia. A head of low geopotential over north-western Europe and the north-east Atlantic, several short-wave troughs move north-eastward.

At lower levels, a cold front has crossed Sweden and Germany and will affect a region from Finland to Poland and the northern Balkans on Thursday. Rich low-level moisture ahead of the front will allow for CAPE around 1000 J/kg in the afternoon hours. Rich moisture will also remain in the Alpine region, whereas drier air spreads into Germany and France.

Most storms will be weakly organized on Thursday, and main threat will be excessive precipitation given the moist profiles ahead of the cold front. Better storm organization due to stronger vertical wind shear can be expected in the vicinity of the cut-off low over Greece and Bulgaria as well as ahead of a short-wave trough entering Finland during the day.

DISCUSSION

Northern and eastern Finland, north-western Russia

Ahead of the north-west European trough, an intense short-wave trough crosses the southern Baltic Sea in the morning hours. This is associated with a strong southerly jet streak, reaching more than 20 m/s at 700 hPa over Finland, resulting in vertical wind shear of 15-20 m/s in the lowest 3 km. Additionally, favourably veering profiles are forecast by lastest models, woth 0-3 km SRH around 100 m²/s².

Latest soundings indicate steep lapse rates in a plume of warm air over the Baltic Sea and southern Finland that will advect further north on Thursday. In the boundary layer, rich moisture is present and continues to spread northward. GFS therefore indicates CAPE of more than 1000 J/kg during the noon and afternoon. Storms have already formed in the warm air advection regime on Wednesday. On Thursday, outflow boundaries of overnights convection will be a focus of new storm development over Finland, before these storms merge to a more linear convective system ahead of a cold front coming in from the Baltic Sea.

Strong vertical wind shear will allow for well-organized storms and supercells are forecast, capable of producing tornadoes (0-1 km vertical wind shear around 12 m/s). Large hail and severe wind gusts are also possible. Increasing coverage of storms during the noon and afternoon poses also a threat of severe wind gusts, especially when a linear systems can form in the afternoon and evening hours that may spread into north-western Russia late in the period.

Greece to Bulgaria and southern Romania

Ahead of the approaching cut-off low, a warm and well-mixed air mass advects into Bulgaria with southerly winds. At low levels, rich moisture is present and diurnal heating will allow for CAPE near 1000 J/kg. First elevated storms are forecast in the morning hours in the warm air advection regime. Storm coverage and intensity will likely increase during the day as storms become surface-based and better organized in 10-15 m/s 0-3 km bulk shear. Later in the day, storms are forecast to merge to mesoscale systems given increasing lift ahead of an approaching cold front from the west.

Multicells and supercells may produce large hail and severe wind gusts. Additionally, the threat of excessive precipitation increases in the afternoon when storms merge to larger clusters. Convective activity is expected to go on during the night as the mid-level cut-off crosses the area, with the main activity moving northward as cold air advection will be associated with some stabilization in the southern portions.

Baltic States, eastern Poland, eastern Czech Rep, Slovakia, eastern Austria, Hungary, and northern and central Balkans

With a most and unstable air mass, CAPE will be likely on Thursday. Lift is expected ahead of a cold front spreading eastward during the day. Storms are forecast to go on along the cold front in the morning hours. During the day, increasing coverage and intensity is expected. With weak vertical wind shear, well-organized storms are not likely. However, slow moving storms may produce excessive precipitation, especially in this moist air mass and due to the rather intense lift ahead of the cold front. Large hail and severe wind gusts are not ruled out, though.

Alpine region

In a moist and unstable air mass, thunderstorms are expected to develop over the mountains. Additional storms may evolve to the north of the Alps along a convergence zone near the approaching cold front. Storms that form will likely be weakly organized due to weak vertical wind shear. Main threat will be excessive precipitation due to the slow storm motion. Locally, severe gusts and large hail are not ruled out. Convective activity is expected to decay after sunset.