Storm Forecast
Valid: Thu 23 Jun 2011 06:00 to Fri 24 Jun 2011 06:00 UTC
Issued: Wed 22 Jun 2011 22:42
Forecaster: GATZEN

A level 2 was issued for the southern Alpine region mainly for large hail and excessice precipitation.

A level 2 was issued fro the northern Balkans mainly for large hail.

A level 1 was issued for southern Finland mainly for tornadoes.

A level 1 was issued for Belarus, western Ukraine, Slovakia, and surroundings mainly for excessive precipitation.

A level 1 was issued for northern Algeria mainly for large hail, severe wind gusts and excessive rain.

SYNOPSIS

A trough migrates across Central Europe. A strong mid-level flow curves around the base of this trough. At lower levels, a tongue of warm air will spread into eastern Europe ahead of a cold front situated from Spain to the Alps and the Baltic States on Thursday.

DISCUSSION

Southern Alpine region

The surface cold front has reached the Alps and has slowed down. To the south of this front, southerly to south-easterly winds are expected on Thursday. Models indicate that the warm air mass will get fairly moist with low-level dewpoints around 18C. Although the lapse rates will be not very steep, diurnal heating will likely create MLCAPE in the order of 1000 J/kg.

Storms are forecast as early as in the late morning given upslope flow and increasing convergence over the southern Alps. These storms will have a rather high potential of becoming severe given the strong deep layer vertical wind shear due to the strong westerly mid-level flow that expands southward. Latest GFS indicates favourably veering profiles and 0-3km SRH around 150 m²/s².

Supercells are expected with the potential of producing large or very large hail and locally excessive precipitation. The storms are forecast to go on during the afternoon and evening while the main activity will likely spread eastward together with the mid-level jet. In the evening hours, the chance of tornadoes is forecast to increase due to increasing low-level vertical wind shear underneath a strong westerly flow at 700 hPa. The storms are forecast to weaken from the west during the night hours as cold air advection sets in.

Northern Balkans

The surface cold front is forecast to move into the northern Balkans on Thursday, and rich low-level moisture is advected with northerly winds. The convergent wind field and upslope flow over the Bosnia region will be associated with moisture pooling, and GFS indicates dew points of more than 20C in the afternoon hours. Although the lapse rates will be rather poor, CAPE up to 2000 J/kg may evolve.

While the synoptic forcing will be weak initially, it will increase in the afternoon due to the approaching mid-level trough. Storms will become likely especially over the mountains and will spread south-eastward later on. Given the strong westerly jet that will be also associated with increasing winds at 700 hPa, favourably veering profiles and 0-3km SRH around 250 m²/s² are possible due to the weak low-level north-easterly winds.

Supercells are expected capable of producing large or very large hail. Especially in the northern regions, tornadoes may occur in the evening hours, when boundary-layer stabilization will be associated with increasing low-level vertical wind shear. During the night hours, the storms are forecast to cluster and may merge to an MCS spreading south-eastward. This system may be capable of producing severe wind gusts and excessive precipitation until the morning hours.

Baltic States, Finland, and surroundings

Latest models indicate a deepening frontal depression along the cold front that will move into southern Finland together with a strong mid-level jet streak on Thursday. Ahead of the low, strong vertical wind shear is forecast associated with curved hodographs (SRH 0-1km 150 m²/s²). A limiting factor will be the weak instability given a dry boundary-layer air mass, and thunderstorms are expected to be mostly elevated. Storms that root to the boundary layer may become severe with a potential of producing severe wind gusts and tornadoes. In the afternoon, some instability may develop over southern Finland near the surface pressure center. Although the vertical wind shear will likely decrease, an overlap of instability and favourably low-level shear may develop resulting in an enhanced tornado risk.

Eastern Central Europe

As the cold front moves into Belarus, the western Ukraine, and Slovakia, diurnal heating will likely lead to increasing convective activity during the day. Current thinking is that these storms will be mostly non-severe and outflow-dominated given the weak synoptic forcing. The best chances for severe weather seem to exist in the late morning hours near a decaying south-westerly low-level jet. Early storms may profit from the strong low-level vertical wind shear and tornadoes are not ruled out. Large hail and excessive rain are forecast to become the prominent severe threat in the noon/afternoon hours.

Northern Algeria

Moist upslope flow during the day and rather cool mid-level air are forecast on Thursday. Storms that form over the Atlas mountains may be capable of producing large hail and excessive precipitation as well as severe wind gusts. Due to the weak vertical wind shear, these storms will likely be short-lived and will cluster later on.