Storm Forecast
Valid: Thu 21 Aug 2014 06:00 to Fri 22 Aug 2014 06:00 UTC
Issued: Thu 21 Aug 2014 06:47
Forecaster: GATZEN

A level 2 was issued for the central Balkans mainly for large hail and to a lesser extend excessive precipitation and tornadoes.

A level 2 was issued for the Adriatic Sea and the surrounding region mainly for large hail and excessive precipitation and to a lesser extend severe wind gusts and tornadoes.

A level 1 was issued for the western central Mediterranean mainly for large hail and to a lesser extend tornadoes and excessive precipitation.

A level 1 was issued for eastern and western and central Romania, western Ukraine into Belarus mainly for large hail and severe wind gusts.

A level 1 was issued for the northern and central Baltic Sea and surroundings mainly for excessive precipitation and tornadoes.

SYNOPSIS

A large long-wave trough dominates the mid-level flow across Europe, pushing the polar jet into the north Mediterranean. A lifting short-wave trough is affecting eastern Europe, and cold air advection sets in from the Balkans to Belarus. At lower levels, a frontal boundary is left behind over the northern and central Mediterranean.

DISCUSSION

The Balkans

In the wake of the east-European lifting trough, a cold front moves southward over the central Balkans. A line of thunderstorms has formed along this cold front and moves over Croatia into the central Balkans in the morning hours. Storms are partly elevated due to the cool boundary-layer in the morning hours, but will become better organized in the next hours due to surface heating ahead of the cold front.

However, the low-level convergence along the cold front will weaken during the next hours, as northerly winds will develop to the south of the cold front. As a consequence, storm coverage will become more dependent on the topography during the day. Scattered storms are forecast even in the wake of the cold front given upslope low-level winds, rather rich moisture, and steepening lapse rates in areas with diurnal heating.

Storms will likely organize into supercells and well-developed multicells as deep layer vertical wind shear is strong below the polar jet stream (around 20 m/s 0-6 km bulk shear). Large hail seems to be the main threat. Tornadoes may be also possible especially near the Adriatic Sea, where low-level buoyancy and low-level vertical wind shear may overlap especially in the morning hours. Additionally, local excessive precipitation may occur. Storms will go on until the evening when they start to decay as weak QG forcing is expected.

Central Mediterranean and western Mediterranean into north-eastern Spain

A frontal boundary is situated from the Balearic Islands to Italy and into the Adriatic Sea, and rich low-level moisture is present over a broad region. Although lapse rates continue to decrease from the north, CAPE can likely exceed 2000 J/kg as indicated by latest LIRE sounding.

Main source of lift will be the land-sea-breeze convergence during the period, although a few storms will initially go on along the cold front over the Balearic Islands and southern Italy in the morning hours. Storms that form will have a high potential to become supercells and well-developed supercells, given a deep layer vertical wind shear of 15 to 25 m/s and locally favourably curved hodographs.

Large or very large hail will be the main threat with supercells, whereas severe wind gusts and excessive precipitation can be expected with multicells and mesoscale convective systems. Tornadoes are also not ruled out, especially along low-level convergence lines. Storms will go on through-out the period, especially over the sea waters.

Baltic Sea

Near the centre of the European long-wave trough, steep low-level lapse rates are present especially over the northern and central Baltic Sea today and may also develop further inland over Finland and Sweden. Rich low-level moisture across the record warm sea surface will lead to high low-level buoyancy. Widespread storms are therefore forecast. Especially back-building storms that move along low-level convergence lines will be capable of producing excessive rain. Additionally, non-supercell tornadoes are forecast, with the highest potential over the Baltic Sea. Convective activity will likely go on through-out the period.

Eastern Europe

Near and in the wake of the lifting short-wave trough that moves into Belarus today, thunderstorms are forecast along a cold front. A bow echo has crossed Hungary and is expected to weaken in the following hours. During the day, new storms will develop, but low-level convergence is weak and moisture is limited over eastern Europe. Therefore, potential of severe storms seems to be limited as well, although some CAPE can overlap with increasing vertical wind shear in the wake of the cold front. Romania and the western Ukraine seems to be mostly affected, especially in regions with upslope flow. Locally large hail and severe wind gusts are not ruled out until the evening, when storms will decay due to weak low-level support.