Storm Forecast
Valid: Thu 13 Jun 2013 06:00 to Fri 14 Jun 2013 06:00 UTC
Issued: Wed 12 Jun 2013 22:25
Forecaster: GATZEN

A level 1 was issued for south-western Ukraine, Romania, and Bulgaria mainly for excessive precipitation and large hail.

SYNOPSIS

Between a quasi-stationary trough centred over Turkey and a slowly eastward propagating frontal zone over north-western Europe, a ridge will affect most of Europe on Thursday morning. During the day, convective initiation is forecast especially across south-eastern Europe. However, additional storms will be possible along a cold front that moves across central Europe during the afternoon and evening hours.

DISCUSSION

Ukraine, Moldova, Romania, Bulgaria, Greece, Turkey, and surroundings

Latest soundings indicate favourable thermodynamics across parts of south-eastern Europe. Due to frequent convective activity, moist-neutral lapse rates have evolved especially from Romania into Bulgaria, whereas deep mixing is evident further east, especially across the Ukraine. On Thursday, this EML will spread westwards and overlaps with the rich boundary-layer moisture especially across Romania and Bulgaria. Across Greece and Turkey, an overlap of moist air in low terrain and steep lapse rates that have formed on Wednesday in higher terrain will lead to similar conditions. This will result in CAPE values around 1000 J/kg over a large area.

Convective initiation is expected during the morning hours and will go on through-out the period. Clusters of storms will develop but MCSs are not expected due to weak QG forcing. However, given the deep moist layer especially in the western regions, excessive precipitation is not ruled out. Additionally, the geopotential gradient between the approaching ridge across Central Europe and the trough over the Aegean Sea will increase during the day, resulting in stronger north-easterly winds especially over eastern Bulgaria and Romania. Storms will profit from the increasing vertical wind shear and multicells and even some supercells are forecast. Large hail is the main threat with these storms, although a tornado is not ruled out in the evening hours when the boundary-layer moisture increases.

In the other regions, slow moving storms and rich moisture wil be capable of producing excesssive precipitation. Large hail is also not ruled out.

Southern Sweden

A strong mid-level jet streak will approach across southern Sweden on Thursday. It will provide strong QG forcing. At low levels, the ageostrophic flow will advect a moist and warm air mass from the Benelux countries into southern Sweden. Main uncertainty will be the development of steep lapse rates. Stratiform rain and thick clouds will limit daytime heating, and chance to get any CAPE are weak. Latest GFS indicates marginal CAPE in the afternoon hours and some thunderstorms may develop. Given a favourably veering profile especially at low levels, mesocyclones may develop with a potential of producing tornadoes given the moist boundary-layer. However, current thinking is that storms will not evolve due to the lack of instability.

France, Germany

A cold front moves across France and Germany on Thursday. The surface front will be localized at the anticyclonically sheared flank of an approaching mid-level jet streak, so that QG forcing is expected to be limited over most places. However, warm air advection will take place especially across south-eastern Germany, where a low-level jet evolves in the evening hours that moves into the Baltic Sea during the night hours.

Increasing boundary-layer moisture and diurnal heating will result in some CAPE ahead of the cold front and storms will initiate across France during noon. Together with the maximum of warm air advection that spreads north-eastward, convection will spread into Germany during the afternoon. Later in the day, storms across France are expected to weaken given weaker forcing. Over Germany, storm coverage and intensity will slightly increase before a cluster of storms will develop along and west of the surface cold front. Large hail is not ruled out as well as excessive precipitation, but overall threat is too marginal for a level 1. After sunset, convective activity will rapidly decay.