Storm Forecast
Valid: Thu 01 Jun 2017 06:00 to Fri 02 Jun 2017 06:00 UTC
Issued: Wed 31 May 2017 19:30
Forecaster: GATZEN

A level 2 was issued for eastern Bulgaria, mainly for large hail and excessive precipitation and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for southern and central Romania, Moldova, north-western Turkey, western and central Bulgaria, northern Greece, FYROM, Serbia, Bosnia and Herzegovina, Croatia, Slovenia, north-eastern Italy, southern and central Austria mainly for excessive precipiation and to a lesser extent large hail and severe wind gusts.

A level 1 was issued for central to eastern France, south-western Germany, Switzerland, north-western Italy mainly for excessive precipitation and to a lesser extent large hail and severe wind gusts.

SYNOPSIS

A cold front has moved east across northern and central Europe. A frontal boundary is left behind from central France to the Alpine region and northern Balkans, where rich low-level moisture has not been removed. Although mid-level lapse rates have decreased in response to mid-level ridging, strong diurnal heating has lead to steep low-level lapse rates and latest models show CAPE to form along the frontal boundary on Thursday, especially from the northern Adriatic towards the northern Balkans and the Black Sea region. The western portions of the frontal boundary are affected by a weakening mid-level trough over southern France that slowly moves east.

DISCUSSION

Bulgaria, southern and central Romania, Moldova, northern Greece, FYROM, Serbia, Bosnia and Herzegovina, Croatia, north-eastern Italy, and southern and central Austria

Along the frontal boundary from northern Bulgaria to central Serbia, central Croatia, and southern Austria, rich and deep moisture is present as indicated by latest Budapest sounding. Diurnal heating will support steepening low-level lapse rates and latest models predict that MLCAPE peaks around 1500 J/kg. Convective initiation is expected during noon over the mountains as well as along the sea breeze convergence over Bulgaria.

Main chance for organized storms is expected along the sea breez convergence over eastern Bulgaria. Due to easterly flow within the sea breeze, vertical wind shear increases, and multicells can form, posing a threat of large hail and excessive precipitation. Stretching of vertical vorticity along the sea breeze convergence may also result in weak tornadoes.

Given mostly unidirectional flow from the north-west and rather short hodographs in the other regions, storms are expected to be weakly organized. However, moderate to high CAPE, rather moist profiles (precipitable water around 35 mm) and rather slow storm motion to the east and south-east increase the chance of excessive precipitation with a threat of local flash floods. Pulse storms can also produce large hail and local severe wind gusts. Storms can cluster and enter the lowlands during the period with a continued threat of excessive precipitation and locally large hail and severe wind gusts. Main reason not to issue a level 2 is rather weak upper-level divergence so that upscale growth of storm clusters and possible widespread excessive rain events are not too likely. An exception is Austria, where upper-level ventilation is increased downstream of the approaching French trough.

Further north, convective storms are also expected especially over Romania, where drier profiles and longer hodographs cause a decreasing threat of excessive precipitation. To the south of the frontal boundary, deeper boundary layers are forecast, together with weaker low-level convergence. Storms are expected to be more isolated but still can produce locally excessive precipitation and severe wind gusts.

Central to eastern France, south-western Germany, Switzerland, north-western Italy

It is especially the frontal boundary from central France to southern Germany that provides rich low-level moisture. Some moisture advection from the Mediterranean will affect south-eastern France in the second half of the day. Although diurnal heating can be limited due to clouds within the mid-level circulation, CAPE around 500-1000 J/kg is expected by all available models. New storms are forecast during the day. Weakly organized storms that can cluster are expected. Given the slow storm motion and moist profiles, excessive precipitation is forecast with the stronger storms. Locally, large hail and severe wind gust are not ruled out.

Further north, warm air advection is expected over northern France. A few thunderstorms are expected in a moist air mass with limited CAPE. With moist profiles as indicated by latest Trappes sounding, excessive precipitation is not ruled out.

Northern Spain and northern Algeria and Tunisia

Sea-breeze and mountain-valley circulation are expected to support storm development in a weakly-shear but rather unstable air mass with MLCAPE around 1000 J/kg. Stronger storms can produce excessive rain and large hail as well as local severe wind gusts. As storms will likely be unorganized and forcing is weak, a level 1 is not issued.