Storm Forecast
Valid: Tue 04 Jun 2019 06:00 to Wed 05 Jun 2019 06:00 UTC
Issued: Mon 03 Jun 2019 20:58
Forecaster: DAFIS

A level 2 was issued for NE France, Belgium and the Netherlands mainly for large hail, severe convective wind gusts and tornadoes.

A level 1 surrounds the level-2 area and covers parts of France, Belgium, Luxembourg, the Netherlands and Germany and it was issued for large hail, severe convective wind gusts and tornadoes.

Level-1 areas were issued for parts of Romania, Moldova, Ukraine, Turkey, SW Russia and Georgia for large hail, large amounts of small hail and severe convective wind gusts.

A level 1 was issued for parts of Slovenia, Croatia, SW Hungary and Bosnia and Herzegovina mainly for large hail.

SYNOPSIS

An amplifying upper/mid-level trough in NE Atlantic is moving east on Tue 04/06 and south-westerly flow is increasing fast in W Europe. At the same time, the low-level advection of warm and moist air masses in France and Germany will create a favourable environment for thunderstorms in the afternoon and during the night and early morning of Wednesday 05/06.

Over the Balkans, a weakening cut-off low will be still able to provide lift to unstable air masses and locally enhanced wind flow in the periphery of the cold drop will provide the ingredients for some severe thunderstorms. More to the east and SE, higher dew points and steep mid-level lapse rates will also create strong instability.

DISCUSSION

.... France, Belgium, the Netherlands, Luxembourg and Germany ....

The approaching trough from the west will be associated with the south-southeasterly flow near the surface, and the veering wind profiles that all available NWP models forecast, will create a dangerous environment for any thunderstorm that will develop. Convective initiation is expected first in Central - NE France after 15z where the PBL mixing ratio is expected to reach values up to 11 g/kg and steep mid-level lapse rates will build large values of CAPE. MLCAPE is forecast to exceed 1.5 kJ/kg inside the level-2 area where more than 15 m/s of 0-3km bulk shear will be available. Storms will be able to quickly form supercells, travelling long distances and producing large hail, severe wind gusts and tornadoes.

The latest outputs of regional models provide different solutions for the convective mode that should be anticipated. If discrete supercells form in NE France and Belgium then the main threat will be large (or very large) hail and tornadoes. If supercells form bow echoes then widespread severe wind gusts will be the main threat.

Given the low predictability of the storm-mode of the storms translating towards Belgium and the Netherlands, it is difficult to exclude any threat. Nevertheless, the high low-level shear of veering winds which create more than 100-150 m2/s2 SREH0-1km and low LCLs, even below 1000 m, mostly inside the level-2 area will increase the probability of tornadogenesis. Convection will become elevated during the night and storms should start weakening before midnight while entering the North Sea.

.... Romania, Moldova and Ukraine ....

Under the core of the cut-off low, strong lift and underlying unstable air masses are expected to form several thunderstorms, most of them driven also by diurnal heating. DLS is minimum below the core of the mid-level low, but more to the north 10-12 m/s DLS will overlap with some hundreds of CAPE. Storms may also get benefit of mid-level helicity over Romania and Moldova, and become short-lived supercells able to produce large hail and severe wind gusts. Large amounts of small hail are as well possible during slow-moving cells, given the low heights of the 0oC wet-bulb isotherm.

.... Slovenia, Croatia, SW Hungary and Bosnia and Herzegovina ....

Locally enhanced DLS according to GFS will overlap with 1-1.5 kJ/kg MLCAPE and diurnally driven storms may become able to produce large hail in the afternoon. The convective activity should stop after sunset, or at least convection will be elevated more to the east side of level-1 area.

.... SW Russia, Georgia and Turkey ....

Southwesterly flow from the Mediterranean and the Black Sea will provide abundant moisture and high PWAT. MLCAPE may locally exceed 2 kJ/kg but DLS will only locally exceed 15 m/s. The level-1 areas show where we expect the best overlap of thermodynamic parameters for thunderstorms to produce large hail and severe wind gusts.