Storm Forecast
Valid: Sun 13 Sep 2015 06:00 to Mon 14 Sep 2015 06:00 UTC
Issued: Sat 12 Sep 2015 19:51
Forecaster: DAFIS

A level 2 was issued for parts of Italy and France mainly for excessive rain, large hail, severe wind gusts and tornadoes.

A level 1 was issued for parts of Germany, Benelux, France and Italy mainly for excessive rain and large hail, severe wind gusts and tornadoes.

SYNOPSIS

A longwave trough reaches the Bay of Biscay during Sunday, associated with several depressions at the surface. Strong wind field in mid and upper troposphere create a strongly sheared environment in Southwestern Europe where again extreme weather events are expected. The main threat is found in SE France and NW Italy where very high amounts of precipitation are forecast, locally exceeding the 300mm/24h. We chose not to issue a level 3 for these regions because of the almost annual iteration of such phenomena and the locally enhanced orographic convection that limits the coverage of the extreme events. In eastern parts of Europe high geopotentials and advection of warm air masses from the South prevent convection.

DISCUSSION

.... Italy and France ....

Flash floods are forecast for a large area of SW France and NW Italy where very unstable air masses are orographically lifted in the southern flanks of the Alps. NWP models predict up to 3kJ/kg of MLCAPE near the coasts with mixing ratios in the BL in the range of 15-20g/kg and dew points up to 17°C. Very strong deep layer shear (0-6km DLS) and SREH (0-3km 300-500 m²/s²) will organize the storm cells into MCSs or supercells able to produce large to very large hail, severe wind gusts and even tornadoes, especially waterspouts at the Gulfs of Genoa and Lyon. Precipitation is forecast to exceed the 300mm in some areas where orography will play a significant role to the persistence of these convective features.


.... Germany, Benelux, France and Italy ....

As a cold front is approaching, DMC is forecast to occur, especially in the Benelux region. DLS will be in the range of 15-20m/s, so we expect some organized storm cells to become severe and produce large hail, severe wind gusts and excessive rain (PW >30mm). Tornado threat is limited concering the strong low level wind field but LLS will form in Benelux and LCL heights will be below 1000m. During the night the rainfall will continue but produced by stratiform clouds.