Storm Forecast
Valid: Wed 28 Oct 2015 06:00 to Thu 29 Oct 2015 06:00 UTC
Issued: Tue 27 Oct 2015 21:14
Forecaster: PUCIK

A level 1 was issued for SE France and NW Italy mainly for excessive precipitation and to the lesser degree for marginally large hail and severe wind gusts.

SYNOPSIS

At mid to upper troposphere, a highly amplified trough is moving from the Atlantic towards France. During the forecast period, it is forecast to continue eastwards, while its southern base detaches as a separate cyclonic vortex and continues towards Central Italy. At the same time a ridge will be stretching over much of Central and Southeastern Europe. Closer to the surface, a deep low will be located over the Atlantic with a decaying frontal system slowly moving over E France / W Germany towards NW Italy. Most of Europe will be under very stable conditions hostile for DMC, the exceptions being parts of France and the Western Mediterranean. Here, a patch of steeper lapse rates thanks to the the cold mid-levels will contribute to latent instability.

DISCUSSION

... SE France towards NW Italy ...

Surface observations as of Tuesday 19 UTC reveal not very high dewpoints over the area, only around 12 deg C in the north to around 16 deg C over the south. NWP does not simulate significant moisture advection from south and with only marginally steep lapse rates (around 7 K/km). Thus, it does not seem likely that CAPE values will exceed several hundreds J/kg. Scattered to widespread initiation is forecast as trough / cyclonic vortex shits southeast along with an ill defined frontal system. DLS is quite strong (over 20 m/s) ahead of the trough, but decreasing with time. Due to the weak CAPE and only slim overlap of CAPE and strong DLS, not much window for intense well organised DMC is expected. Near the sea, almost saturated conditions with moderately moist airmass may help in a few marginal excessive rainfall events. Due to the cold mid troposphere, hail may occur with any stronger cell, but should stay mostly non-severe. A belt of stronger low-tropospheric flow is simulated near the Rhone valley, perhaps supporting a severe wind gust event in case that convection reaches the area by 12 UTC. After this time, flow will quickly weaken. Overall, Level 1 seems to be sufficent for this setup.