Storm Forecast
Valid: Sat 19 Nov 2016 06:00 to Sun 20 Nov 2016 06:00 UTC
Issued: Fri 18 Nov 2016 22:51
Forecaster: TASZAREK
A level 1 was issued for NW France mainly for severe wind gusts.
A level 1 was issued for parts of CNTRL Mediterranean mainly for excessive precipitation.
SYNOPSIS
A strong mid-level cyclonic vortex covers waters between Iceland and Norway. A broad Rossby wave extends from Greenland, through NW and CNTRL Europe, up to NW Russia and is surrounded by a jet with 300 hPa wind speeds of around 50-80 m/s. A ridge is placed over E and SE Europe. Shallow cut-off travels eastwardly S of Sicilly. A quickly deepening low-level vortex approaches NW France and SW British Isles and creates a threat for severe wind gusts in the end of the forecast period. Thunderstorms developing in weakly sheared environment in CNTRL Mediterranean area are capable of producing locally an excessive precipitation.
DISCUSSION
...NW France...
Within a passage of a shortwave, a quickly developing low is predicted by NWP models to hit NW France and partly SW British Isles in the nighttime hours. Although instability will be marginal, a strong QG-lift in the SE flank of the low may trigger shallow convection capable of enhancing already strong wind gusts resulting from horizontal pressure gradient. Given 20-25 m/s low-level flow, convection in the warm sector and along cold frontal zone may organise into "lines" and bowing segments. Damaging wind gusts of 25-30 m/s are expected to occur in the nighttime hours over NW France and continue in the morning hours over N France. However, due to rather marginal or event absent thermodynamic instability, a convective enhancement of wind gusts and the presence of any lightning activity is uncertain.
...parts of CNTRL Mediterranean...
A mixture of 7-8 g/kg MIXR and ~ 6.5 K/km LR provides 400-600 J/kg CAPE in a weakly sheared (DLS ~ 10-15 m/s) environment. Convection is triggered by local convergence zones and orographic lift within a perpendicular SWerly airflow. Thanks to weak shear, an isolated convection clustering locally into multicells is expected. Given such mode, low CAPE and shallow parcel layer depth, a severe weather potential is limited. However, given slow motion of the cells and locally superimposing stratiform and convective precipitation, a local excessive precipitation event cannot be ruled out.
