Storm Forecast
Valid: Tue 22 Nov 2016 06:00 to Wed 23 Nov 2016 06:00 UTC
Issued: Mon 21 Nov 2016 22:24
Forecaster: DAFIS

A level 2 was issued for E Spain and S France mainly for excessive rainfall, severe wind gusts and waterspouts.

A level 1 surrounds level 2 area for the same threats but with less coverage.

SYNOPSIS

A big trough covers West Europe while a ridge remains over the eastern parts of the continent, with a strong anticyclone on the surface prevailing over Russia. Several short-wave troughs are visible on the maps around North Sea and at the surface, the deep low pressure system over the UK is progressively moving NE, towards Norway. The big baroclinic wave on the west side of our domain seems that will interact with the mountain range of Atlas in NW Africa, resulting in cyclogenesis near the surface, with a deepening cyclone moving over Spain during Tuesday. Unobstructed S-SE flow in South France and East Spain will bring moist and unstable air masses near the shores, where we forecast some severe weather events. Moreover, excessive rainfall is forecast for NW Italy but the absence of CAPE suggest that no organized DMC activity will take place.

DISCUSSION

Low level forecast trajectories suggest a constant flow of moist air masses towards the coasts of France and Spain in NW Mediterranean and MLCAPE is expected to exceed 1000 J/kg, along with steepening mid-level lapse rates. Training thunderstorm activity from the early morning may involve into multicells and even isolated supercells into this highly sheared environment (DLS 20-25 m/s and SREH0-3 300 - 600 m²/s²). Low-tropospheric winds (at 850 hPa) in South France are expected to reach 15-20 kts offshore but 25-30 kts due to orographic channeling, towards the Rhone Valley. Excessive precipitation with flash flood risk (PW 30-35 mm) and severe wind gusts will be the main threats throughout the period covered by this outlook. In addition, during the night, thickness lowers over the level 2 area and a mid-level jet streak will cross NE Spain, let alone the long-lived low level convergence zones that high resolution models forecast (WRF). There will be enough lift and high shear juxtaposed with 1000 - 1200 J/kg MLCAPE for any storm to become severe. NCAPE values preclude the formation of large hail but smaller sizes cannot be ruled out (<2cm). Finally, waterspout threat is increased due to strong LLS near the coasts, slightly curved forecast hodographs due to veering winds and cold mid-level air masses.