Storm Forecast
Valid: Tue 13 Sep 2016 06:00 to Wed 14 Sep 2016 06:00 UTC
Issued: Mon 12 Sep 2016 20:23
Forecaster: DAFIS

A level 2 was issued for parts of N Spain, France and UK mainly for excessive precipitation, severe wind gusts, tornadoes and to a lesser extent large hail.

A level 1 surrounds level 2 area for the main threats but with less coverages.

SYNOPSIS

A positive tilted trough breaks from the synoptic flow and turns into a cold pool over the Bay of Biscay during Tuesday. The WAA results in warm mid-levels over West Europe, the strong mid-upper level vorticity advection and many ingredients at the surface, converge to issue a level 2 area for widespread thunderstorm activity. Instability is also found in South Europe especially in Italy and Greece where numerous pulsating storms are expected, while in the rest parts of the continent a high pressure system will suppress convective phenomena.

DISCUSSION

During the first half of the day we expect a strengthening convergence line to move from Portugal eastwards and excessive precipitation is forecast, as well as large hail in N Spain where the steep lapse rates will feed some strong updrafts even though global models do not produce more than 500 J/kg of MLCAPE. Moreover these storms will be formed inside a highly sheared environment with a speed maxima (30 m/s) at 850 hPa over SW France suggesting severe to damaging wind gusts at the surface. After 12-15z the eastbound moving cold front on the surface increases the severe risk over France and UK, where NWP models predict a tongue of high effective PW (>30 mm) and strong 0-6km shear (directional and speed). Forecast soundings suggest deep saturated profiles, and modest SREH0-3km values (limited supercell threat) so large hail risk is rather limited or at least we expect very few events of hail larger than 2cm. Tail-end storms and training MCS activity also increases the flash flood risk. As storms near the coasts of France and UK will cluster in the late afternoon, the overlap of LLS/CAPE also increases the tornado threat. Thunderstorm activity will progressively move east during the early morning of Wednesday, with South France having an increased risk for serious flash floods.