Storm Forecast
Valid: Sun 27 Mar 2016 06:00 to Mon 28 Mar 2016 06:00 UTC
Issued: Sat 26 Mar 2016 15:13
Forecaster: TUSCHY

A level 1 was issued for NW France/SE UK mainly for severe wind gusts and an isolated tornado during the overnight hours.

SYNOPSIS and DISCUSSION

Prominent Rossby wave remains anchored over the NE Atlantic. Progressive pattern with numerous short-waves circling that wave boost thunderstorm probabilities over NW Europe. Ridge/trough pattern over CNTRL-Europe with rather dry/stable air precludes thunderstorm probabilities over most parts of remaining Europe. Greece and Turkey may see isolated thunderstorm activity beneath that trough, but nothing severe is forecast.
Strong westerlies impact the Alps and cause splitting flow pattern over that region. The outcome will be a healthy short wave, which affects N/CTRL Italy during the night.

Scattered to widespread thunderstorms occur over NW Europe with an onshore peak during peak heating. Offshore activity starts early but winds down after sunset as WAA from another depression causes more stable conditions during the night. Forecast soundings show thin/elongated CAPE profiles with weakly capped 200-500 J/kg SBCAPE. Hodographs feature compact appearance with marginal curvature. Widespread 100 J/kg SRH-1, LCLs aoa 1000 m and 50-100 J/kg LLCAPE cause a low-end risk for an isolated tornado/funnel event. Deviating storm motions may see temporarily enhanced probabilities for better organization due to larger SRH. However, weak speed shear and missing straight hodographs in forecast data indicate a low risk for splitting storms. Strongest cores may contain graupel/marginal hail. Most prominent threat however will be strong to isolated severe wind gusts with 20 m/s LL flow. The overall risk rapidly diminishes after sunset.

Another area of interest will be over the Ligurian Sea. As a response of the passing short-wave, strong southerly flow with modest BL moisture occurs during the evening/overnight hours. Combined with strong lift and mid-level cooling, a progressive but weakly organized MCS may evolve over the Ligurian Sea with a gradual motion to the SE. Heavy rain may affect the coastal areas. Progressive nature precludes level area for now.

Another depression over the N-Bay of Biscay will move to the NE during the night while gaining strength. 1PVU maps show a steep horizontal potential temperature gradient and partial overlap of SE-ward surging cold front with eastward spreading dry mid/upper level air. A few limiting points exists however: the cold front outruns steeper mid-level lapse rates during the end of the forecast and the surface front seems to be affected by strong subsidence at mid-levels. However a small corridor of overlap may exists and a convectively enhanced line with showers/isolated thunderstorms could evolve beneath the cyclonically curved 45 m/s mid-level speed max. Shear would be supportive for severe wind gusts and isolated tornadoes. Due to ongoing uncertainties, a level 1 was issued.

It has to be mentioned that past model runs (f.ex. GFS) showed a classic pattern for an organized convective line with more widespread enhanced convection. The development of this depression has to be monitored closely in case of better overlap of ingredients compared to current data.
The level 1 was expanded to SE UK, which could also be affected by the cold front.