Storm Forecast
Valid: Sun 03 Mar 2019 06:00 to Mon 04 Mar 2019 06:00 UTC
Issued: Sat 02 Mar 2019 20:05
Forecaster: TUSCHY

A level 1 was issued across N France and Belgium/Netherlands mainly for severe wind gusts and a tornado risk.

SYNOPSIS

A broad upper trough is analyzed over SE Europe. Numerous smaller-scale waves circle that trough and cross the far E Mediterranean. Upstream ridging covers a region from Spain to Italy while a very dynamic pattern evolves over NW Europe as prominent cyclonic Rossby Wave breaking will be underway. Accompanying decent cyclogenesis is forecast to affect UK during the day with an healthy CJ event. A second impulse approaches the English Channel from the W in the wake of the deep vortex. Dampened by the CAA, this wave starts to split with a weakening and lagging southern part and a more progressive northern part. The latter one exits the E-English Channel east until 06Z.

The deepening cyclone over UK pushes a warm front north to Denmark with a trailing cold front crossing parts of France and Benelux during the night. The wave interacts with the cold front and delays the eastbound motion of the front. This wave also pulls higher potential temperature WCB air west beneath cold mid-levels and therefore results in a CAPE/shear overlap and a confined corridor with enhanced severe probabilities during the night.

DISCUSSION

... E Mediterranean ...

An eastbound moving / deamplifying upper trough crosses the E Mediterranean and with cold mid-levels atop SSTs in the 17-19 C range, 400 to 800 J/kg weakly capped offshore SBCAPE evolve. Strongest synoptic-scale forcing departs east until noon, so the main activity probably occurs beneath the trough's base due to the enhanced vertical temperature gradient and weak CIN. Scattered thunderstorms bring heavy rain and isolated hail, but missing foci like pronounced convergence zones peclude an upgrade. With 100-200 J/kg LLCAPE in place, a waterspout risk is also present. During the night the activity diminishes from W to E.

... N-France to Belgium and the Netherlands ...

A nocturnal severe risk is forecast. Driving features for this confined area with enhanced severe were already described in the SYNOPSIS part. A splitting low-tropospheric trough results in enhanced post (cold) frontal convergence, which induces lift in a weakly capped low LCL environment. 200-400 J/kg SBCAPE evolve as the westward pulled WCB air mass gets overtaken by cold mid-levels and steep mid-level lapse rates. 25-30 m/s LL shear overlaps with the CAPE plume and forecast soundings reveal an equal mixing depth, so severe wind gusts in that speed magnitude are possible with any thunderstorm. This risk is reinforced along short line segments, as near parallel background flow to the storm motion favors downstream seeding of precipitation cores (probably resulting in a SW-NE aligned LEWP pattern). Strongest cores may also be accompanied by some hail.

During the trough passage more backed LL flow results in better LL hodograph curvature. Also deviating storms may ingest enough SRH for LL rotation. Combined with sub 800-1000 m LCL heights, an isolated tornado event is forecast with tail-end or more discrete thunderstorms.

Further south over CNTRL France, additional convection rapidly crosses the mid-level jet axis, which however features rather cold mid-levels. In addition numerous short-waves and finally the main wave from the west add enough lift for enhanced convection along the cold front. Rather warm EL temperatures question the chance for lightning, but with the forecast mid-tropopsheric temperature profile in place at least a low-end chance exists. Despite intense winds at all levels no upgrade was issued that far south as the shallow nature of the updrafts probably won't increase the fraction between the background wind and the gust speed noticeably. Additionally enhanced LL divergence along the cold front lowers the confidence of strongly forced convection. Still more concentrated swaths of severe wind gusts may occur along line segments and in addition a low-end tornado threat is also possible.