Storm Forecast
Valid: Mon 07 Apr 2014 06:00 to Tue 08 Apr 2014 06:00 UTC
Issued: Sun 06 Apr 2014 21:25
Forecaster: TUSCHY

A level 1 was issued for parts of UK mainly for an isolated tornado event.

A level 1 was issued for NE France, Belgium, parts of the Netherlands, Luxembourg and parts of extreme W/NW-Germany mainly for an isolated tornado, severe wind gust and large hail event.

SYNOPSIS

A positively-tilted trough, located over W-Ireland and the extreme western Bay of Biscay at 06Z should become neutrally or even slightly negatively oriented while crossing UK/Scotland and entering the North Sea during the end of the forecast period. This allows rather strong mid-layer height falls to affect N-France, Benelux and UK. Advancing surface pressure trough deepens over the English Channel and over the North Sea. Global models show an ongoing large spread in where and when to initiate weak low-tropospheric cyclogenesis. Combining run-to-run consistency and model spread, the most likely track of this depression will be from the N-UK to the N-North Sea. The dynamics at mid/upper levels won't support strong deepening. Despite ongoing uncertainties in track and strength of this depression, a plume of modified subtropical air advects to the NE and affects regions from N-France to Denmark. This air mass will be found in a warm sector, which is framed by a leisurely eastward moving cold front which affects UK, France and Benelux and a sharpening warm front over N-Germany/Denmark.
A strong ridge has a lock on the western part of the Mediterranean and Central Europe. Quiescent conditions prevail.
Long-lived cut-off over the Central Mediterranean starts to open up and it becomes incorporated in the large-scale cyclonic flow over NE Europe. This supports a persistent N-erly flow regime over SE-Europe. Rather dry continental air advects far south and remains mostly separated from steep mid-layer lapse rates over Turkey. Still, some areas over SE-Europe see an overlap of both ingredients with regionally enhanced CAPE-build up.


DISCUSSION

... E/SE-UK ...

Incoming wave still offers a neutral or even a slightly positive tilt, which raises concerns about warm sector quality in respect of CAPE-build-up. Combination of rich BL moisture and neutral mid-layer lapse rates should keep the atmosphere in the proximity of breaking the cap, although GFS favors an even strenghtening cap around noon with faint warming of the 850-700 hPa layer. Therefore, isolated and elevated thunderstorm activity proceeds the cold front and affects the warm sector until noon. Nothing severe is forecast .
Focus for enhanced convection remains along the eastbound moving cold front, which crosses E/SE-UK during the late afternoon/evening hours. Cooling mid-layers and incoming forcing (synoptic-wise and due to the front itself) with rich prefrontal moisture probably assist in 200-500 J/kg MLCAPE along the front. Strong 0-6km bulk shear in excess of 20 m/s, enhanced BL shear (directional and speed shear) and low LCLs cause a wind gust and isolated tornado risk mainly with more discrete cells ahead/along the cold front. Marginal angle of the deep-layer shear vector to the front keeps the risk of longer-lived discrete structures low however. As the cold front leaves the forecast area to the east around sunset, thunderstorm probabilities drop rapidly from W to E.

... Central/N-France, Benelux and W/NW-Germany ...

The aforementioned cold front seems to undergo a temporal speed-up over NE France and Benelux before slowing down again over Germany/Denmark during the end of the forecast. EZMWF and GFS both agree in a surge of prefrontal moisture all the way to NW-Germany and despite GFS/WRF running slightly higher with BL dewpoints, confidence in lower tens is high, as dewpoints of 13-14 °C already cover W/NW-France. This moisture counteracts weak mid-layer lapse rates, so combination of some diurnal driven diabatic heating and the approaching front with increasing (synoptic) background forcing enhance thunderstorm probabilities over France and to the NE.

Initiation is forecast over central/northern France around noon with thunderstorms spreading east/northeast during the afternoon and evening hours. Shear at low to mid troposphere support a few better organized pulsating storms or multicells with strong wind gusts and marginal/isolated large hail. After sunset, storms continue well into the night over E-France as they approach the Vosges and Jura Mountains. Despite a slowly cooling BL, ongoing advection of moist air assists in a weak cap, so gusty winds and marginal hail may occur over E-France well after sunset. Bursts of heavy rain are possible with effective PWs between 10-20 mm, but progressive nature of convection should keep the rain risk in check.

Further to the NE over NE France, Benelux and NW Germany, stronger forcing and BL mixing ratios in excess of 8 g/kg but weak mid-level lapse rates and quite strong capping create a complex forecast regarding coverage of storms. Latest idea is that a temporal increase of the front's forward motion and a rather large angle between the deep-layer shear vector and the front itself might support a more organized line of deep convection over NE France and Benelux. A broken line with bowing line segments could evolve. Any more discrete cell could bring marginal/isolated large hail and strong wind gusts. We can't rule out an isolated tornado event as low LCLs and 25-30 kt LL shear (with enhanced directional shear) overlap. Unseasonable depth of the BL moisture partially offsets more negative impact of time of the year and late daytime hours, so near surface based convection is possible well after sunset over Benelux into far W/NW Germany. As BL flow/shear remains enhanced, an isolated nocturnal tornado event can't be ruled out.
Elevated but non-severe storms (on an isolated scale) spread far east and affect most of S/E Germany during the night.

... SE-Europe ...

An overalp of regionally enhanced BL moisture and mid-layer lapse rates around 7K/km support a few spots with 500-800 J/kg MLCAPE. Diurnal driven pulsating storms evolve within a weakly sheared environment. Marginal hail and locally heavy rain will be the main hazard. An isolated large hail event is possible over W-Turkey, where steep lapse rates cause some fatter mid-tropospheric CAPE profiles in forecast soundings.

... Ireland ...

Enhanced marine convection may turn thundery during the afternoon hours. Nothing severe is anticipated.