Storm Forecast
Valid: Wed 16 Sep 2015 06:00 to Thu 17 Sep 2015 06:00 UTC
Issued: Wed 16 Sep 2015 00:23
Forecaster: VAN DER VELDE

A level 2 was issued for parts of France, Belgium, Luxemburg, southeastern UK, Netherlands and western Germany mainly for severe convective wind gusts and tornadoes.
A level 1 extends over Denmark mainly for tornadoes and severe convective wind gusts.

SYNOPSIS

A low with a core pressure approaching 980 hPa moves from the Bay of Biscay rapidly through the English Channel region. Very tight mid level height gradients are in place across much of western Europe, with a 40 m/s 700 hPa wind maximum over central France. The left exit region of a 65 m/s 300 hPa jet passes over northwestern France, Belgium, southeastern UK and the Netherlands. This region of strong vertical ascent is more visible as the leading edge of a marked dynamic tropopause/PV feature.
GFS and other models develop 500-2000 J/kg of SBCAPE or 200-1000 J/kg MLCAPE, mainly advected from the Atlantic over France and onwards to the Netherlands. Meanwhile, 1-3 km average GFS model wind speed reaches 25 m/s over a wide area and low and deep level shear support organized thunderstorms.

DISCUSSION

Three areas of focus can be discerned: 1) The left exit / PV notch region with a fast cold front running over northwestern France, southeastern UK, Belgium, Netherlands and Denmark, 2) the occlusion area over the SE UK, and 3) the dragging cold front over eastern France to Luxemburg.

In region 1, convection-resolving models seem to forecast fast-moving isolated cells rather than a squall line. These may also produce severe wind gusts in their paths, and there a chance of tornadoes given 0-1 km low-level shear values over 10 m/s. SREH seems to be missing here which is sign of weaker or less curved hodographs in this region as a result of strong low-level cold air advection setting in quickly. This might also kill widespread convection chances if a clear west-east convergence zone at the surface cold front does not form. If it does, the threat of gusts would be more substantial if a linear system with bow echoes manages to form. This is not atypical for the area ahead of such strong PV intrusions.

In region 2 (UK) the occlusion stalls at the surface from 10-15Z with CAPE, a clear convergence zone and enhanced storm-relative helicity and low level shear, even if a bit displaced toward central England while convective initiation chances seem best in the southeast. This region and the English Channel area is highlighted by the steepest mid and low level lapse rates, with cold air advection aloft before the low levels stabilize. If low level S-R helicity and thunderstorms occur in the same location this region has decent chances of seeing a tornado. The 18Z GFS run shifts the incoming PV trigger more towards the UK.

In region 3 (central to northeastern France) a more consistent overlap of CAPE and SREH is predicted, while storms roughly drag along the same track, mainly in northeastern France where SREH could reach 400 m²/s² and 0-1 km shear 15-20 m/s, very favorable for mesocyclone-bearing storms with significant chances of tornadoes. Large hail is also possible. Tue 12Z-initialized mesoscale WRF models produce a squall line moving into western Germany, although in 18Z GFS the amount of CAPE is rapidly decreasing in the direction of its motion, which would not be favorable.