This Saturday evening a lot of potential atmospheric energy was waiting to explode. A CAP needed to be broken; and this happened late in the afternoon at the expected western part of Belgium. The gross convection soon got the characteristics of a supercell on plain radar. Observations from the field confirmed my diagnosis.
The next hours I was in doubt 'to go or not to go' and checked the radarimages and the journey of the ongoing cell. At 18 UTC I decided to drive southward to catch at least a glimpse.
I was impressed by the structure that showed the updraft-area, precipitation area with 'stroboscope lightning' and a huge inflowtail. Anvilcrawlers were inviting me to come closerby, or were they chasing me away....?
Estofex:
" A level 1 was issued for northern France, Belgium, Netherlands mainly for severe wind gusts and tornado chance.
SYNOPSIS
Between a large low with a center pressure below 974 hPa some 500 km west of Scotland and a ridge of high pressure over central Europe, warm air is transported far northward over western Europe. Its warm front lies approximately over Denmark with the unstable warm airmass over Benelux and France, confined on the west side by a long cold front. The cold front will be over the UK at the start of the forecast period and over Denmark and Germany, and stationary in central France, by the end of the period.
DISCUSSION
...northern France, Belgium, Netherlands...
500-1500 J/kg MLCAPE is predicted by various models for this area. Based on GFS model, the LCL and LFC are not so close together for boundary layer parcels and much of the day can be considered capped. However, the uncapped parcel layer method does show parcels with low enough CIN for initiation. Initiation will depend rather strongly on forcing, which comes from the cold front and associated advection of low dynamic tropopause (PV band) and low level convergence. Several models hint at afternoon development of storms near the coastline with a second development later in the evening. The vertical wind shear environment can be classified as moderate, given 20-25 m/s 0-6 km shear vector magnitude and 100-200 m²/s² 0-3 km storm-relative helicity. A large part of the shear is concentrated in the lowest 1 kilometer (7-17 m/s). Some storms will have the potential for developing mesocyclones and can produce tornadoes (LCL heights are low as well) and large hail. Due to shear and storm system propagation parallel to the triggering front, linear mode is preferred and while not ideal for gusts, some bow echoes will likely develop and can cause local severe wind gusts. The more so since 1-3 km altitude winds reach 15-20 m/s and delta-theta-e (downdraft instability) is high: 16-20°. "
