Storm Forecast
Valid: Sat 10 Sep 2011 06:00 to Sun 11 Sep 2011 06:00 UTC
Issued: Sat 10 Sep 2011 02:42
Forecaster: VAN DER VELDE
A level 1 was issued for northern France, Belgium, Netherlands mainly for severe wind gusts and tornado chance.
A level 1 was issued for the United Kingdom mainly for a chance of severe wind gusts and tornadoes.
A level 1 was issued for Finland, Estonia, Latvia, western Russia mainly for spout-type tornadoes.
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 mē/sē 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°.
...UK...
CAPE is lower than over the European mainland and cloud tops are warmer (lower), it is a more messy situation, but several bands of forcing pass over the region, GFS produces surface heating signals around noon, LCL heights remain very low, and low-level shear is rather strong with 12 m/s, supporting tornadoes. During the afternoon a clear overlap of shear, mid level winds >22 m/s, and instability is sketched over the southeastern UK. Isolated severe wind gusts and isolated tornadoes are the main threat.
...Finland, Estonia, Latvia, western Russia...
Very slow cell motion, a moist, low shear environment and a few hundred J/kg CAPE concentrated in the lower levels should lead to several occasions of spinning-up of vertical vorticity in the form of land- and waterspout type tornadoes. A convergence zone is depicted by GFS at the southwest coast of Finland. Local models should be inspected for more precise locations. Isolated excessive rain sums may occur due to slow storm motion.
...central Europe...
Models indicate several isolated spots of convective precipitation in the unstable airmass, mainly along south side of the Alps and Czechia. Deep layer shear can reach around 15 m/s, partially due to local terrain winds and large hail is possible, but given low storm coverage no level was drawn.
