Storm Forecast
Valid: Sun 15 Jul 2007 06:00 to Mon 16 Jul 2007 06:00 UTC
Issued: Sat 14 Jul 2007 19:54
Forecaster: DAHL
SYNOPSIS
Quasi-stationary longwave trough is positioned just W of centinental Europe ... in a favorable position for setting the stage for a convectively active period. Plume of steep mid-level lapse rates is advecting across France, the SE UK, Benelux, and W Germany ahead of this trough during the period. Main LL frontal boundary should extend from W Iberia across the Gulf of Biscay and the southern UK towards the North Sea and N Germany during the afternoon hours. A vort max at the E periphery of the large-scale trough and a SFC frontal wave attendant to it, will cross the British Isles on Sunday. Downstream upper trough exists over N Scandinavia, maintaining influx of moist and unstable air mass into this region and a persisting threat for partly organized deep convection.
DISCUSSION
... SE British Isles ...
It seems that an MCS will form late on Saturday night ahead of the developing fronal wave over the Biscay, which will affect the southeastern British Isles during the first half of the day. At the moment, it is not quite certain how well the air mass will recover in the wake of the MCS before the cold front crosses the region. However, at least isolated surface-based development should be possible ahead and along the cold front in the afternoon and evening hours. Although instability is likely to be somewhat meager ... strong shear profiles will be in place, and a few well-organized multicellular storms as well as supercells may occur. These storms may produce large hail and severe wind gusts, though a few tornadoes cannot be excluded either, given 0-1 km shear on the order of 10 m/s courtesy of the frontal wave.
... France ... Benelux ... N Germany ... N Poland ...
Additional, pre-frontal storms will likely develop across France, the Benelux countries as well as N Germany in the afternoon hours. This activity is expected to develop at the E/S periphery of the upper jet, so that the deep-layer shear will vary accordingly over the region of interest, ranging from about 15 m/s to 30 m/s. LLS will increase over N France, Benelux, as well as over W and N Germany during the day. The main issue will be whether or not storms will initiate, due mainly to strong capping/insufficient mesoscale ascent. Thinking is that allover coverage will remain rather low. Also, this activity may fail to become well organized given a lack of strong mesoscale ascent. However, intense shear profiles should aid some of the storms in becoming severe, main threats being large hail and severe wind gusts. However, increasing LLS over N France, Benelux, as well as over W and N Germany and N Poland suggests that a few tornadoes are also possible.
... western France ... Benelux, late Sunday evening/night ...
More widespread activity may occur towards late Sunday evening/night along the cold front over western France. Upscale growth into an MCS may occur, which appears to develop into increasingly weaker sheared air as it moves northeastwards, so that its degree of organization should decrease after about midnight. Still, 10 m/s LLS will likely exist in its path, and isolated surface based cells will have some potential of producing a tornado. Otherwise, marginally severe hail and win gusts should still be possible.
... NW Russia ...
Scattered storms should persist ahead of the Scandinavian upper trough. Deep shear of 20 m/s should maintain a threat mainly for isolated large hail and severe wind gusts.
