Forecast Update
Valid: Fri 05 Sep 2008 12:00 to Sat 06 Sep 2008 06:00 UTC
Issued: Fri 05 Sep 2008 12:08
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Thu 04 Sep 2008 18:13.

DISCUSSION

... E-France and W-Germany ...

Airmass over extreme W-Switzerland/E-France should stay capped with warm mid-levels and hence much of the forecast instability release will stay untouched.Moving away from this airmass to the west and north, a moist BL, cooler mid-levels but weaker lapse rates create an environment with some low-end instability release. If an intense forcing mechanism would cross the area, things could become very interesting, but strongest forcing will be well displaced ( Belgium/Netherlands). Shear at all levels becomes very intense with 0-3km shear of 25-30m/s and 15-20m/s LL shear and if low-topped convection evolves, a distinct tornado/ severe-damaging wind gust risk would arise. As a result the level-area was expanded well to the north, but the thunderstorm area was reduced as it will be questionable if shallow nature of thunderstorms supports enhanced lightning activity. This outlook remains very uncertain as even topographic effects could locally result in better thunderstorm/svr.wx. chances, which can't be forecast that far out.

... N-France, S-UK ...

Latest radar loops over NW-France show a broken line of enhanced convection, moving rapidly to the east/northeast. This trend should continue during the day over N-France as upper-level trough axis spreads northeastwards and as winds at 850hPa increase to 20-25m/s, the risk for severe wind gusts will be augmented. A broad level-1 area was introduced and well expanded to the NE.

Latest satellite images indicate that the core of the strong depression reveals strong convective activity, which spreads northeastwards during the next few hours. LL shear at 10m/s and at least some low-end veering at lowest levels could result in a few rotating updrafts with a tornado risk and a level area was issued.