Forecast Update
Valid: Tue 05 Feb 2008 22:00 to Wed 06 Feb 2008 06:00 UTC
Issued: Tue 05 Feb 2008 21:57
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook issued at Tue 05 Feb 2008 10:04.

DISCUSSION

... Belgium, the Netherlands and NW Germany...

18Z IR2 and IR3 images show the upper-level trough over Ireland with the attendant upper-level streak entering the western English Channel. Placement of the jet and wind speeds derived from the satellite all indicate a good handling of the global forecast model on the current synoptic pattern over NW Europe.An area of enhanced convective activity currently evolves over the western English Channel and NW France with the first lightning reports.Eastward spreading of a compact but gradually weakening mid-/upper-level streak likely supports current activity and this continues during the rest of the forecast period through the level-1 area.

During the past few hours a few storms over SE UK exhibited deviant storm motion and tendencies for bowing structures. Local VWP data assists this with nice veering at lowest 2-3km. T-Td spread is low and as a vort max rotates through the area during a 21Z-06Z time-frame helicity values are forecast to increase again to at or above 200J/kg. Less moisture in the well mixed BL and rapidly departing lift will suppress convection during the next few hours with the main activity being confined along the SW / west coast of UK.

Further towards the east over Belgium and the Netherlands, dewpoints gradually rose during the day and the 8°C isodrosotherm now covers the whole area of interest.This is in accord what GFS forecast and the chance for storms to root into the boundary layer increases.Thickness values drop rapidly during the next few hours as cold air at upper-levels streams southeastwards and in combination with increasing lift of the approaching trough axis, scattered thunderstorms are still expected to develop.Latest VWP has some veering at lowest levels accompanied by an intense wind field of more than 30m/s just 1km above the surface. Despite the weakening trend of GFS during the next few hours, winds will still be strong with 20-30m/s at 850hPa and the more robust values over extreme NE France and Belgium.
There is a trend in a separation of the strongest instability and best shear fields, but nevertheless, helicity values are strong enough for storm rotation as thunderstorms become surface based. A few tornadoes are possible during the forecast period but strong wind field also favors a severe wind gust risk.

As thunderstorms move eastwards over NW Germany they will encounter an area with lower dewpoints and a more stable boundary layer due to better nocturnal radiation. Current thinking is that storms tend to become more elevated and weaken as instability eases betimes, but the risk for strong-severe wind gusts, marginal hail and an isolated tornado continues in the level-1 area.