Forecast Update
Valid: Fri 10 Apr 2009 14:00 to Sat 11 Apr 2009 06:00 UTC
Issued: Fri 10 Apr 2009 14:04
Forecaster: GROENEMEIJER

SYNOPSIS

Refer to the original convective forecast

DISCUSSION

Northern France, Benelux...

Wind shear in the lowest 3 kilometres is stronger than anticipated in the original forecast, with the radiosonde of De Bilt and the wind profiler at Zaventem indicating around 20-25 m/s at 700 hPa. As a result, the chance of storm organization has increased. Two important requirements for severe wind gusts are now met, which are i) a deep dry boundary layer and ii) strong low-level wind shear. Models and soundings suggest that an axis of instability (500-1000 J/kg) is developing from the Netherlands SSEward across Belgium, Luxemburg to NE France, ahead of a surface trough. Along this axis and to it west towering cumulus clouds are developing. It appears likely that scattered storms will initiate soon. Strong evaporational cooling below cloud base, downward transport of horizontal momentum, possibly aided by some storm organization has the potential of causing some strong and a few severe (>25 m/s) downbursts. Additionally, an isolated large hail (2-3 cm) report is not ruled out. Clustered weakening convection should move onto the North Sea in the (late) evening, mitigating the threat over land.

Spain...

Along a cold front tracking eastward over the Iberian Peninsula, several linear segments of deep convection have formed. The Zaragoza sounding at 12 UTC indicated that about 20 m/s and veering/strengthening low-level winds with altitude are in place ahead of that system. Updraught rotation is therefore a possibility, which enhances the risk of some hail and some isolated severe winds (25 m/s) are possible. A brief tornadic spin-up is not ruled out in the Ebro Valley. The overall threat, being limited by very modest amounts of CAPE (500 J/kg), marginally warrants a level 1. Model guidance suggests that the storms will likely loose intensity as they approach the Mediterranean. Overnight, however, new convective development is expected over the northwestern Mediterranean Sea and Catalonia. This acitivity has the potential of producing very heavy rainfall in places.