Storm Forecast
Valid: Sat 11 Apr 2009 06:00 to Sun 12 Apr 2009 06:00 UTC
Issued: Fri 10 Apr 2009 20:05
Forecaster: GROENEMEIJER

SYNOPSIS

An elongated north-south oriented trough stretches from Iceland to southern Iberia. A mid/upper cyclone cuts off form the base of the trough and moves from SE Spain to N-central Algeria during the forecast period. A large ridge is located over Scandinavia and the European continent. Within the ridge though, two small cut-off lows are present, one centered near St. Petersburg, the other over the Polish/Belorussian border region. A lobe of warm air flows northward across western parts of the European continent.

DISCUSSION

Tunesia...

As a strong jet streak rotates around the southern flank of the aforementioned cut-off low, an active surface cyclone is expected to develop across the Great Erg, which moves northward and reaches northern Tunisia on Sunday morning. As the left exit of the jet streak starts to interact with lingering low-level moisture over central and northern Tunisia on Saturday evening, convective storms are expected to initiate, that will likely organize linearly. Given about 25 m/s deep-layer shear, these may include rotating updraughts. Some threat of large hail and severe winds is expected with the system, warranting a level 1.

France, Germany, Benelux...

Within the thermal ridge over the area a few storms will likely develop. Rather limited low-level moisture -dew point temperatures will be in the 5-10 C range- will probably keep CAPE values rather low, on the order of 500 J/kg. The storms will likely not be very intense. Some risk for strong gusts and (mostly small) hail however exists, mainly across the northern Netherlands and western Lower Saxony, where wind shear is highest (i.e. 10-15 m/s in the 0-3 km layer). Current thinking is that this risk is too low for a level 1.

Spain...

Associated with the upper low over SE Iberia is a weak cyclone centered near the Balearic Isles. On its northern flank widespread stratiform and partly convective precipitation is forecast. In areas of onshore/upslope flow across Catalonia and Valencia, high precipitation amounts are expected of 25-50 mm in some locations, and locally even more, which could locally cause flooding problems. Otherwise, the threat of convection should be low however.