Storm Forecast
Valid: Tue 03 Apr 2007 06:00 to Wed 04 Apr 2007 06:00 UTC
Issued: Tue 03 Apr 2007 01:27
Forecaster: VAN DER VELDE

SYNOPSIS

Northwestern Europe is dominated by a large high pressure area with its center northwest of Scotland. At its eastern flank, colder air is transported southward and forms a cold front reaching from Bretagne to southern Poland. The stagnant warmer, slightly unstable airmass is expected to yield isolated to scattered thunderstorms over France and southern Germany, probably somewhat more than yesterday as the cold front may provide a more organised source of lift. Insolation should not be a problem. Isolated to scattered thunderstorms are likely to occur over the Iberian Peninsula, parts of the Mediterranean Sea, as well as Italy and the western Balkan, where sea breeze circulations may trigger isolated convection. Vertical shear is mostly weak to moderate, leading to single cells and some clustering storms with negligible severe weather threat.


DISCUSSION

The strong upper jet over southern Iberia and the Tunesia-Srn Italy area increases chances that convection may produce a few severe weather events:

southern Portugal and southwestern Spain:

The secundary core of the upper low near Portugal should trigger convection also over land during daytime, with 0-6 km bulk shear values of 20-30 m/s. Although there is not much veering with height or increased low-level shear and only a few hundred J/kg CAPE, the area could yield more organised and/or more sustained convection with better chances of marginally large hail.

Tunesia, Malta:

Up to 25 m/s 0-6 km shear is forecast to be present, but here low-level winds should be veering with height so that 0-3 km storm-relative helicity of more than 250 m2/s2 could be present in areas with CAPE of a few hundred J/kg and steep mid level lapse rates. This area lies at the left exit of the jet and strong low-level convergence is forecast within the 9-10 g/kg moisture pool. Supercells may develop (Tunesia after 12Z) with a chance of large hail, though the strong linear forcing may favour more of a squall line later in the period with perhaps a few severe gusts. The southern parts possibly remain capped by heat advecting off the Sahara.

north of Sicily:

Stronger instability and convergence near a low pressure core likely will cause some thunderstorms, while enhanced storm-relative inflow under moderate shear conditions is expected to result in long-lived storms, capable of a few large hail events.