Storm Forecast
Valid: Sat 06 Aug 2011 06:00 to Sun 07 Aug 2011 06:00 UTC
Issued: Fri 05 Aug 2011 19:20
Forecaster: GROENEMEIJER

A level 1 was issued for parts of France, Switzerland and southwestern Germany, mainly for heavy rain and secondarily for large hail.

A level 1 was issued for parts of eastern and southeastern Germany and the western Czech Republic, mainly for large hail.

A level 1 was issued for parts of northern Germany, northeastern Netherland and extreme southern Denmark, mainly for isolated tornadoes.

A level 1 was issued for extreme NE Algeria and Tunisia, mainly for large hail and damaging wind gusts.

SYNOPSIS

Upstream of a broad ridge over eastern Europe, a trough moves into western Europe. A trough/vorticity maximum initially located to the southwest of Ireland is expected across the southern North Sea on Sunday morning.

DISCUSSION

...France, Switzerland, SW Germany...

Ahead of the mid/upper-level vorticity maximum a stable frontal wave is expected to move northeastward across the area. Forcing for upward motion of a moderately warm and quite humid air-mass should lead to widespread convection across the southeastern half of France, whereas stratiform rain will likely dominate further to the north and west. The high precipitable water content of this air-mass suggests some threat of excessive rainfall will exist. In addition, locally a marginal large hail event cannot be discounted, for example over the Massif Central.

... E and SE Germany, W Czech Rep...

Isolated thunderstorms in an environment of somewhat steeper lapse rates and about 1000-1500 J/kg MLCAPE may prove sufficient for some marginally severe hail events in the afternoon and evening.

... N Germany, NE Netherlands, extreme S Denmark ...

Near a surface warm-front that lifts northward, a few weak tornadoes may develop. Reasons include moderate 0-1 km shear around 10 m/s per GFS and the presence of a very humid boundary layer. Moreover, a strong wind shift should exist along the front and numerical guidance produces copious amounts of convective precipitation. Poor deep-layer shear prevents a more rubust threat.

... Extreme NE Algeria and Tunisia ...

Model guidance suggests convective initation will occur in presence of about 1000 J/kg MLCAPE and weak to moderate deep-layer shear. Lapse rates will be steep and LFC high, so that deep broad updraughts capable of producing some large hail may form. Additionally, strong evaporative cooling should be possible, creating a modest threat of downbursts.