Storm Forecast
Valid: Thu 31 Jan 2013 06:00 to Fri 01 Feb 2013 06:00 UTC
Issued: Wed 30 Jan 2013 23:25
Forecaster: PISTOTNIK

A level 1 was issued for the Netherlands, Belgium and parts of Germany for severe wind gusts.

SYNOPSIS

To the South of an almost endless chain of deep cyclones from the Northern Atlantic via the British Isles into Scandinavia, a pronounced zonal flow continues over much of Europe. An occlusion which rapidly moves eastward across North-Central Europe provides a possible focus for a strongly forced convection event.
Meanwhile, a subtropical ridge extends into most of the Mediterranean region and presents it with calm and dry weather.

DISCUSSION

...North-Central Europe...

The occluded frontal system of a mature low over Scotland gets reinforced in the left exit region of a jet streak (60 m/s at 500 hPa) as it moves from the English East coast via Germany into Poland in the 06 to 18 UTC time frame. The accompanying signal in the surface fields seems subtle, as it is dwarfed by the overall strong pressure gradient. However, the combination of a strong background wind field and slightly backed surface winds is sufficient to drive the 0-3 km storm-relative helicity into the 200-300 m^2/s^2 range, overlapped by synoptic lift support and neutral to marginally unstable profiles in the lower troposphere.
The lack of a pronounced frontal structure and the slantwise orientation of the vorticity lobe to the mid-level flow suggest that a convective line (if any) will soon give way to a small flock of shallow multicells as the dominant convective mode. Showers and isolated thunderstorms are expected to make landfall in Belgium and the Netherlands around 10 UTC and to move into Northwestern Germany in the following hours, and they might attain briefly rotating updrafts in this helical environment. Sub-severe soft hail is locally possible and even an isolated tornado is not ruled out. However, all in all this threat is limited by a lack of low-level buoyancy and by a displacement of the deepest instability (further North) from the strongest vertical wind shear (further South). The only uniformity in this highly dynamic setup is the strong low-level wind field (30-35 m/s at 850 hPa), which makes the severe wind threat solid enough to issue a level 1 area, even though the majority of convection will be too shallow for electrification.
The favorable overlap of strong vertical wind shear, lift support and marginal instability is quite limited in space and time, hence the wind risk will taper off in Central Germany, Poland and the Czech Republic in the late afternoon and evening hours. A further involvement of convection becomes doubtful, too.