Storm Forecast
Valid: Sun 22 Jun 2008 06:00 to Mon 23 Jun 2008 06:00 UTC
Issued: Sat 21 Jun 2008 23:25
Forecaster: GROENEMEIJER

SYNOPSIS

A very intense SWly jetstreak over Britain at 6:00 UTC with over 40 m/s winds in its core at 500 hPa is translates eastward into the European continent. Over most of France, the Benelux and Germany, a warm and humid air-mass is intially present ahead of a cold front that moves eastward. An important feature is a shortwave trough with its axis about 500 km SW of Ireland initially that crosses southern Britain in the early evening and will then proceed eastward to reach southeastern Sweden Monday morning.

DISCUSSION

Benelux countries, N and central Germany, southern Denmark, Extreme S Sweden, Baltic Sea, Poland..

During the morning and early afternoon, ample low level moisture is advected northward by an intensifying southwesterly flow. This results in MLCAPE values near 1000 J/kg in the northern Benelux to more than 2000 J/kg across central France. A zone of low-level convergence effectively marking the surface front is expected near a line from central Jutland via Groningen and Paris to Nantes around 12 UTC. At that time, convective initiation should be underway despite upper-level forcing for upward motion being not very strong.

Strong deep-layer shear (0-6 bulk shear ~25 m/s, increasing to 35 m/s across N Germany in the evening) will aid in rapid storm organization. Some storms that manage to stay isolated for a while may become strong supercells capable of producing large hail and damaging winds. Low LCL heights and low-level speed shear will allow for a few tornadoes also, some of which may be strong (F2 or higher). Given the nature of the forcing, the storms will likely merge relatively rapidly into one or more clusters of linear convection.

During the mid-evening, these convective systems will likely start an eastward acceleration on approach of the shortwave from Britain. It appears likely that one or two bowing quasi-linear systems with damaging winds will then move eastward across the area labelled level 2. A few embedded supercells are well possible given the strong shear so that some short-lived tornadoes and large hail will be a continuing threat.

In the late evening and overnight, convective activity should weaken somewhat. However, the strong low-level wind field indicates that the threat of damaging winds will likely continue as the systems move onto the Baltic Sea and into Poland.

South Germany and north, central and SW France...

Somewhat weaker forcing is expected in this area, and the cooler air will probably not protrude much further southward than a line from Bordeaux to Basel to Salzburg Monday morning. High values of CAPE of 1000-2000 J/kg are forecast across France and later SW Germany. Scattered storms are expected to develop during the late afternoon near the approaching frontal zone. With about 20 m/s deep-layer shear these cells will likely include a few supercells with large hail, flash floods and strong downbursts. Overnight, storms will probably continue but become elevated with the severe weather threat gradually diminishing.

NW Spain...

An small but intense mid-level trough filled with cold air has its axis along the Iberian west coast on Sunday morning. Ahead of this system, isolated to scattered storms are forecast to develop across the mountains of northern Spain during the late afternoon and evening. Although CAPE values are expected to be relatively low, strong shear and storm-relative helicity will be sufficient for the development of rotating updrafts, which may result into a few large hail events and local strong winds.

Southern Alps...

Despite the fact that deep-layer shear is rather weak, high values of CAPE could produce a few severe events with storms developing here in response to strong diurnal heating. The main threat will be local large hail.

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