Extended Forecast
Valid: Sun 22 Jun 2008 06:00 to Mon 23 Jun 2008 06:00 UTC
Issued: Fri 20 Jun 2008 23:40
Forecaster: GROENEMEIJER/TUSCHY

SYNOPSIS

On Sunday morning a broad ridge in the mid and upper-level flow extends from southern Scandinavia to the central Mediterranean. Upstream, over Britain, the English Channel and the Celtic Sea an intense 35 m/s mid-level southwesterly jet translates eastward moving over the Benelux, NW France and NW Germany during the afternoon. In the lower troposphere, warm and humid air over these areas will be replaced by cooler air as a cold frontal system moves eastward.

...NW and N France, Benelux countries, much of Germany, Denmark...

Relatively high values of low-level moisture (with dew points around 20 C are expected, which should render the atmosphere quite unstable in a zone ahead of the cold-frontal system. There, mixed-layer CAPE of 1000 - 2000 J/kg should be typical values. Numerical models suggest that the forcing from upper-level troughs may be small during the afternoon, but naturally, the details are quite uncertain at this time. Despite the relatively weak forcing, GFS and UKMO both develop scattered storms in vicinity of the front. Current thinking is that storms will already form over Benelux and northwestern France early in the day and will do so further south and east in the late afternoon and evening.

The environment in which the storms are predicted to initiate is characterized by strong to very strong deep-layer shear (30 m/s bulk shear in the 0-6 km layer, and 10-15 m/s in the 0-1 km layer). Such strong shear supports supercell development with intense rotating updrafts with large hail. With southwesterly 850 hPa winds of 20 m/s expected across the Benelux and later NW Germany, the storms would have a high threat of producing severe winds, especially after the cluster into bowing segments. Wind damage may well be the greatest hazard of this episode, even though the small angle between the front and the jet (at least intially) is not favorable of long-lived bow echoes. Further south and east, across E France, SW and central Germany where shear is weaker, the threat of severe winds and hail is more localized. In addition to wind and large hail, a few tornadoes are possible too, which appear most likely across northern parts of the threat level area where cloud bases are lowest.

A level 2 will likely be issued for those areas that appear to be most a risk late Saturday evening.

... N 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.


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