Storm Forecast
Valid: Mon 25 Jun 2007 06:00 to Tue 26 Jun 2007 06:00 UTC
Issued: Sun 24 Jun 2007 22:09
Forecaster: GROENEMEIJER

SYNOPSIS

An important feature on the weather maps of monday morning at 06:00 UTC is a cut-off low over the English Channel. This feature is expected to move eastward and the lift northeastward later on Monday. Ahead of the system, a strong southwesterly jet at mid- and upper levels is present.

DISCUSSION

France, Germany, Poland, Czech Republic, Switzerland, northern Austria...

Scattered convective activity is expected to be ongoing over parts of the Benelux, Germany, eastern France and Switzerland early Monday morning. This activity is associated with rising motion in the warm advection regime and partly with activity initiated by an upper vorticity maximum over far northeastern Germany.
Within the wake of the vorticity maximum, convective coverage is expected to decrease slightly during the early morning before storms reform ahead of the cold front associated with the approaching a upper trough/cut-off low.

As a rather intense surface cyclone forms across England, SWly low level flow should increase over the western Europe continent leading to rather strong low-level shear.

Strong forcing for vertical motion is expected to especially affect the Benelux countries, that should find themselves below the left exit of an increasingly strong SSWly jet streak. Forcing will be less further to the south and east.

Instability however may be somewhat higher over Germany, Poland, the Czech Republic and Austria that will likely see the formation of a squall line moving rapidly eastward through the region during the afternoon and evening. Strong winds -in excess of 20 m/s winds at 850 hPa- suggest that strong and damaging wind gusts could be rather widespread. The strong shear additionally suggests that a few short-lived tornadoes are possible as well along the squall. Isolated storms that may form ahead of the squall line will have the possibility of producing large hail.

The wind threat associated with the squall line should slowly diminish during the late evening and night when it is over S Poland , the E Czech Republic and Austria.

Alps, Extreme northern Italy...

More storms are likely within the Alpine region and on the southern Alpine flanks. There the strongly sheared flow suggests that strongly rotating storms are likely. A number of storms with a main threat of large hail and potentially some damaging downbursts are expected.

Benelux and NW Germany...

Over NE Germany and the Benelux countries, strong upward vertical motion in the dry slot of the system suggests that considerable destabilization will likely occur in the postfrontal environment. This likely leads to steep low-level lapse rates that, in concert with strong low-level shear could principle create a favourable environment for tornadic low-topped supercells. The currently forecast magnitude of low-level shear appears a bit marginal for tornadoes with 0-1 km shear around 8-10 m/s. If stronger low-level winds occur than currently forecast, an upgrade to level 2 may be necessary. In any case, some large hail around 2 cm in diameter appears possible. The storms coverage and intensity should weaken during the late evening.