Storm Forecast
Valid: Tue 08 May 2007 06:00 to Wed 09 May 2007 06:00 UTC
Issued: Tue 08 May 2007 10:02
Forecaster: VAN DER VELDE

SYNOPSIS

A strong zonal flow pattern is in place between a low over Scandinavia and higher pressure over the Mediterranean. Especially in the middle and upper levels a lot of wind (polar jet) is present in a wide band stretching from British Isles into Belarus, in combination with slight instability. The subtropical jet branch affects the southern Balkan and Turkey.
Within the flow some shortwave troughs will enhance convection over central/western Europe.

DISCUSSION

...areas in western/central Europe...

Convective cloud top temperatures colder than -20C and convective precipitation has been indicated by GFS over large parts of Europe during the forecast period. Areas in central Europe where strong deep layer shear and low level shear are present in unstable air, as well as troughs coming through, have been given a level 1 for threats of isolated severe gusts and isolated tornadoes. Storms will likely tend to organize into lines with bowing segments and fast storm motion (>20 m/s) increasing chances for severe gusts, while embedded mesocyclones may develop as result of the strong (20-40 m/s) deep layer shear and decent (8-15 m/s) 0-1 km shear. Low LCLs and locally strong low level buoyancy (as visible in e.g. De Bilt, NL sounding of 00Z) are also supportive of tornadoes.

The German/Polish area is mostly for afternoon convection. The area of northeastern France and southwestern Germany is not included in a thunder area for the reason that forecast cloud tops are rather warm, though lifted index will reach under 2 indicating marginal conditions... when GFS indicates strong low-level instability around 00-03Z, within very strong low-level shear and >250 m2/s2 0-3 km SREH under the jet axis, 2000-4000 meter lapse rates show significant stabilisation. Lower topped convection (say 4 km deep) may produce severe gusts and a threat of tornadoes.

Earlier, during the afternoon, slight instability may occur in northeastern France with about the same shear conditions but weaker low-level instability but colder cloud top heights at the passage of a slight mid-level trough and associated frontal wave. GFS precip supports this. The same threats apply.
At the other side of the Alps some instability may develop in the Hungary area, mostly during late afternoon/evening. When the wave/jet exit passes this could force the same type of convection as described before.


...Greece and Turkey, southeastern Bulgaria...

GFS indicates a band of instability crossed by at least moderate deep layer shear but no low level shear of importance. High LCL heights around 2000 meter (00Z soundings show even higher...) and cold downdrafts induced by dry mid level air (surface downdraft temperature drop possibly more than 12 degrees), together with the mid level wind field, create a threat of severe gusts with any convection that may develop... GFS model is not very enthusiastic about precipitation over southeastern part until in the evening. Will barely mention a chance of large hail when storm updrafts prove long-lived, but instability should be only marginal.