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

SYNOPSIS

A low deepening to below 960 hPa north of the British Isles at 00Z Tuesday is forecast by GFS to track towards northern Sweden/Finland by 06Z Wednesday. The accompanying pressure gradient field is likely to cause damaging non-convective severe winds. Some convection is anticipated on the cold front or perhaps a postfrontal line, given consistent signal of cold GFS equilibrium level temperatures near the nose of the mid level potential vorticity intrusion that moves through the level 1 area. NMM model highlights a consistent CAPE area as well. In general, lightning activity is expected to be sparse unless a more dynamic convective system manages to form ahead of the PV max.

Another area of thunderstorms, in a less dynamical setting, is created by an upper cut-off low southeast of Italy. The jetstream rounds the area with CAPE without overlapping, so more than an isolated waterspout is not expected.

DISCUSSION

...Southern Norway, central Sweden, southern Finland...

A level 1 has been issued because the threat of tornadoes seems significantly enhanced over climatology: a few events are possible. In addition, already potentially damaging strong low-level winds plus organized (with some uncertainty) convective activity could yield gusts up to 60-70 kts.
The tornadic threat is created by very strong low level shear (0-1 km) over 20 m/s, LCL heights under 500 m, LI under 1, under a jet axis that creates 50 m/s (no, not knots!) deep layer shear (0-6 km). SREH values range over 250 m2/s2 so there is definately potential for rotation in storms. Strong dynamic lifting ahead of the mid level vorticity maximum in the area of low lifted indices should be sufficient to generate some active storms: Norway around 12Z, Sweden towards 18Z, 21-02Z, Russia and perhaps northern Estonia 03-06Z, if GFS 18Z ini model is on the right track.

...southern UK...

GFS and NMM both give some possibilities for convection for late in the forecast period (say after 21Z), around a not much developing frontal wave. Though instability and lift don't look that exciting, the area will have adequate low-level and deep layer shear and SREH for isolated tornadic activity if convection is able to form.