Storm Forecast
Valid: Sat 17 Mar 2007 06:00 to Sun 18 Mar 2007 06:00 UTC
Issued: Fri 16 Mar 2007 19:36
Forecaster: TUSCHY

SYNOPSIS

Significant changes in the upper troposphere / lower stratosphere will occur during the next few days. The western part of an elliptical shaped polar vortex will broaden and will expand towards Europe, which will produce a complete shift in the prevailing weather pattern, compared to the past few months.
Pretty busy period is already underway for northern Europe with one upper-level trough crossing an area between Norway and Belarus from the west, while another, even more powerful upper-level trough will approach Europe from the NW and will affect Ireland, Scotland and United - Kingdom during the latter part of my forecast period.
A pretty well defined cold-core upper-level low will continue its SW-ward shift, crossing Spain and S-Portugal from the NE, while high geopotential heights over central Europe will suppress any convective activity.
Cool and stable conditions prevail over eastern / northeastern Europe.


DISCUSSION

... Spain, Portugal, Sardinia and the coastal areas of Algeria and Tunisia...

A region of low geopotential heights over SW Europe will stretch from Portugal towards the east, while a constantly weakening cold-core upper-level low over Spain will shift towards the SW.
Compared to yesterday, lapse rates at mid-levels will relax somewhat and this should limit storm coverage, although strong diabatic heating will be sufficient again for a few thunderstorms to evolve.
A marginal cool-down of the lower tropospheric thermodynamic profile south and southeast of Sardinia will be the conducive for a few thunderstorms to develop, too.
Predominating thermodynamic / kinematic parameters won't justify any severe thunderstorm threat.

...Scotland...

A strong cold front will traverse the region from the NW during a 22Z - 03Z time-frame.
Right now there are no signs of any robust instability release along the frontal zone.
The main focus for a few thunderstorms arises in the postfrontal airmass, where up to 200 J/kg SBCAPE and rapidly steepening lapse rates will support a more favorable environment for thunderstorm development.
LL shear of more than 17m/s and SRH 1km values of up to 200 J/kg will be fine for an isolated severe wind gust and local tornado threat.
Uncertainties regarding the magnitude of the thunderstorm coverage ( best environment will approach during the end of my forecast ) and regarding the final airmass quality ( pretty high LCLs / low dewpoints ) don't justify any higher probabilities....at least for the moment.

... Parts of Poland...

An eastward rambling upper-level trough will bring an intense cool-down at 500 hPa over most parts of Poland with an attendant increase in mid-level lapse rate strength.
Modified soundings from this area indicate some low-end instability release . Convective activity should stay low-topped, also indicated by EL-heights in the range of 3.5 - 4.5 km.
Despite the fact that only a trickle of expected 25-40m/s DLS can be utilized, 25m/s at 850hPa should be easily mixed down in stronger convective segments. Beside the severe wind gust risk, there will also exist an isolated tornado risk...especially in storms, which tend to move off the hodograph.
A level-1 was issued for N-Poland, where the strongest instability release can be expected ( about 100-200 J/kg SBCAPE ).
Would not be surprised to see a few SFLOC reports beyond the general thunderstorm area, but sporadic coverage of storms would not support a broader region.