Extended Forecast
Valid: Thu 21 Feb 2008 06:00 to Fri 22 Feb 2008 06:00 UTC
Issued: Tue 19 Feb 2008 20:06
Forecaster: TUSCHY

Predominant blocking pattern completely alters into a high index situation as belt of intense upper-level winds spreads eastwards south of a compact polar vortex. Attendant rise of geopotential heights SW of the UK and over France makes sure that highly E-W elongated upper-level trough splits up over SW Europe with a discrete upper-level cut off evolution west of Portugal. The rest of this trough rapidly translates eastwards. All those features will pose a risk for thunderstorm initiation.

The first area will be south / southwest of Portugal in the range of the developing cut-off low. Shear will stay weak, LL lapse rates relax and hence no severe weather threat is anticipated right now. Thunderstorm activity should ease after sunset.

The second area for a few thunderstorms arises southeast of Italy as the upper-level trough shifts eastwards mainly after 15Z. GFS has nice LL CAPE in this area during the morning and early afternoon hours with an attendant waterspout risk. In addition, GFS develops a convergence zone in this area which could serve as another focus for waterspouts. Too many uncertainties that far out so only a small thunderstorm area was issued.

The third area will be along the coastal areas of northern Norway after midnight as a very cold mid-level airmass overspreads an occlusion just west of central Norway. Very cold EL temperatures and some instability release could support a few vigorous updrafts and thunderstorms. A general thunderstorm area may be needed but right now confidence is low that thunderstorm coverage could justify a general thunderstorm area.

GFS has a few spots where instability and shear overlap over Scotland and SW Norway but warm ELs, meager instability release and placement under the anticyclonic side of the intense polar jet should preclude deep convection. Those areas have to be monitored in later model runs.