Storm Forecast
Valid: Sun 17 Feb 2008 06:00 to Mon 18 Feb 2008 06:00 UTC
Issued: Sat 16 Feb 2008 22:14
Forecaster: DAHL

SYNOPSIS

High-amplitude upper flow pattern is present over Europe ... with a large and broad upper ridge covering the western half of Europe, and an intense longwave trough over eastern Europe. At low levels, an equally large high pressure area has established, advecting continental polar air masses deep into southeastern Europe. Comparatively weak but persistent upper low is residing west of the Iberian Peninsula, maintaining rather extensive surface low over the eastern Atlantic. The eastern periphery of this feature will affect the W Iberia on Sunday.

DISCUSSION

... southwestern Iberian Peninsula ...

Weak instability is simulated to be present in a warm/moist plume that is expected to overspread western Iberia on Sunday. Though bulk of convection should remain off shore, isolated thunderstorms will likely occur over the peninsula as well. Very weak low-level lapse rates suggest that storms may struggle to lift SFC-based parcels and consequently remain somewhat elevated. If surface-based air is ingested, kinematic fields suggest that chance for an isolated mesocyclone or two exists. However, deep shear is expected to remain rather weak, so mesocyclonic structures are unlikely to persist long enough to become organized to a significant degree. It thus seems that even surface-based storms would have limited potential for severe weather. However, probability for marginally severe hail as well as for a brief/weak tornado is somewhat enhanced, but not sufficiently large to warrant a categorical risk.

... Aegean region ...

Quite cold and dry well-mixed polar air mass will overspread the Aegean and the central Mediterranean Sea late on Sunday. Strong DCVA-forced ascent ahead of the SE European upper trough may aid in deepening the convectively-mixed layer. GFS suggests that CAPE deep enough for thunderstorms will eventually evolve, but substantial uncertainty exists on how quickly/effectively the low-level moisture can be accumulated by evaporation from the warm sea surface. The deep convective mixing, as well as anticipated turbulent mixing in response to rather strong low-level flow may impede the build up of sufficient low-level moisture to support deep moist convection. Models seem to agree on at least weak precipitation, though turbulent flux parameterizations are still far from perfect ... but this does not constitute a compelling reason to distrust the model solutions either, devoid of any other guidance, and a thunderstorm area will be issued. Strong flow and high LCL's should counteract the favorably steep low-level lapse rates, so that it does not seem that a significant threat for waterspouts exists.