Storm Forecast
Valid: Sat 26 Dec 2009 06:00 to Sun 27 Dec 2009 06:00 UTC
Issued: Fri 25 Dec 2009 23:08
Forecaster: TUSCHY
A level 1 was issued for the coast of Algeria mainly for excessive rainfall.
SYNOPSIS
As it is the case with major cyclones, phase diagrams have problems in identifying the main center, but all indications are present for a deep and very dynamic cyclonic vortex to evolve over the NE Atlantic during the following 24h and onwards, as a strong Rossby wave moves towards the east. As this highly amplified pattern evolves, strong geopotential height rises are expected over extreme SW-Europe during the forecast. Supergeostrophic winds along the crest of the developing ridge cause the broad and ill defined upper trough over the western Mediterranean to re-sharpen and amplify. Hence, overall eastward progress of this trough is slow, before accelerating during the night. At the surface, yesterday's depression over Spain reforms further east over the western Mediterranean and evolves into the dominant surface feature during the forecast. As a surface high builds in over Spain, a favorable pattern for persistent CAA evolves, affecting most parts of N-Africa. Unseasonably warm conditions persist over the eastern Mediterranean but air mass is well capped.
Another vortex over NE-Europe causes unsettled conditions with snow and rain for E/NE-Europe.
A southeastward moving trough also affects the western English Channel with an isolated thunderstorm event although available moisture is very low. A 50m/s mid-level streak to the SW and a potent vorticity lobe ought to offset the scarce moisture content, so we went with a low probability area. An adjustment to the south may be needed, but convection may occur after 06 UTC down there. Further east, over the English Channel and south North Sea, convective signals in GFS/ECMWF increase after midnight with cold ELs and some marginal moisture. Despite producing some headache to issue such a large thunderstorm area, spread of model solutions and flat nature of the upper trough enforce such a huge, highlighted area.
DISCUSSION
... Western and central Mediterranean ...
During the morning hours, scattered thunderstorms east of the Strait of Gibraltar and over S/SE-Spain start to decay, as the pressure at all levels increases. The focus therefore shifts eastwards, where another surface depression gradually intensifies just southeast of the Balearic Islands. Some moisture enters the center of the depression from the south with cool mid-levels, so some marginal instability is forecast over Corsica and Sardinia. Despite limited instability build-up, shear is sufficient for an isolated severe wind gust, large hail and tornado risk. Global models agree in this scenario but still indicate some deviations in respect of the final track, so overall confidence and expected risk is too low for a level area. This depression also affects central Italy and the central Adriatic Sea thereafter, but both, limited BL moisture and slight warming of the mid-levels all hint on weak convection. This system could be a rain-maker over Albania, but mainly stratiform in nature. Despite issuing a low probability thunderstorm area, heaviest rain occurs onshore and therefore diplaced from potential thunderstorm activity. If CAPE/QPF maxima merge in future runs, a subsequent upgrade may be needed.
Another spot for enhanced thunderstorm probabilities is the N-coast of Algeria, where the surface cold front slows down during the forecast. Adequate moisture and steep lapse rates beneath the broad upper trough keep shower/thunderstorm activity moderate. Locally high rainfall amounts caused the issuance of a level 1.
