Storm Forecast
Valid: Sat 13 Dec 2008 06:00 to Sun 14 Dec 2008 06:00 UTC
Issued: Fri 12 Dec 2008 21:48
Forecaster: DAHL

SYNOPSIS

Quite an intense upper long-wave trough will dig into the western Mediterranean towards the end of the forecast period, while the upper low, located over the N Maghreb States by the beginning of the period, will accelerate eastwards, reaching the Ionian Sea early Sunday morning. Eastern Europe and Russia are covered by a large-scale deep anticyclone, maintaining quite stable conditions over these regions. Ahead of the Atlantic cyclone attached to the digging large-scale trough, a narrow region of warm advection will affect western portions of Europe as well as the western Mediterranean.

DISCUSSION

... western Mediterranean ...

Marginal but weakly capped instability is expected to exist ahead of the eastward-accelerating upper low which will cross the south-central Mediterranean during the afternoon and night hours. Currently, it does not seem that an appreciable amount of shear will be available in the storms' environment.

Ahead of the large-scale trough, reaching the W Mediterranean late in the period, some 20 m/s DLS are simulated over the SW Mediterranean Sea ... overlapping with a region where cellular post-frontal convection should develop. However, the convection is likely to be rather shallow over this region, so that the effective shear may be somewhat reduced. The allover severe threat seems to be too low for a categorical outlook, though especially over the SW Mediterranean, a marginally severe wind/hail event cannot be discounted towards early Sunday morning.

... Gulf of Biscay ... extreme northern Spain ...

Farther NW towards over the Gulf of Biscay, more vigorous convective mixing is anticipated in the deep polar air mass. Though deep shear is expected to be minimal where convection is deepest, shear rapidly increases towards the western Biscay and western Iberia. In addition, strong low-level shear is anticipated over the Iberian peninsula, so that the landfalling convection may have some chaces of spawning a brief tornado or two. Also, marginally severe hail/wind may occur. The lack of decent DLS reduces the risk somewhat, but given some tornado threat along the coast, a level-one area seems to be marginally warranted.