Storm Forecast
Valid: Thu 14 Jan 2010 06:00 to Fri 15 Jan 2010 06:00 UTC
Issued: Wed 13 Jan 2010 21:58
Forecaster: GATZEN

A level 1 was issued for northern Algeria mainly for tornadoes.

A level 1 was issued for southern Turkey mainly for excessive precipitation.

SYNOPSIS

A high-over-low flow blocks most portions of Europe and dry low-level air has spread across the continent with easterly and northerly winds. Some influence of Atlantic westerly winds can be felt over the British Isles, where low-level moisture is slightly better. Rather rich low-level moisture is present in the range of the main frontal boundary that stretches across the Mediterranean with low-level mixing ratios of about 7 g/kg. Some of this moisture has its origin over the subtropical Atlantic and is advected into the Mediterranean in the range of surface lows moving eastward. One of these lows is actually moving across northern Iberia and is expected over Sardinia at the end of the period. At mid-levels, a strong jet curves around the associated short-wave trough that ends as a cut-off low late in the period. Ahead of the trough centre, QG forcing is expected to lead to sufficient lift to create neutral lapse rates in the moist air mass.

DISCUSSION

West Mediterranean

The occluded frontal system of the Iberian low will move across the west Mediterranean Sea during the period. In the range of this occlusion, a tongue of rich low-level moisture spreads eastward. At mid-levels, a strong westerly jet streak is forecast and QG forcing will lead to mid-level cooling and steepening lapse rates. Moist neutral to slightly unstable lapse rates will likely evolve in this warm air advection regime. The greatest uncertainty exists about the development of deep moist convection, as the low-level air mass may stabilize ahead of the occlusion given rather dry low-levels. Latest models predict mostly stratiform precipitation ahead and along of the occlusion except for the southernmost portions along the African coast. As the trough cuts off across the west Mediterranean, continuing lift creates more instability along the occlusion that slows down to the west of Italy and thunderstorms seem to be more likely. Additionally, deep moist convection is also expected in the range of the trough axis as it moves across the rather warm sea surface.

Current thinking is that a frontal rain band including some embedded convective cells will spread across the west Mediterranean Sea. The best potential for thunderstorms seems to exist along the western flank of this rain band, where low-level moisture is best. Late in the period, the precipitation mode along the occlusion may turn more into convective as the lapse rates will further steepen in response to the lift.

Given the weak instability, rather weak low-level buoyancy, and also rather weak low-level convergence in the range of the occlusion, it seems that severe convection will not build although the vertical wind shear is rather strong in the morning hours. The best potential is expected along the coasts of Algeria, where the warm moist air mass will be surface-based just south of the occlusion point. Storms that form in this region will likely profit from strong vertical wind shear of about 10-15 m/s in the lowest kilometre as well as favourable veering profiles, and mesocyclones are not ruled out at the southern flank of the frontal rain band, capable of producing tornadoes. As the mid-level jet spreads southward, the strong lift and low-level convergence is expected to weaken, and the chance of severe convection decreases.

Late in the period, increasing low-level convergence along the occlusion may favour a potential of excessive rain along the south-western portions of Italy, but most of the precipitation will likely be stratiform and not convective.

Southern Turkey

A broad trough is forecast to spread across Turkey on Thursday. Ahead of this trough, an occlusion and rich low-level moisture affect the southern coasts with excessive precipitation. Although most of the precipitation may be of stratiform nature, embedded convective cells are expected posing a threat of local flash flooding. In the range of the trough axis, deep moist convection will be more widespread, but coast-parallel flow will lead to a weakening threat of excessive precipitation.