Storm Forecast
Valid: Thu 01 Jan 2009 06:00 to Fri 02 Jan 2009 06:00 UTC
Issued: Wed 31 Dec 2008 14:58
Forecaster: GATZEN

SYNOPSIS

The westerly is situated over the Mediterranean during the period. Troughs are present to the south of Greece and Turkey and over Iberian Peninsula. High pressure covers western Europe and north-eastern Atlantic and ridges into Greenland. Downstream, large arctic trough amplifies over Scandinavia, causing widespread cold air advection over northern and central Europe. At the periphery of the northern trough, a cut-off low moves into northern Balkans and Adriatic.

DISCUSSION

Northern Adriatic

Latest observations and soundings indicate that very dry boundary-layer is stable in the range of the cut-off low. Even steep mid-level lapse rates cannot lead to significant instability. On Thursday, rather strong lift is forecast in the range of the cut-off low as it moves across the Alps. This will affect the northern Adriatic, and the latest models indicate widespread precipitation. Current thinking is that stratiform rain is very likely, while embedded convection will likely be quite shallow. Although lightning is not completely ruled out, warm EL temperatures are expected and thunderstorms are not forecast.

Portugal

An upper trough moves eastward across the Iberian Peninsula during the day. Best instability is likely near the Atlantic coast of Portugal, where quite moist low-levels are forecast to be slightly unstable. Latest satellite images indicate some convection off the coast spreading eastward. Current thinking is that some convection will affect the coast of Portugal in the range of the trough axis. Some convergence especially near the coasts and quite cold EL heights may be sufficient for isolated thunderstorms. Severe convection is not expected.

South-western Mediterranean

In the range of the south-western Mediterranean, latest soundings show relatively rich low-level moisture in the boundary-layer, while this air mass is capped by warm mid-levels. During the period, upper trough axis spreads across the Iberian Peninsula that will likely be associated with upper height falls and increasing instability. Due to the lift and some WAA, models also indicate falling surface pressure at the lee side of the Iberian Peninsula, leading to more westerly winds over western Mediterranean, advecting dry air mass from Spain eastward. Instability will therefore move to the south-east during the period. Main problem for initiation may be the weak low-level convergence over western Mediterranean, and it is questionable if the cap will really break. Additionally, EL temperatures are forecast to be really warm during the period, and thunderstorms are expected to be rather unlikely at this time.