Storm Forecast
Valid: Fri 05 Dec 2014 06:00 to Sat 06 Dec 2014 06:00 UTC
Issued: Thu 04 Dec 2014 22:22
Forecaster: PISTOTNIK

A level 1 was issued for the W Balkans and W Greece mainly for excessive precipitation.

SYNOPSIS

Weak gradients prevail across Europe. A diffuse area of rather low 500 hPa geopotential covers western, southwestern and central Europe, whereas an upper-level ridge stretches from the Black Sea towards Finland, slowly moving east.
This broad "no man's land" is framed by two well-established upper-level jet streams: the subtropical jet runs straight from northern Africa to Greece and Turkey, whereas the polar jet with embedded frontal systems meanders across Iceland and Scandinavia into northern Russia.

DISCUSSION

... central Mediterranean region ...

At the forward flank of the diffuse upper-level trough, a surface cyclone remains almost stationary near Corsica. An upper-level vorticity maximum travels across the Tyrrhenian Sea, southern Italy, the Ionian Sea and the Western Balkans with large-scale lift. Cooling upper levels on top of seasonably mild and moist near-surface air enable the buildup of almost uncapped CAPE, possibly up to 1000 J/kg per forecast model guidance.
Within a weak to moderate southwesterly flow, scattered to widespread thunderstorms are expected. A level 1 is issued for possible excessive precipitation over the western Balkans and western Greece, where orographic lift of moist air is maximized throughout the forecast period and rainfall accumulations up to 100 mm are predicted by numerical models. 0-6 km vertical wind shear is generally weak, only towards the southeast it increases to roughly 20 m/s in closer vicinity to the subtropical jet. A few storms may attain a better organization near the southern rim of the highlighted areas, but a stronger cap and weaker large-scale lift make initiation in this better shear regime more questionable. Hence the hail and wind risk seems to be limited.

Behind an ill-defined cold front that crosses the Tyrrhenian Sea and Italy till the afternoon, still low CAPE will be present and further thundery showers will follow, though their intensity should decrease with the advection of gradually drier air. One or two flash floods are possible especially along west-facing coast of Sardegna and southern Italy, and a few waterspouts may spin up near the cyclone's center or along other convergence zones. Due to the lack of obvious foci, it was decided not to issue a level 1 in both cases.