Storm Forecast
Valid: Sun 27 Dec 2009 06:00 to Mon 28 Dec 2009 06:00 UTC
Issued: Sun 27 Dec 2009 02:12
Forecaster: SCHLENCZEK

A level 1 was issued for most parts of Greece, the Bosporus region and W Turkey mainly for excessive rainfall.

SYNOPSIS / DISCUSSION

Numerous low pressure systems will affect most portions of western and N-central Europe on Sunday with a maritime polar airmass. A 980 hpa low over central England is forecast to move eastward towards NW Germany and W Denmark. A strong pressure gradient between this low and a ridge over Iberia will lead to an enhanced risk of (mostly non-convective) severe wind gusts in parts of southeastern England, northern France, the Benelux countries and western Germany. Most convection in this low pressure system should be confined to the trough region where both vertical shear and instability are too marginal for severe weather. Some small thunderstorm areas highlight the regions where electrified convection is most likely.

A low pressure system over the central Mediterranean shifts eastward during the period, affecting the S Tyrrhenian Sea area in the morning / afternoon and later in the period also S Italy, Adriatic Sea, Greece and S Balkans. Along its cold front and also near the upper cold core, thunderstorms are forecast in an environment with a few hundred J/kg MLCAPE. Some instability is also forecast near a 50 m/s upper jet streak over the Ionian Sea / SW Greece. Latest runs of GFS and latest soundings indicate that the warm sector airmass should be strongly capped and only some elevated storms may evolve in a large precipitation area further inland. Nevertheless, an isolated severe wind gust / tornado event is not ruled out with the strongest storms and the precipitation sums in parts of Greece and the Bosporus region should exceed the level 1 threshold.

Another intense surface low is present over the Azores, moving slowly eastward towards the Bay of Biscay. Some convection should be possible over W Iberia and parts of the E Atlantic during the late evening / night hours where low-end instability will overlap with some 20 - 35 m/s deep layer shear. A strong pressure gradient will lead to 850 hPa winds around 25 m/s which may lead to an enhanced risk of (mostly non-convective) severe wind gusts. Recent model outputs don't show a source of strong QG forcing which may create a narrow line of thunderstorms or organised multicells / supercells and therefore no threat level is issued.