Storm Forecast
Valid: Thu 10 Sep 2009 06:00 to Fri 11 Sep 2009 06:00 UTC
Issued: Wed 09 Sep 2009 20:55
Forecaster: GATZEN

A level 1 has been issued for portions of the Tyrrhenian and Adriatic Sea mainly for water spouts.

SYNOPSIS

Europe is dominated by a broad region of high pressure that intensifies across the western portions. This is associated with northerly low-level winds from the British Isles across central Europe, the Baltic States, and most of eastern/south-eastern Europe. As cold air spreads into northern and north-western Europe, a cold front extends from France and Germany to northern Poland. Some low-level moisture is present in the range of this front, leading to weak instability, while stable air masses are likely to the north of this front. Over the Mediterranean, a region with rather low geopotential includes a weak cut-off low centred over the southern Adriatic. Given the warm sea surface and rather rich low-level moisture over the sea, instability will likely remain during the period.

DISCUSSION

Central Mediterranean

Rather steep lapse rates are present across the central Mediterranean over the rather warm sea surface. This will result in low-level CAPE given rich low-level moisture. CAPE is expected to be deepest underneath the mid-level cut-off low that slowly propagates eastward from the Tyrrhenian to the Adriatic Sea during the period. On Thursday, some thunderstorms are likely given rather weak CIN and some low-level forcing. Best potential seems to be present in the morning hours when models predict rather high low-level buoyancy and offshore low-level convergence lines are likely. Convection that develops will be mostly unorganized as vertical wind shear is weak, while the thermodynamic environment seems to be favourable for some waterspouts especially in the morning hours. These may develop on Thursday especially over the Tyrrhenian Sea, while the potential is expected to be higher over the southern Adriatic on Friday.

Aegean

Rich low-level moisture is present across the Aegean Sea. The approaching mid-level cut-off will likely be associated with some QG forcing and steepening lapse rates, creating some instability. Expect a few low-level convergence lines forcing will likely be strong enough for a few thunderstorms. While these storms will be mostly unorganized given the weak vertical wind shear, increasing deep layer vertical wind shear is forecast over the southern portions, where multicells are more likely to develop. However, large hail seems to be not very likely over the sea with these rather weak thermodynamic profiles.

Western Iberian Peninsula

Latest surface observations indicate locally rich low-level moisture over western Spain and southern Portugal. On Thursday, some moisture will be again present given upslope flow over this region. While the lapse rates are relatively steep underneath the weak mid-level trough, diurnal heating may be sufficient for weak CAPE. A few showers and thunderstorms are forecast. Given weak vertical wind shear, severe thunderstorms are not expected. Storms will decay quickly after sunset.