Storm Forecast
Valid: Thu 23 Nov 2006 06:00 to Fri 24 Nov 2006 06:00 UTC
Issued: Wed 22 Nov 2006 20:07
Forecaster: GATZEN

SYNOPSIS

Long-wave trough over northern Atlantic continues to ampliy, and strong cyclone is forecast over northern British Isles on Thursday. Over Europe, south-westerly flow starts to dominate the weather pattern, and upper trough over Adriatic continues north-eastward while weakening. At lower levels, stable and unseasonable warm air mass is advected into France and Germany, while cold air mass remains over Adriatic Sea. Moist and slightly unstable air mass is present over Aegean, where thunderstorms are expected. Another region with convective activity is forecast over the Atlantic, North Sea, and British Isles.

DISCUSSION

Aegean

Cold front of low pressure moving from Greece into Aegean is expected to be the focus of convective activity. While upper trough is not expected to propagate further east into Aegean, surface low is forecast to weaken gradually. Nevertheless, models predict strong southerly low-level jet to form ahead of the cold front. While deep layer vertical wind shear will be weak, low-level wind shear is expected to increase during the period along the coast of Turkey. Given steep low-level lapse rates and weak CIN, tornadoes may develop in association with mesocyclones. However, given weak deep layer shear, mesocyclones seem to be not very likely. Current thinking is that strong low-level convergence and warm air advection will support a MCS along the cold front that slowly moves eastward over Aegean during the day. A few embedded mesocyclones may pose a threat of tornadoes. A few severe wind gusts are not ruled out either given strong southerly low-level jet reaching 25 m/s at the 850 hPa level. High precipitation may also occur.

British Isles

Over northern British Isles, intense cyclogenesis has set in, and latest satellite images show both well developed warm conveyor belt and dry intrusion. On Thursday, focus of convective activity is unstable air mass of the dry intrusion in the range of strong synoptic forcing at the cyclonic flank of strong jet streak and associated short-wave trough. Models indicate convective precipitation underneath the trough axis, and it is not ruled out that thunderstorms even merge into a comma cloud. Given strong surface pressure gradient, severe wind gusts will likely occur. At this time, it is not expected that convection will merge into a system that is that well-organized to warrant a risk category. However, situation has to be monitored closely for a possible upgrade. Convective acticity is expected to spread rapidely north-eastward over British Isles during the day reaching Norway at the end of the period, where it is expected to weaken.