Storm Forecast
Valid: Sun 20 Dec 2009 06:00 to Mon 21 Dec 2009 06:00 UTC
Issued: Sun 20 Dec 2009 05:33
Forecaster: SCHLENCZEK
A level 1 was issued for SW Spain and Portugal mainly for excessive rainfall and to a lesser extent for severe wind gusts.
A level 1 was issued for large portions of the Aegean Sea region and W Turkey mainly for severe wind gusts and large hail and to a lesser extent for tornadoes and excessive rainfall.
A level 1 was issued for NE Greece, Macedonia and the Bosporus region mainly for excessive rainfall and to a lesser extent for severe wind gusts.
SYNOPSIS
As very cold air has overspread most parts of continental Europe, there are only a few places where deep moist convection is possible. Some showers and weakly electrified thunderstorms could be expected in the vicinity of a low pressure complex over the North Sea. A developing low pressure system over the Azores will affect western Iberia in the second half of the forecast period. Strong southerly LL flow will advect warm and moist air with 8 - 10 g/kg average LL mixing ratio towards southern Spain and Portugal. Especially in the late evening / night hours, thunderstorm activity is forecast to increase as the SFC low deepens.
Another region of interest ist the Aegean Sea including S Greece, Crete and SW Turkey. An occluding SFC low moves across this region in the morning / afternoon hours and thunderstorms will develop in a strongly sheared environment with some hundred J/kg CAPE. Although shear and instability will decrease during the period, severe weather should be possible.
DISCUSSION
...Scottish Sea, N North Sea...
Ahead of a small upper low over the northern North Sea, steepening lapse rates will lead to a few hundred J/kg CAPE. Overall shear is considerably weak and severe thunderstorms will be unlikely. Nevertheless, some briefly organised multicells may develop and produce some isolated flashes. Only a small region with locally more than 3 g/kg LL mixing ratio is highlighted. An isolated waterspout may occur in a region with relatively calm winds and steep lapse rates.
...SW Iberia...
Ahead of the approaching upper trough, warm and moist air with a few hundred J/kg CAPE is advected towards SW Iberia. Especially near the cold front which should pass the region on Monday morning, deep layer shear in order of 20 - 25 m/s and locally enhanced LLS is expected near the left exit of a 50 m/s upper jet streak that points at N Portugal. Some QG forcing may allow convection that is mostly elevated and may be accompanied by isolated severe gusts. A local maximum in the effective precipitable water over this region is forecast by GFS and heavy rainfall is likely to occur. If a storm can maintain a rotating updraught on its way to the coastline, an isolated tornado may be possible.
...Aegean Sea, SE Greece, Bosporus region, W / SW Turkey...
A plume of moderately unstable air is advected towards S Greece and the Aegean Sea during the morning hours. Even though large fields of mid-level clouds will hinder insolation, thunderstorm initiation should be possible due to strong DCVA ahead of the upper trough. In this case, most of the storms will stay elevated which reduces the probability of tornadoes in an environment with 15 m/s LLS and 20 - 30 m/s DLS. Winds at 850 hPa will speed up to 20 - 25 m/s and severe wind gusts will become the primary threat. The main storm mode should be well-organised multicells and a few supercells with possible large hail, given strong deep layer shear and around 200 mē/sē SRH3. In the northern part of this level 1 area, excessive rainfall will become the primary threat as storms will persist there for several hours.
