The polar low season started early this year. In this picture, which was taken on Tuesday 28 October, 2008, we see a pretty specimen centered roughly over Shetland. This polar low is part of the first proper cold-air outbreak this winter, yielding October snow in London for the first time since the 1930s.
The polar lows season in the North Atlantic usually starts in October/November and ends in March/April. The season that ended earlier this year was characterized by weak activity and few cold-air outbreaks.