Polar low hits the UK


This picture shows a nice polar low when its centre was north of Scotland in the early hours of 6 December 2011. (Image downloaded from the Dundee Satellite Receiving Station.) It later moved towards the south-east and led to snowfall in western Norway – snow still on the ground outside my window. The same cold air outbreak, which covered large parts of the Nordic Seas, also left some snow in the UK. You can see the cloud streets stretching all the way up to the East Greenland coast in the same image. It’s quite rare that polar lows move this far south, and this is a particularly nice specimen, with the beautiful spiral form near the low centre. Let’s hope that we get more of these this winter…

Here’s a StormGeo surface wind speed forecast for 0800 in the morning on the same day (click on the picture for a larger version):

A band of pretty strong winds, up to 24 m/s, hit Shetland according to the forecast. Although I don’t know how correct the forecast was, it’s very interesting to look at the structure of the winds. The area with strong winds is quite small – this is typical for polar lows. Eyewitness accounts report that they see a wall of clouds coming towards them from the north-east in otherwise calm weather. This is not what would happen in a regular storm coming in from the south-west. Then you can see the storm advancing hours ahead – typically you see high, wispy cirrus clouds first, followed by a denser, more stratified layer of clouds, and then you get the rain, snow and/or hail in the cumulus clouds or thunderstorms. Polar lows just sneak up on you, and that’s exactly why they’re so dangerous.

I’ve read lots of accounts of shipwrecks in northern Norway, and one pattern seems to stand out. The fishermen would stay ashore while large storms rolled by from the south, and then when the weather cleared, they were understandably anxious to go out in their boats. Thr trouble is that polar lows almost always form in the cold air outbreaks that follow behind these storms, and then they would get surprised by very strong, sudden winds. This can still happen; polar lows are notoriously difficult to forecast, but it is encouraging to see that we at least got the general structure of this one right.

Advertisements