Polar low in the wake of storm

A terrific storm moved north along the coast of northern Norway on 25 and 26 November 2011, causing massive damage to houses and boats. It was well forecast, so one can only hope that no one was out at sea during its passage. A cold air outbreak in the wake of the storm was also well forecast, and even the development of a polar low in those cold air masses.

This is the forecast for midnight on Saturday, where the storm, which was called “Berit” by the way, is clearly visible just off the coast of Lofoten in northern Norway:


If you look at the isobars (the black curves) circling around the centre of the storm, you can tell that there were northerly winds coming down from the Svalbard region and the Barents Sea. This is because the wind blows roughly parallel to the isobars, and the direction is always counterclockwise around the low centre.

Now take a look at the “analysis”, which was computed after the fact, taking all observations into account, for midnight on Sunday:

There seems to be a dual feature in pretty much the same location as the storm was in the previous picture. (Also note that there seems to be a polar low in development west of Iceland.) These are polar lows that have formed as the icy cold winds blew down from the north and interacted with the much warmer ocean surface. The process is the same as what happens when you blow on a bowl of hot soup. The air is a lot colder than the soup, and therefore gets heated from below and starts rising and producing steam. Over the ocean, the steam turns into clouds, and nasty ones at that if the temperature difference between the water and the air is large enough. The perfect recipe for polar lows.

The picture on the left is a satellite image taken at 3 in the morning on Sunday, three hours after the analysis above. Click on the image for a larger version. You’ll have no trouble picking out the polar low here, but I’ve marked it in the picture all the same. You can also see the original storm, which has moved up into the Barents Sea now. There’s also a new storm coming in from the south, which is producing heavy rain here in Bergen as I write this.

The current forecast predicts that the PL will move towards the south-east and hit the coast of Norway later today. It might also merge with the new storm coming in.

The consequences will probably be thunderstorms, heavy snowfall, and a drop in temperatures. These are common features of polar lows, and is why they have always represented a major hazard for the coastal communities in northern Norway, Iceland and Japan, which are the regions that are most prone to polar lows. The odd polar low also strikes the UK and the island communities further north (Shetland, the Faeroes), sometimes causing panic on the roads.

I’ll try to get a good capture from the MODIS satellite as well.

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