The October case of 2012

This was an exceptional polar low in that it developed very early in the winter, in late October. This is well outside the normal season of polar lows, which starts in November and lasts until April. The low was well forecast on account of an unusually clear signal in the difference between the sea surface temperature and the temperature at 500 hPa, which is seen as a strong indicator of the potential for Polar Low development. This temperature difference was 54 deg C in this case, which is one of the highest values ever seen in this area. The situation lasted for several days. In such an environment, the development of a polar low is almost inevitable. The main Polar Low dissipated southwest of Bjørnøya, but several smaller centers formed later on, and caused very heavy snowfall and considerable problems for transportation in a large area including Trøndelag, Nordland and Troms. During a four day period, the snow depth increased by 82 cm in one station at 700m altitude in the Lyngen Alps in Troms. The event generally produced from 30 to 50 cm of snow at lower altitudes. This Polar Low was the first to be forecast by the newly developed tracking methodology at the Norwegian Meteorological Institute, involving the new Harmonie 2.5 km model in an EPS configuration. The Polar Low was extensively covered on the public web portal of www.barentswatch.no, also a novelty in this context. As of late February 2013, this low is one of only two fully developed Polar Lows recorded this season. In the seasons 2001 till 2011, on average 9 Polar Lows were recorded during the early season from September through February. The last two seasons of 2011-12 and 2012-13 have been exceptional in that there were only two Polar Lows in the same period each year.

The 24th October low at its mature state. The low then followed a trajectory north, then west around Bjørnøya before it subsequently dissipated at 75 N and 08E in the afternoon on the 25th.

SST-T500 from the Hirlam 8km model, showing an exceptionally large area of values above the threshold of 44 deg C. Blue and red lines gived the temperature difference between the sea surface and 500 hPa, with red indicating values above 44 deg C. The geopotential og the 500 hPa is shown in green.