Remarkably Dry and Warm Winter Due to Record Extreme Jet Stream Configuration

Flowers are sprouting in January in New Hampshire, the Sierra Mountains in California are nearly snow-free, and lakes in much of Michigan still have not frozen. It’s 2012, and the new year is ringing in another ridiculously wacky winter for the U.S. In Fargo, North Dakota yesterday, the mercury soared to 55°F, breaking a 1908 record for warmest January day in recorded history. More than 99% of North Dakota had no snow on the ground this morning, and over 95% of the country that normally has snow at this time of year had below-average snow cover. High temperatures in Nebraska yesterday were in the 60s, more than 30° above average. Storm activity has been almost nil over the past week over the entire U.S., with the jet stream bottled up far to the north in Canada. It has been remarkable to look at the radar display day after day and see virtually no echoes, and it is very likely that this has been the driest first week of January in U.S. recorded history. Portions of northern New England, the Upper Midwest, and the mountains of the Western U.S. that are normally under a foot of more of snow by now have no snow, or just a dusting of less than an inch. Approximately half of the U.S. had temperatures at least 5°F above average during the month of December, with portions of North Dakota and Minnesota seeing temperatures 9°F above average. The strangely warm and dry start to winter is not limited to the U.S–all of continental Europe experienced well above-average temperatures during December.

December 2011 jet stream pattern the most extreme on record
The cause of this warm first half of winter is the most extreme configuration of the jet stream ever recorded, as measured by the North Atlantic Oscillation (NAO). The Arctic Oscillation (AO), and its close cousin, the North Atlantic Oscillation (which can be thought of as the North Atlantic’s portion of the larger-scale AO), are climate patterns in the Northern Hemisphere defined by fluctuations in the difference of sea-level pressure in the North Atlantic between the Icelandic Low and the Azores High. The AO and NAO have significant impacts on winter weather in North America and Europe–the AO and NAO affect the path, intensity, and shape of the jet stream, influencing where storms track and how strong these storms become. During December 2011, the NAO index was +2.52, which was the most extreme difference in pressure between Iceland and the Azores ever observed in December (records of the NAO go back to 1865.) The AO during December 2011 had its second most extreme December value on record, behind the equally unusual December of 2006. These positive AO/NAO conditions caused the Icelandic Low to draw a strong south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward over the U.S. and Europe.

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