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.)And the swing in this index! Dizzying!:
Meanwhile, the hoped-for forecast change in California has fizzled out, so January looks like it will be a miserable month too. There is a chance for a weak storm on the 15th. Then, that's about it on the horizon.
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