© National Oceanic and Atmospheric Administration
An unusual region of atmospheric pressure over the Arctic has kept the polar jet stream (green) locked up at far northern latitudes, causing a warm, dry U.S. winter.
A little snow and rain are falling in a few states today, but the 2011 - 12 winter has been extremely warm and dry across the continental U.S. Meteorologists think they have figured out why.
First, a few records: The initial week of January was the driest in history. And more than 95 percent of the U.S. had below-average snow cover - the greatest such percentage ever recorded - according to some intriguing data maps generated by the National Oceanic and Atmospheric Administration. During December, approximately half of the U.S. had temperatures at least 5 degrees Fahrenheit above average, and more than 1,500 daily record highs were set from January 2 to 8. Europe has seen similar extremes.
The chief suspect behind the mysterious weather is an atmospheric pressure pattern called the Arctic Oscillation, which circles the high Northern Hemisphere. Its lower edge is known as the North Atlantic Oscillation (NAO). Together, the related features influence the path and strength of the jet stream. The jet itself is an air current that flows west to east across the northern latitudes of the U.S., Europe and Asia, altering temperature and precipitation as portions of it dip southward or crest northward. A strong jet stream that flows in a somewhat straight line from west to east, with few southward dips, prevents cold arctic air from drifting south. "The cause of this warm first half of winter is the most extreme configuration of the jet stream ever recorded," according to Jeffrey Masters, a meteorologist who runs the
Weather Underground, a Web site that analyzes severe weather data.
