Winter Is Finally Releasing Its Chilly Grip on the U.S.

After more than a month of colder-than-average weather in the U.S., warmer days are finally on the way. According to the Climate Prediction Center, which is part of the National Oceanic and Atmospheric Administration (NOAA), above-average temperatures are expected across the eastern U.S. from April 8-16, and above-average temperatures are also favored in the East, South, and Southwest for the April-June time period. 

Click image to enlarge. Credit: NASA.

The warmth will be especially welcome after an unusually cold March gripped much of the U.S., and affected Europe and parts of Asia as well. The above graphic from NASA tells the story, showing the colder-than-average temperatures between March 14-20 (compared to average of the same dates from 2005 to 2012). The image is based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite, and shows land surface temperature anomalies .

Areas with above-average temperatures appear in red and orange, and areas with below-average temperatures appear in shades of blue. Much of Europe, Russia, and the U.S. saw unusually cool temperatures, while Greenland was surprisingly warm for the time of year.

As Climate Central has reported, the long-lasting cold was related to a strong blocking High pressure system over Greenland, which was associated with a particular configuration of an atmospheric pressure pattern known as the Arctic Oscillation, or AO. The AO is a measure of the difference in relative air pressure between the Arctic and the mid-latitudes, and the configuration of air pressure patterns can have profound impacts on weather patterns in the Northern Hemisphere. 

During March, the AO hit rock bottom.

Plot of the Arctic Oscillation Index during the 2012-2013 winter season.
Credit: Image adapted from NASA's Earth Observatory.

When the AO index is in its “negative” phase, air pressure over the Arctic is higher than average, while pressure over the mid-latitudes is relatively low, and prevailing winds allow extremely cold air to spill out of the Arctic, as if opening the Northern Hemisphere's refrigerator door. 

Warmer times ahead! NOAA's latest temperature outlook for the April-June period. 
Credit: NOAA.

The AO Index fell all the way to -5.6, which was one of its lowest readings on record, dating back to 1950. As this occurred, near record-breaking cold broke out in the U.K., which had its fourth-coldest March since 1962. Germany saw its coldest March since 1883, and Moscow had its coldest March since the 1950s, according to NASA and news reports.

The AO, which is still in negative territory, is expected to return to its positive phase by mid-April, raising the odds of a major weather pattern shift, possibly to warmer-than-average conditions.

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