Winter Is Finally Releasing Its Chilly Grip on the U.S. 		 						
- Published: April 3rd, 2013, Last Updated: April 3rd, 2013
 
 						 						 							 							 							 							 							 							 						
  						
	
	
	
		
		
		
		
	
	
  						 							
 							 						
 		 						 							 								 									
	
									By 
Andrew Freedman 								
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 						 						 													 						 		 							 		 							 		 							 		 							 		 							 		 							 		 							 		 							 		 							  							 								 	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.