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Air pressure in the lower atmosphere compared to the 1981-2010 average during February 2016 (top), when the PNA was positive, and in February 2019 (bottom), when it was negative. The location of highs and lows and the flow of the jet stream around them often produce a sharp warm-cold split in temperatures in the western and eastern halves of the United States. NOAA Climate.gov, based on data from the Physical Science Lab.