Figure 1. Temperature rankings for spring 2012 in the Contiguous U.S. Thirty-one states were record warm for the 3-month period, and an additional eleven states had top-ten warmth. Spring 2012 beat the previous record for hottest spring on record, set in 1910, by an remarkable 2°F. Image credit: NOAA/NCDC.
U.S. heat over the past 12 months: a one in half-a-million event
But the heat of spring is not the entire story. The U.S. record for hottest 12-month period fell for the second straight month in May. The June 2011 - May 2012 temperatures smashed the previous record by a startling 0.4°F. The past twelve months have featured America's 2nd warmest summer, 4th warmest winter, and warmest spring on record. Thirty-two states were record warm for the 12-month period, and an additional ten states were top ten warm. Each of the 12 months from June 2011 through May 2012 ranked among the warmest third of their historical distribution for the first time in the 1895-present record. According to NCDC, the odds of this occurring randomly during any particular month are 1 in 531,441. Thus, we should only see one more 12-month period so warm between now and 46,298 AD--assuming the climate is staying the same as during the past 118 years. The unusual warmth was due, in part, to a La Niña event in the Pacific that altered jet stream patterns, keeping the polar jet stream much farther to the north than usual.
Figure 2. Three of the top ten warmest 12-month periods in the contiguous U.S. since 1895 have occurred since April 2011. Image credit: NOAA/NCDC.
Figure 3. The average temperature during January - May 2012 was the warmest on record: 5°F above the 20th century average for the period, and 1.3°F above the previous record set in 2000. January - May temperatures have been rising at about 1.8°F per century since 1895. Image credit: NOAA/NCDC.
Second warmest May, warmest year-to-date period on record
May 2012 was the second warmest May in the contiguous U.S. since record keeping began in 1895. Twenty-six states had a top-ten warmest May, and no states had a top-ten coolest May. The January - May 2012 period was the warmest January - May period since record keeping began in 1895, with temperatures 5°F above the 20th century average for the period. This broke the previous record set in 2000 by an unusually large margin--1.3°F.
Figure 4. NOAA's U.S. Climate Extremes Index (CEI) for spring (March - April - May) shows that 2012 had the most extreme spring on record, with 44% of the contiguous U.S. experiencing top-10% extreme weather.