U.S. experiences warmest 12-month period on record

The past twelve months were the warmest twelve months in U.S. history, said NOAA's National Climatic Data Center (NCDC) on Tuesday, in their monthly "State of the Climate" report. Temperatures in the contiguous U.S. during May 2011 - April 2012 broke the previous record for warmest 12-month period, set November 1999 - October 2000, by 0.1°F. The past twelve months have featured America's 2nd warmest summer, 4th warmest winter, and warmest March on record. Twenty-two states were record warm for the 12-month period, and an additional nineteen states were top ten warm. NOAA said that the January - April 2012 period was also the warmest January - April period since record keeping began in 1895. The average temperature of 45.4°F during January - April 2012 was 5.4°F above the 20th century average for the period, and smashed the previous record set in 2006 by an unusually large margin--1.6°F.

Figure 1. The ten warmest 12-month periods in the contiguous U.S. since record keeping began in 1895. Image credit: NOAA/NCDC.

Figure 2. The average temperature of 45.4°F during January - April 2012 was the warmest on record: 5.4°F above the 20th century average for the period, and was 1.6°F above the previous record set in 2006. January - April temperatures have been rising at about 1.9°F per century since 1895. Image credit: NOAA/NCDC.

April 2012: 3rd warmest on record

April 2012 was the third warmest April in the contiguous U.S. since record keeping began in 1895. Ten states had a top-ten warmest April, and no states were cooler than average. But what's really remarkable about April was that eight states--Illinois, Indiana, Kentucky, Michigan, Wisconsin, Ohio, Pennsylvania, and West Virginia--had average April temperatures cooler than their March temperatures, even though their April temperatures were still above the long-term average for the month. These statistics show just how remarkably warm March 2012 was. Most extreme was Illinois, where April 2012 temperatures ranked in the top 20% for warmest Aprils, yet were cooler than March 2012 temperatures.

Figure 3. Temperature rankings for April 2012 in the Contiguous U.S. Ten states had a top-ten warmest April, and no states were cooler than average. Image credit: NOAA/NCDC.

Figure 4. NOAA's U.S. Climate Extremes Index (CEI) for January - April shows that 2012 had the most extreme weather on record.

Most extreme January - April on record

NOAA's U.S. Climate Extremes Index (CEI), an index that tracks the highest 10 percent and lowest 10 percent of extremes in temperature, precipitation, and drought, was 42% during the January-April period, over twice the average value, and the greatest on record. Remarkably, 82% of the contiguous U.S. had maximum temperatures that were in the warmest 10% historically, and 68% had warm minimum temperatures in the top 10%, with records going back to 1910. The previous records were 56% (2000) and 57% (1992) for maximum and minimum temperatures, respectively. The percentage area of the U.S. experiencing top-10% drought conditions during January - April was 19%, which was the 17th greatest since 1910. Extremes in precipitation as computed by the CEI were near average for the January - April period.

Jeff Masters - Weather Underground