September 2012 the 23rd warmest on record for the U.S.

September 2012 was the 23rd warmest September on record for the contiguous U.S, said NOAA's National Climatic Data Center (NCDC) in today's State of the Climate report. The month was also the driest on record for Montana, North Dakota, South Dakota, and Minnesota, and was a top-ten driest month for six surrounding states. The warm September temperatures helped make the year-to-date period of January - September the warmest such period on record for the contiguous U.S.--a remarkable 1.2°F above the previous record. Even if the remainder of 2012 ranks historically in the coldest one-third of October - Decembers on record, 2012 will beat out 1998 for the warmest year on record in the U.S. The first week of October has been one of the coldest weeks of the year, relative to average, with record cold lows outpacing record warm highs by a ratio of four-to-one in the contiguous U.S. However, next week will be warmer than average for the U.S., and it is highly unlikely that October 2012 will rank in the coldest one-third of Octobers on record. The October 2011-September 2012 period was the warmest such 12-month period on record for the contiguous U.S., and was the 3rd warmest 12-month period on record. The six warmest 12-month periods since record keeping began in 1895 have all ended during 2012.


Figure 1. Year-to-date temperatures for the contiguous U.S. through September, compared to the previous record warmest years in U.S. history. Outcome scenarios based on persistence of temperature from October through December, the remaining three months of 2012, are shown. Even if the remainder of 2012 ranks historically in the coldest one-third of October - Decembers on record (dark blue line), 2012 will beat out 1998 for the warmest year on record. The data for 2012 are preliminary. Image credit: NOAA/NCDC.

Most extreme January - September period on record
The year-to-date period was the most extreme in U.S. history, according to NOAA's U.S. Climate Extremes Index (CEI), which tracks the percentage area of the contiguous U.S. experiencing top-10% and bottom-10% extremes in temperature, precipitation, and drought. The CEI was 45% during the year-to-date January - September period. This is the highest value since CEI record-keeping began in 1910, and more than double the average value of 20%. Remarkably, 86% of the contiguous U.S. had maximum temperatures that were in the warmest 10% historically during the first nine months of 2012, and 79% of the U.S. of the U.S. had warm minimum temperatures in the top 10%. Both are records. The percentage area of the U.S. experiencing top-10% drought conditions was 24%, which was the 8th greatest since 1910.


Figure 2. NOAA's U.S. Climate Extremes Index (CEI) for January - September shows that 2012 had the most extreme first nine months of the year on record, with 45% of the contiguous U.S. experiencing top-10% extreme weather.

Jeff Masters