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Tuesday, June 12, 2012

Jeff Masters: Spring 2012 -- the most extreme season in U.S. history [not good, not good at all] -- the old record being crushed by a full degree is a stunning and unparalleled event in U.S. meteorological history

Spring 2012 -- the most extreme season in U.S. history

by Dr. Jeff Masters, WunderBlog, June 8, 2012

Spring 2012 in the contiguous U.S. demolished the old records for hottest spring and most extreme season of any kind, said NOAA's National Climatic Data Center (NCDC) on Thursday. With the warmest March, third warmest April, and second warmest May, the March-April-May spring season was 5.2 °F above average -- the largest temperature departure from average of any season on record for the contiguous United States. What's truly remarkable is the margin the old record was broken by: spring 2012 temperatures were a full 1 °F above the previous most extreme season, the winter of 1999-2000. All-time seasonal temperature records are very difficult to break, and are usually broken by only a tenth of a degree. To see the old record crushed by a full degree is a stunning and unparalleled event in U.S. meteorological history. 

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
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, which is a huge margin to break a record by for a 1-year period. 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. However, it is highly unlikely that the extremity of the heat during the past 12 months could have occurred without a warming climate. Some critics have claimed that recent record warm temperatures measured in the U.S. are due to poor siting of a number of measurement stations. Even if true (and the best science we have says that these stations were actually reporting temperatures that were too cool), there is no way that measurement errors can account for the huge margin by which U.S. temperature records have been crushed during the past 12-month, 5-month, and 3-month periods.

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.

Most extreme Spring and January-May period on record
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, was 44% during the spring March-April-May period. This is more than twice the average value, and spring 2012 was the most extreme season of any kind in U.S. history. A list of the top five most extreme seasons since 1910, as computed using the CEI, show that two of the three most extreme seasons in U.S. history have occurred in the past 12 months:

Spring 2012: 44%
Winter 1979: 42%
Summer 2011: 39%
Fall 1985: 39%
Spring 1934: 38%

Remarkably, 81% of the contiguous U.S. had maximum temperatures that were in the warmest 10% historically during spring 2012, and 71% of the U.S. of the U.S. had warm minimum temperatures in the top 10%. The percentage area of the U.S. experiencing top 10% drought conditions during spring was 18%, which was the 19th greatest since 1910. Extremes in 1-day, spring, heavy precipitation events were the 8th largest on record. The year-to-date January-May period was also the most extreme such period in U.S. history, with a CEI of 43%. Climate change theory predicts that, in general, the climate should warm, wet areas should get wetter, and dry areas should get drier. The spring 2012 Climate Extremes Index reflects this pattern.

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