May 2011 to April 12 ranked as the warmest 12-month stretch over the U.S. since records began in 1895 (NOAA NCDC)

The record-setting 12-month period edged out November 1999-October 2000, the 2nd warmest 12-month period, by 0.1°F. The average temperature was 2.8 degrees F above the 20th century average.

Link: State of the Climate, National Overview, April 2012

In the last year, the U.S. has experienced its second hottest summer, fourth warmest winter (December through February) and warmest March on record. And NCDC announced April 2012 was third warmest on record.

Twenty-two states experienced their warmest May 2011-April 2012 (12-month) period, including Maryland, much of the Northeast and the Upper Midwest. Virginia and several other states had their second warmest May-April period on record.

Calendar year 2012 (January-April) also record warm

As warm as it’s been, it’s no surprise that the calendar year 2012 (spanning January to April) is off to a record warm start, 5.4 degrees F above average across the Lower 48. Twenty-six states east of the Rockies logged their warmest start to the year on record, including Virginia and Maryland.

Washington, D.C.’s average temperature in 2012 is warmest on record year to date as illustrated by this time series showing the top 5 warmest years (NOAA NCDC)

The degree by which some states and regions have exceeded their norms so far this year is incredible, and record-setting.

North Dakota’s temperature so far this year is 10.4 degrees above normal. That’s the biggest warm anomaly for the first 4 months of the year on record for any state. Other states with exceptionally warm starts to year include Minnesota (+9.6 degrees), South Dakota (+9.0 degrees), Wisconsin (+8.6 degrees), and Iowa (+8.6 degrees). These are all amongst the biggest temperature anomalies in the last 117 years for the first four months of the year.

The U.S. Climate Extremes Index - that tracks extremes in temperatures, precipitation and tropical cyclones - showed a record 42 percent of the country experienced extreme weather during the first four months of the year, primarily exceptional warmth.

April third warmest on record, and more

In addition to reporting the month finished third warmest on record, NCDC’s April climate report chronicled numerous other eye-opening climate facts from across the U.S.:

* At least 332 locations (mainly concentrated in the Rockies and Southern Plains) soared to their warmest April temperature on record

* Few April monthly temperature records remain from the 20th century after a decade of warmer than average Aprils

NOAA visualization showing age of April monthly maximum temperature records. Relatively few records remain that were set prior to 2000. A large fraction have occurred since 2000. (NOAA)

Behind the exceptional warmth

This onslaught of record-breaking warm weather over the Lower 48 this year is related to a jet stream pattern which has locked up cold air in the high latitudes and favored sprawling heat domes over the mid-section of the country which have meandered east and west at times.

Although the contortions of the jet stream established the unusually warm weather patterns, the intensity and persistence of the warmth compared to the past is very likely related to the elevated greenhouse concentrations in atmosphere from human activities.

Related links

Epic March heat wave in Midwest, Great Lakes and Northeast: link to global warming?

Greenhouse gases and global warming activate the steroid era of our atmosphere

April colder than March in many cities across Northeast and Midwest including Washington (Dulles)

Behind the present global warming pause