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The last 12 months also have been the warmest since modern record-keeping began in 1895, narrowly beating the previous 12-month period that ended in May 2012.
Every state except Washington in the contiguous United States had warmer-than-average temperatures for the June 2011-June 2012 period.
The recent blistering heat wave broke records across much of the United States, threatening the Midwest's corn crop and helping to fan destructive wildfires.
June was 2 degrees Fahrenheit (1.1 degrees Celsius) warmer in the lower 48 states than the 20th-century average, but still just the 14th hottest June in the record books, NOAA's National Climatic Data Center said in a statement.
June 1933, during the calamitous Dust Bowl period, was the hottest.
More than 170 all-time warm records were broken or tied during June's second half, NOAA said.
Temperatures in South Carolina and Georgia of 113 degrees F (45 degrees C) and 112 degrees F (44 degrees C) respectively are under review as possible all-time statewide temperature records.
Such record-high temperatures are in line with a long-term warming trend in the 48 contiguous states, said Jake Crouch, a scientist at the National Climatic Data Center.
Climate change spurred by carbon dioxide emissions may not be the primary cause, but these extreme conditions are consistent with what scientists see as a "new normal," Crouch said by telephone.
"It's hard to pinpoint climate change as the driving factor, but it appears that it is playing a role," he said. "What's going on for 2012 is exactly what we would expect from climate change."
This past month was also the 10th driest June, with drought spreading to 56 percent of the contiguous U.S. states, up from 37.4 percent in May, making it the largest drought footprint of the 21st century.
The heat and drought put pressure on the corn crop, with analysts suggesting that the U.S. Department of Agriculture should lower its yield forecast in a monthly report due on Wednesday.
In early June, before the highest temperatures hit the U.S. grain belt, USDA forecast a record-large yield of 166 bushels per acre. Since then, hot, dry weather has baked much of the corn-growing region just as the crop was starting pollination, the key growth phase for determining yield.
Wildfires claimed 526,000 hectares, mostly in the West, the second-largest area to be charred during any June on record.
But it wasn't all dry. Tropical Storm Debby dumped so much rain during its slow pass across Florida that the state's monthly statewide precipitation total for the month was 33.4 cm, or 5.7 cm above average, making it Florida's wettest June on record.
Maine, Oregon and Washington state each had a top-10 wet June.
A broader look at global weather conditions for 2011 by U.S. and British scientists is set for release on Tuesday by NOAA and the United Kingdom's Met Office.