Past 12 months warmest ever in U.S.

The mainland United States, which was largely recovering Monday from a near-nationwide heat wave, has experienced the warmest 12 months since record-keeping began in 1895, a top government science and weather agency announced Monday.

The report from the National Climatic Data Center, which is part of the National Oceanic and Atmospheric Administration, does not take into account blistering heat from this month, with 2,116 high temperature marks either broken or tied between July 2 and 8 in communities nationwide.

But it does incorporate the warmest March ever recorded as well as extreme heat in June, which also helped make the first six months of 2012 the warmest recorded of any January-June stretch.

In the last half of June alone, 170 all-time temperature records were matched or smashed in cities across the lower 48 states. The U.S. State Climate Extremes Committee also is reviewing whether 113-degree temperatures in South Carolina and 112-degree recordings in Georgia qualify as all-time records in those two states.

"There are a lot of things going on that have been very unusual over the last several months," said Dev Niyogi, earth and atmospheric sciences professor at Purdue University.

That includes an outbreak of particularly large wildfires in Colorado, which had its warmest June ever, according to NOAA. Most of that state is experiencing extreme to exceptional drought, which is also true in places as far afield as Arizona and Georgia, according to the U.S. Drought Monitor, run out of the University of Nebraska.

That's a pressing problem in Indiana, where Niyogi is the state climatologist and where the National Weather Service says conditions in central and southern parts of the state "resemble the Dust Bowl days of the 1930s."

Niyogi said it is "very hard to root out" the role played by short-term weather patterns and longer-term trends in creating conditions recorded over a given stretch, like the period from June 2011 to June 2012 measured by NOAA. But he said, "They're all tied together" and can contribute to major issues, pointing specifically to the interdependent relationship between heat and drought.

"That is an intriguing and fairly dangerous kind of event that obviously has affects on humans," he said, referring to drought.

The average temperature for the U.S. mainland in June was 71.2 degrees -- two degrees above the 20th-century average. It was the 14th warmest June on record.

Besides the Midwest and Rocky Mountain region, coastal states have also felt the heat of late: The state of New Jersey, for instance, has experienced 16 straight months (not including data from June) in which average monthly temperatures measured above the norm.

Stepping back further in time, the calendar year 2010 tied with 2005 as the warmest ever, based on global surface temperatures, according to NOAA. In fact, a March report from the Intergovernmental Panel on Climate Change noted that "climate change has led to changes in climate extremes such as heat waves, record high temperatures and, in many regions, heavy precipitation in the past half-century."

Millions in the central and eastern United States who experienced the latest heat wave got somewhat of a reprieve Monday. But the break in the heat could mean severe storms in some areas, forecasters said.

A cold front working its way through the Mid-Atlantic states helped drop high temperatures into the 80s in cities including Chicago, New York, Boston and Washington. Even the 91 degrees recorded Monday in St. Louis was a welcome relief in a city where temperatures topped 100 degrees for 10 straight days.

The heat wave that roasted much of the country for more than a week left scores dead, including 13 in Maryland and 10 in Virginia, according to state officials. Millions also lost power at one point or another, many of them for several days, due to severe storms that swept east from Indiana to the Mid-Atlantic states starting on June 29. As of Monday, just under 100,000 customers in 11 states and metropolitan Washington still lacked electricity.

The nation's capital set another record Sunday with a high of 102 degrees, the city's 11th straight day of temperatures over 95.

Other high temperatures Sunday included 105 in Raleigh-Durham, North Carolina, which tied an all-time record; 103 in Richmond, Virginia; 101 in Charlotte, North Carolina; and 100 in Baltimore.

The cold front, while welcome, brings a chance of severe weather, particularly across the eastern Carolinas and southeastern Virginia, CNN meteorologist Sarah Dillingham said. More than 160 wind reports were received nationwide as the front worked its way eastward, she said.

Severe thunderstorms that hit part of Virginia, North Carolina and South Carolina late Monday afternoon potentially included "quarter-size hail and damaging winds in excess of 60 mph," according to the National Weather Service.

On Sunday, damaging winds struck Fredericksburg, Virginia, and collapsed a building, injuring two people, according to the National Weather Service's Storm Prediction Center. The storms have complicated power restoration efforts in some spots and caused even more damage. One person died Saturday in storms in Cuba, Missouri, the prediction center said.

"It has been a tough few weeks for many Virginians," Gov. Bob McDonnell said late Sunday. "They have suffered from record-breaking temperatures and an historic storm that brought widespread damage and power outages. Now, many have lost power again. I ask Virginians to remain patient and to continue to help each other get through this latest storm."

Still, in terms of temperatures, things are better in the Midwest, Southeast and Northeast. The same can't be said for the U.S. Southwest: The National Weather Service has issued excessive heat warnings for parts of Arizona and California starting Monday, predicting temperatures from 110 to 115 degrees.

"The combination of hot afternoon temperatures and very warm overnight lows will result in oppressive conditions," the weather service said.

High temperatures Monday were expected to reach 102 in Redding, California; 98 in Sacramento, California; 104 in Fresno, California; 100 in Salt Lake City; 115 in Yuma, Arizona; and 113 in Phoenix.

CNN