(CNN) -- Even as the Midwest, Southeast and Northeast cool a bit, the U.S. Southwest is practically boiling.
The National Weather Service has issued excessive heat warnings for parts of Arizona, California and Nevada through Wednesday, with temperatures forecast to peak Tuesday at 113 degrees in Las Vegas; 113 in Phoenix; 116 degrees in Yuma, Arizona; and 125 degrees in Death Valley, California.
In the Las Vegas Valley, local officials are opening up cooling stations Tuesday to give residents a place to beat the heat. Forecast highs in the 100s will continue into next week.
It's 100 degrees outside (or worse). How's a fur-covered pup to cope? CNN iReporters from around the United States have been sharing photos of their best friends cooling off, and let's face it: They're too cute not to post.
Here, Prim the Afghan hound manages to look elegant while relaxing in a baby pool in Gahanna, Ohio. Beautiful standard poodle Gracie has water up to her shoulders in her family's above-ground backyard pool in Barberton, Ohio. Dexter, a Cavalier King Charles spaniel, decides it's too hot even to swim in Constant Lake in Ontario, Canada. Four-month-old German shepherd Zhik naps on a float in his family's pool in Tulsa, Oklahoma. Sierra's family says they can't keep her out of their Loxahatchee, Florida, pool. Here, the sopping wet yellow lab takes a rest on a float. Flossy, a 5-year-old Maltese, captains a boat raft off the shores of Destin, Florida. Year-old black lab Jackson enjoys his baby pool in Fort Wayne, Indiana. It's not just American dogs that need to cool off. Sandro Capannolo spotted this pup taking a dip in a fountain in Rome. And Istvan Szemes of Hungary shared a video of his German shepherd gleefully splashing around in a baby pool.
Dog days of summer The scorching temperatures inflamed the passions of some parents whose sons started preseason football practice in Las Vegas on Monday.
"I just feel that at 111 degrees ... I don't feel that it's even reasonable," a local mom told CNN affiliate KTNV. "I can't believe the school district is even allowing this because it is so dangerous ... These kids should not be playing."
The Clark County School District said it's up to each coach to determine their practice schedule.
"If it is too hot to practice and I feel like kids can't handle it, then we will adjust our plan and either cut practice short or move inside," Liberty High School football coach Richard Muraco said.
Las Vegas Fire and Rescue said it gets as many as 10 extra calls a day when the extreme heat kicks in. But the calls tend to involve the homeless and people not acclimated to the region, not people used to working outside during the summer, affiliate KVVU reported.
Hydration is key, with officials recommending three-times normal water intake.
"When you start to feel thirsty, the body is already damaged," said Las Vegas Fire and Rescue's Tim Szymanski.
Some may think that the excessive heat warnings are, well, a bit excessive given they're being issued for the desert where 100-plus temperatures aren't uncommon.
Not so, says Gary Woodall of the National Weather Service in Phoenix.
"We look at our high temperatures and the low temperatures that we're forecasting," he told affiliate KPHO. An average temperature of 100 degrees for the day is key, since that would limit the body's ability to cool itself.
"We have tried to recognize those cases where we see more of a human impact, and that's what we're trying to represent with those excessive heat warnings," he said.
As the Southwest heated up, the rest of the nation began returning to normal summertime temperatures.
A cold front pushed through the Midwest, as well as parts of the South and Mid-Atlantic states, sparking severe storms and heavy rain in some areas.
Flash flooding hit downtown Greensboro, North Carolina, on Monday night as up to five inches of rain fell, said Darin Figurskey, a meteorologist at the National Weather Service's Raleigh bureau.
"Motorists are encouraged to stay off the roads, if possible," the city said in a statement.
Nearby on Interstate 40, between 20 to 30 cars were stranded due to flooding rains, said Figurskey.
Severe lighting and heavy rain also raked metro Atlanta Monday night.
The front cooled things down.
High temperatures Monday were in the 80s in cities including Chicago, New York, Boston and Washington, where temperatures on Sunday hit 102 degrees and surpassed 95 for the 11th straight day. Even the 91 degrees recorded Monday in St. Louis was a welcome relief.
This cool-down follows a heat wave that roasted much of the country for more than a week and comes as the National Climatic Data Center reported the mainland United States has experienced the warmest 12 months since record-keeping began in 1895.
The report does not take into account blistering heat from this month, with 2,116 high temperature marks either broken or tied between July 2 and July 8 in communities nationwide.
But it does incorporate the warmest March 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, 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.
The average temperature for the U.S. mainland in June was 71.2 degrees -- 2 degrees above the 20th-century average. It was the 14th warmest June on record.
The conditions contributed to dozens of heat-related deaths, including 18 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, because of severe storms that swept east from Indiana to the Mid-Atlantic states starting on June 29.