(CNN) Holiday barbeques and pool parties might get bumped due to severe weather, tornado watches and potentially catastrophic flooding.
From Wyoming to Ohio, more than 25 million people are at risk for severe weather Monday, CNN meteorologist Dave Hennen said.
The areas at greatest risk include parts of Nebraska, Indiana, Ohio and Illinois, where the Windy City of Chicago and parts of Colorado are at risk of tornadoes.
And while the South bakes in record-setting heat, the Grand Canyon could get blanketed in snow.
Here's what different parts of the country are facing this Memorial Day:
About 10 million people are spending the holiday under flood warnings. Many are in states already pummeled by last week's severe weather, which killed at least 12 people.
"Another round of heavy rain is on tap today for parts of the Midwest and Plains," Hennen said.
"Many rivers are already in major flood (stage) across the region, and the Arkansas River is expected to exceed record flooding over the coming days."
Residents in several Arkansas counties already started evacuating over the weekend, officials said.
Parts of the Arkansas River could crest over 4 feet above the record, meaning "catastrophic flooding is possible in the towns of Van Buren and Fort Smith," Hennen said.
In Oklahoma, where six people were killed in severe weather last week, the situation "still could get worse," Gov. Kevin Stitt said Monday.
"We still have water still rising in the east," he said. "We are not out of the woods yet."
A state of emergency was still in effect for all 77 counties on Monday. More than 100 injuries have been reported at local hospitals, the state health department said.
The governor said he toured the destruction left by a tornado that struck El Reno on Saturday night, noting it was "unbelievable how violent" it was.
Some of the mobile homes, he said, "look like they were blown up."
It's still three weeks before summer, but Memorial Day temperatures in the South have soared past average summer temps.
Cities such as Columbia, South Carolina, and Augusta, Georgia, are expected to hit triple-digit, record-breaking temperatures Monday.
And dozens of other cities from Tennessee to Florida could shatter their records for high temperatures.
Out West, tourists hoping for a warm, sunny trip to the Grand Canyon will instead face brutal winds and unusually cold weather.
Clear views of America's most beautiful ditch could be interrupted by something rarely seen this late in the year -- a blanket of snow.
'It's common for snow to fall in Flagstaff in April and May, but to have accumulating snow after May 26th is quite unusual," said the National Weather Service's office in Flagstaff, Arizona.
In fact, "it's only happened 8 times since we've been keeping records" back in 1898, the NWS Flagstaff office said.
Flagstaff is about an hour south of Grand Canyon National Park, which about 5.9 million tourists visit every year.
Across northern Arizona, "unusually cold" weather is paving the way for dangerous travel conditions, the NWS office said.
"We've had several slide-offs in this area and a report of hail," the Arizona Department of Transportation tweeted. "Please slow down in northern Arizona because roads are extremely slick and snow is starting to fall."
There is a tornado watch for northeastern Colorado until about 9 p.m. local on Monday, CNN meteorologist Taylor Ward said. Areas in western Colorado that are at higher elevation are under a winter storm watch for Tuesday, Ward said.
The Army Corps of Engineers said it was accelerating the release of water at Keystone Dam to 275,000 cubic feet per second (CFS) Monday morning, "with the expectation to leave it at this level through Thursday."
Tulsa Mayor G.T. Bynum said the Oklahoma Guard was patrolling the city's levee system 24 hours a day.
"We continue to urge residents along the river to plan for the flood of record (the worst flood in Tulsa history), which was equivalent to 305,000 CFS," the mayor said.
"The levees continue to operate as designed, but this will place an even greater load on them," he said, urging residents living behind Levees A and B to relocate.
Tulsa city officials issued a warning for residents to brace for significant flooding:
"Due to the additional release at Keystone Dam, flood water is expected to rise and residents and businesses along the Arkansas River must remain vigilant and take precautions."