In Arizona, heavy rains caused flooding that crippled infrastructure, closed roadways and led to the death of a woman at Grand Canyon National Park, officials said.
Rebecca Copeland, 29, of Ann Arbor, Michigan, was found in the Colorado River on Thursday after a flash flood struck during a rafting trip.
The city of Flagstaff and Coconino County opened a joint emergency operations center and Gov. Doug Ducey issued an emergency declaration for the area Friday.
"Severe post-wildfire flooding is creating dangerous challenges for communities in northern Arizona," he said. "The flooding is causing road closures, damaging property and putting Arizonans’ safety at risk."
Some of the flooding occurred in neighborhoods previously impacted by the 2019 Museum Fire.
The National Weather Service said many areas in the Copper State have received more rain in the past month than in the entire 2020 monsoon season.
In nearby Utah, flooding in Iron County contributed to a train derailment on Thursday.
The crash happened near Lund, injuring three workers, though not severely.
Monsoon season has also hit New Mexico. Highway 70 near the state's White Sands Missile Range reopened after rains covered the area in four feet of mud for more than seven miles.
Paved roads were decimated as an inch and a half of rain fell in less than 15 minutes last Sunday.
One driver told the Las Cruces Sun News on Thursday that her car had suddenly been swept away in the mudslide.
In Colorado, The Aspen Times reported Friday that mudslides and flash flood warnings had also led to the continual closures of Glenwood Canyon's Interstate 70 due to the burn scar of last year's 32,631-acre Grizzly Creek Fire.
Midwestern states have also been pounded by flash floods and thunderstorms over the last few days.
In eastern Wisconsin, floodwaters from heavy rain restricted travel, closed roads and filled basements with water.
A state police officer used a boat to rescue a man stranded downtown on top of his submerged car.
The NWS said Saturday that more heavy precipitation and thunderstorms are expected in the Northeast, with low-lying areas extremely susceptible to flash flooding.
The agency warned of "excessive rainfall" in parts of the Northeast and New England, and warned of the potential for isolated tornadoes and damaging wind gusts between western Connecticut and northeast Maryland.
Across the Southeast, Lower Mississippi Valley and Southern Plains, isolated downpours and flash flooding remain a possibility.
The NWS said the southwestern monsoon could continue to produce an isolated flash flood threat, with additional rainfall and thunderstorms through the weekend.
More heat warnings and watches have also been issued across Montana and the Intermountain West, and the Northern Plains and Northern Great Basin can expect to see above-average temperatures once again.
The heat will exacerbate the drought felt through the region, increasing wildfire danger.
The Associated Press contributed to this report.