Oklahoma and Arkansas were bracing for their worst-ever flooding as a new wave of storms forecast to roll through the region threatened to further bloat the Arkansas River that already has reached record crests in some areas.

Forecasters reported tornadoes, high winds, hail and heavy rain across the region on Monday, triggering evacuations and high-water rescues. The storms are the latest to rip through the Midwest over the past two weeks, leaving at least nine dead and a trail of damage from high winds and flooding.

In Tulsa, the Oklahoma National Guard patrolled the city's stressed levee system.

"The levee system is still operating as designed," Mayor G.T. Bynum said Monday. But he said that could change: "We are asking for everyone to prepare for the worst-case scenario ... the worst flood in our history."

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Bynum urged residents near the levees to "proactively relocate," and the city has opened multiple shelters. He said authorities were reviewing how such flooding would impact the city's infrastructure.

The river is forecast to reach a record crest Tuesday evening, according to the National Weather Service. Tulsa increased its releases of water from the Keystone Dam, adding to the woes downstream in Fort Smith, Arkansas, where the river already surpassed its historic crest Sunday.

Some residents were forced to evacuate. Fort Smith Mayor George McGill said the city is experiencing record flooding, and high-water rescues were underway.

McGill warned residents to be careful traveling around the city. But he said residents are known for their grit and expressed confidence the city would overcome and thrive.

"It's a sight that we've never seen before, but just like we recovered from other record-breaking floods we will recover from this," McGill said. "There is nothing you can do about Mother Nature."

Meteorologists on Monday recorded 45 tornadoes, primarily in Colorado, Iowa and Indiana. Twisters also touched down in Minnesota, Illinois and Idaho, according to preliminary reports by the weather service's Storm Prediction Center. Several houses were damaged, but no injuries were immediately reported.

Flash flood watches and warnings persisted late Monday in parts of northeastern Indiana, western Nebraska and southeast Wyoming, according to the National Weather Service said.

Additional heavy rain and severe thunderstorms were forecast through Wednesday before the region sees a possible respite. But it might not last long.

"There are early indications this weather pattern could return next weekend and into the following week with more rounds of severe weather across the central U.S," AccuWeather Meteorologist Brett Rathbun said.

Meanwhile, the Southeast may see record highs through Wednesday, the National Weather Service said, with temperatures soaring well into the 90s and lower 100s for some areas.

Contributing: Kristin Lam, USA TODAY