(CNN) — After heavy rain and flash floods in parts of the Southwest this weekend, more than nine million people across the southern Plains are under flood watches Sunday, including the Dallas-Fort Worth metroplex and Shreveport, Louisiana.

Heavy rain falling across the Texas/Oklahoma border Sunday morning is expected to continue into Monday, with 4 to 8 inches of rain forecast overall across the region. Some areas may get more than 8 inches and flash flooding.

“We don’t know which areas will receive the 8+ inches of extreme rainfall, but if it does occur near you, significant flash flooding will result,” the National Weather Service office in Fort Worth/Dallas warned.

“Everyone in the watch area should plan on extra commuting time, especially on Monday morning,” the weather service said.

A Level 3 out of 4 “moderate risk” of excessive rainfall has been issued by the Weather Prediction Center, mainly across northern Texas, where as much as 3 inches of rain could fall per hour during the heaviest storms on Sunday.

“Much of this rainfall will be beneficial and welcome due to the effects of an ongoing drought, but the potential still exists for instances of flash flooding in urban areas and places with poor drainage,” the Center said.

More than 90% of the state of Texas is currently experiencing drought conditions with nearly 62% under extreme or exceptional drought conditions, the highest two categories.

Additional flood watches could be added in surrounding locations later Sunday.

Some rainfall persisted in parts of Arizona and New Mexico Sunday following floods in parts of the Southwest Saturday.

In Utah, several hikers were “swept off their feet” Friday in Zion National Park by a flash flood. Search and rescue team members were still working Saturday to find a missing hiker near the Virgin River, the park said.

In New Mexico, about 160 people had to shelter in place for several hours at Carlsbad Cavern National Park Saturday because of flash flooding, the City of Carlsbad said in a Facebook post.

The park will be closed Sunday, the National Park Service said.

“Maintenance crews will begin to assess and clean debris from the roadway,” the National Park Service added.

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