Intense rain and hail battered parts of Oklahoma on Saturday afternoon and tornadoes touched down in Texas and Colorado, but the greatest threat of severe weather across the Plains was expected to come later.
Major thunderstorms, including the potential for tornadoes, threatened tens of millions of people in parts of Kansas, Colorado, Texas, and much of Oklahoma.
The National Weather Service said tornadoes were confirmed to have touched down near Cisco, Texas, and Truckton, Colorado. There were no reports of serious damage.
The threat for severe weather goes through Sunday — Mother's Day — extending a run of violent conditions for the region.
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Golf ball-sized hail and deadly floods battered Oklahoma late Friday, and there were preliminary reports of three twisters in Texas.
Oklahoma City officials said Saturday that 6 single-family homes, 3 businesses, the Maranatha Assembly of God church and 86 mobile homes and RVs were destroyed in severe storms that struck the city on Wednesday. Seventy-six buildings and mobile homes suffered major damage in those storms.
Saturday afternoon, the National Weather Service in Norman, Oklahoma, posted a tornado watch that stretched 400 miles, from San Angelo, Texas, to Oklahoma City. The watch extended till 10 p.m. ET.
Another tornado watch was posted for eastern Colorado and western Kansas.
"All the ingredients are coming together for a major severe thunderstorm and tornado outbreak Saturday for the central and southern Plains," said Weather Channel forecaster Michael Palmer. Saturday's threat was greatest in western Kansas and western Oklahoma, he said.
"Golf ball- or even baseball-sized hail is possible from some of the thunderstorms, with winds of 60 to 70 mph," he added.
The stormy weather caused havoc with air travel. Departures destined for Dallas/Forth Worth International Airport were averaging delays of more than two hours on Saturday afternoon.
Worries of a major outbreak of long-track tornadoes eased some Saturday morning with the arrival of cloudy skies.
"Big tornado outbreaks almost always need mornings with sunshine to heat up and destabilize the atmosphere. We have the opposite this morning," said Bill Karins, MSNBC meteorologist. "Tornadoes will still occur, but the big ones that stay on the ground for a long time are less likely."
The storm system was set to move slowly eastwards, primarily affecting the Ohio Valley down to Mississippi on Sunday.
The northern fringes of the system were due to bring snow to Wyoming and Colorado — and even blizzard conditions were possible in parts of South Dakota. Up to five inches of snow could fall in the Nebraska Panhandle, and South Dakota could receive up to a foot in some areas, according to the National Weather service.
In Shawnee, Oklahoma, the Red Cross opened a shelter Friday night because officials said a Granada Lake dam was close to being topped after heavy rains, threatening homes in one neighborhood. Officials also closed some roads, including part of Interstate 44 in Tulsa.
Heavy rain has swollen Oklahoma creeks and rivers, dramatically increasing the likelihood of flash flooding.
One deluge was so heavy that a 43-year-old Oklahoma City woman drowned after becoming trapped inside her underground storm cellar.
"It just flooded with her in it, and she couldn't get out because it was like a river coming down on top of her," police Sgt. Gary Knight said. "I don't recall it ever raining like that before."
Skylyna Stewart's body was discovered in an older, underground shelter detached from the home.
The 7.1 inches that fell in Oklahoma City was the third-heaviest rainfall for any day on record, dating back to 1890, said state climatologist Gary McManus. Radar data from the part of the city where Stewart's body was recovered indicated as much as 8 to 12 inches may have fallen.
The Associated Press contributed to this report, which originally appeared on NBCNews.com.