The threat of tornadoes loomed across parts of the Central U.S. as warnings were posted in parts of Kansas, Oklahoma and Texas amid severe thunderstorms and high winds throughout the Great Plains on Saturday.

The severe weather followed a day in which tornadoes tore through parts of Nebraska and Iowa, leveling dozens of homes on Friday.

A rare “particularly dangerous situation” tornado watch was issued on Saturday afternoon for western Oklahoma and parts of northern Texas.

Watches issued with this language mean forecasters have confidence that a few tornadoes will form and that some could be relatively intense and track a long distance. Watches are issued to remind the public to be aware that tornadoes could form and to be mindful of warnings which mean to take cover.

A notice issued by the Weather Service office in Norman, Okla., on Saturday warned residents that “dangerous supercell thunderstorms” were possible that could produce strong tornadoes.

“Many public events are occurring today and with people out of the office/school, there is a high concern in our office of people out on the roads or away from shelters,” it said.

The notice also urged residents to remain near shelters, saying that the “environment resembles that of previous historic tornadic outbreaks.”

A ‘very unstable’ outlook

The potential for tornadoes was expected to increase throughout the afternoon and early evening, the National Weather Service said.

As rich moisture surged northwestward, the outlook was “already very unstable,” said Ryan Jewell, a forecaster at the Weather Prediction Center of the Weather Service.

“It’s a fairly complicated scenario today because of the sheer number of storms,” he said. “They start interacting and there’s several different pockets of potential.”

In Norman, a city of about 130,000 people that is 20 miles south of Oklahoma City, the air was thick with moisture on Saturday morning as flag lines knocked against their poles.

That moisture was on a northwestward path, bringing with it the potential for an evolving system that could change in character as the day continues.

Excessive rainfall was likely in Kansas, Oklahoma, Texas and parts of Missouri, creating conditions for potentially damaging flash flooding.

By Saturday afternoon, nearly 5 million people were under a tornado watch.

“We could really have quite a range of weather phenomena,” Mr. Jewell said. Hail over two inches in diameter and strong winds, some greater than 70 miles per hour, were expected.

More than 100 reports of tornadoes

The far-reaching storm system comes on the heels of tornadoes on Friday that struck several areas of Nebraska and Iowa, where at least nine people were injured as winds battered the region. Dozens of homes were razed, and an industrial building collapsed.

At a news conference on Saturday in Douglas County, Neb., where more than 150 homes were damaged, Chris Franks of the Weather Service described extensive damage from winds of up to 165 m.p.h.

“These are strong tornadoes, rare tornadoes,” he said, describing a system that started in the Lincoln area, and another tornado that formed over Eppley Airfield in Omaha. “To have tornadoes that are in the less than 10 percent chance of occurring, happen in and around the metro area is extraordinary, again, with the lack of fatalities and the critical injuries.”

The service said that it had received more than 100 reports of tornadoes in at least five states in the Great Plains on Friday.

Gov. Jim Pillen of Nebraska said that he visited several hard-hit areas, which he described as “extraordinarily sobering.” He noted that strong storm cells were expected later on Saturday.

“Let’s not panic, let’s follow the rules, follow the process and make sure that we’re all safe,” he said. “Let’s just make sure we get through these next 24 hours.”

Phil Enke, an elder at Harvest Alliance Church in Minden, Iowa, said that the place of worship was leveled in Friday’s storms. Mr. Enke, 65, walked over splintered wood and debris on Saturday afternoon, looking for documents and photographs that he could salvage.

“We were just trying to get stuff that can’t be replaced,” Mr. Enke said.

“It’s a hassle and a mess, but you just have to pick up the pieces and move on,” he added.