People in two states took shelter amid wailing warning sirens Sunday as tornadoes touched down in Kansas and Oklahoma as part of an extreme weather system plowing through the nation's midsection.
The system, which stretched from North Texas to Minnesota, also heaved hail -- dime- to softball-sized -- as well as heavy rainfall.
Near Oklahoma City, a half-mile-wide tornado was reported, prompting an an unusually blunt alert from the Weather Service: "You could be killed if not underground or in a tornado shelter," the advisory said.
Around Shawnee, Okla., three large tractor-trailer rigs flipped over, one that had apparently been blown off a highway overpass, NBC station KFOR TV in Oklahoma City reported.
Across central Oklahoma, where multiple twisters were seen, homes were blown apart and off their foundations with some of the worst damage seen in the Twin Lakes area just outside Wellston, according to KFOR. Power lines were downed and trees uprooted.
Oklahoma Gov. Mary Fallin declared a state of emergency in 16 counties.
St. Anthony Shawnee Hospital in Shawnee, Okla., treated 11 patients, hospital information officer Carla Tollett said. One victim was in critical condition, she said; the remaining 10 were to be treated for minor injuries and released.
Oklahoma's Department of Emergency Management confirmed four injuries in Lincoln County, but no fatalities. Officials were still surveying damage in many areas. Damaged buildings were confirmed in Edmond, Norman, Lincoln County and Pottawatomie County, which declared a state of emergency.
Residents in downtown Wichita, Kan., were told to seek shelter Sunday afternoon after a tornado was confirmed on the ground – with its presence cloaked by thick thunder clouds and heavy rain.
The National Weather Service in Wichita warned of a large and “extremely dangerous and potentially deadly” tornado late Sunday. Weather spotters confirmed the tornado 7 miles northwest of Haysville and moving northeast at 30 mph, the Weather Service said.
The tornado later passed south of the city in Sedgwick County in southern Kansas, but rain and thunderstorms continued to batter the area, NBC station KSN-TV in Wichita reported.
The warning, which covered downtown Wichita as well as the surrounding area that includes Haysville, was lifted in early evening, KSN reported.
Power lines were down and at least three homes were damaged near Wichita, one with its roof blown off, KSN reported. Authorities said there were no injuries to report.
Other tornadoes were confirmed near Udall and Emporia, and danger remained in many parts of southcentral Kansas with residents told to seek refuge in storm shelters.
At least one massive tornado was confirmed on the ground near Oklahoma City, KFOR reported. The Weather Service reported that that twister was seen by spotters near Luther and was moving east at 30 mph.
The Lincoln County sheriff's office reported damage from three tornadoes that touched down, but the extent of the damage was not immediately known.
The Weather Service's Storm Prediction Center in Norman, Okla., is forecasting tornadoes, large hail and damaging winds over parts of the central Plains into the week.
Low pressure in the Plains states will keep things "very unsettled and stormy" as the week goes on, The Weather Channel reported.
On Monday, the severe storms threat moves down to North Texas and Oklahoma, through northwest Arkansas, southeast Kansas and Missouri into parts of the Upper Mississippi Valley and Great Lakes, according to the Weather Channel. Large hail and damaging winds are possible.
By Tuesday the large system is expected to be moving slowly to the east, from eastern Texas to the southern Great Lakes.
The storms are being generated by a dip in the jet stream combined with moisture moving north from the Gulf of Mexico, Kim Cunningham of The Weather Channel reported on NBC Nightly News.
The danger follows a series of tornadoes that struck northern Texas on Wednesday night, leaving six people dead and dozens injured. One of the twisters was preliminarily classified EF-4 by the National Weather Service, meaning it could have had winds up to 200 miles per hour.
Overall, tornadic activity has been slow this May, typically a bad month for twisters, said the Weather Channel’s Tom Moore.
Reuters contributed to this report.