JOPLIN, Mo. – When the tornado hit, Staci Perry, a scrub technician at St. John’s Regional Medical Center, had just left the operating room to grab a piece of equipment for a surgery in progress. An urgent announcement came over the loudspeaker: “Execute condition gray.” That was the hospital’s code for an impending disaster, though in drills, the command was always preceded by “Prepare for condition gray.”

There was no time to prepare. As she heard the glass walls crack, Perry, 33, dashed back to surgery. “The pressure in everyone’s ears was just tremendous,” she said. A physician’s assistant threw himself against the door so it wouldn’t blow in and destroy the operating room. The lights went out. The wind howled.

“Literally, the hospital imploded,” said Dr. Jim Riscoe, an emergency room physician at the 230-bed facility. There is an emergency plan for disasters, he said, “but they don’t anticipate the emergency being the hospital.”

When it was over, just after 5:30 p.m. Sunday, a six-mile swath roughly a half-mile wide had been gouged. At least 116 people died. Five were hospital patients and one was a visitor.

The apocalyptic after-images were depressingly familiar, reminiscent of those from the April tornadoes in the South: rubble as far as the eye could see, cars buried under pieces of houses, trees wrenched from the ground with their huge roots reaching toward the sky, columns of smoke rising from gas fires, emergency vehicles with lights flashing. And everywhere, people stunned by nature’s violence mourned their losses, counted their blessings and told their harrowing stories.

In torrential rain, lightning and heavy winds, rescuers went door-to-door Monday, gingerly avoiding debris and downed power lines that ignited fires fueled by leaking gas. They pulled seven survivors from the rubble, officials said. Joplin officials said more than 2,000 structures were ripped apart and whole neighborhoods were obliterated. Power remained out Monday on most of the city’s west side. Residents were advised to boil water.

“We still believe there are people to be saved in the rubble,” said Missouri Gov. Jay Nixon, who deployed about 140 National Guard troops to help with the rescues.

The devastating tornado that hit Joplin, Mo., may have been just one blow in an outbreak of severe weather that could strike Plains states again today. Forecasters said Monday that people in Kansas, Oklahoma, northern Texas and western Missouri should monitor the weather closely today.

“This is a very serious situation that’s brewing and it is possible, if not likely, that we’ll be moving to our highest alert status sometime during the next 12 to 24 hours as we gather more information,” said Russell Schneider, director of the Storm Prediction Center in Norman, Okla.

Cities affected, he said, include Joplin, Wichita, Kansas City, Topeka, Oklahoma City and Tulsa.

President Obama, visiting Ireland, expressed his condolences in a telephone call to Nixon. Federal Emergency Management Agency Administrator Craig Fugate was on his way to Joplin to coordinate federal disaster relief, said White House spokesman Nick Shapiro.

The National Weather Service said Monday that the Joplin tornado, with winds as high as 198 mph, was the deadliest single tornado since 1953, when 116 were killed in Flint, Mich. It was rated F-4 on the Fujita scale, one step below the strongest tornado. (Multiple tornadoes were responsible for about 340 deaths over four days in late April, mostly in Alabama.)

The weather service also said that more than 100 tornadoes have occurred in May, the most active month for tornadoes. The May record of 542 tornadoes was set in 2003.

A tornado also touched down Sunday in North Minneapolis, Minn., killing two people.

Dr. Jason Persoff, 39, a Florida internist and storm chaser, had been in southeast Kansas on Sunday with his storm-chasing partner, also a physician, when they realized they were seeing something huge. “This storm will do incredible things,” Persoff remembered thinking.

But as they headed into Missouri, they heard on the radio that a “debris ball” had been spotted on radar — a mass of material torn from the ground, carried along by the turbulence. Persoff’s excitement turned to terror. He pulled off the highway into Joplin and saw crumpled semi trucks. He flagged down an emergency worker and got directions to the nearest functioning hospital, where he and his friend helped treat patients all night, amazed at the dedication of the hospital staff.

“They didn’t know what happened to their families,” Persoff said, “and yet they were focused 190 percent on keeping people alive.”

A few blocks from St. John’s Hospital, where a helicopter had been blown off the roof, Zach Simonds was cleaning floors at the Greenbrier Nursing Home when he heard a “code red” announcement on the intercom. About a dozen staffers tried to gather up the 85 patients into the central hall, since the home had no basement. Some refused to budge. But most clustered in the hall.

The tornado tore the roof off the nursing home. Simonds could see cars being tossed around overhead. “Everybody was praying, you could hear people praying, ‘Please God, don’t kill me,’” he said. Then, the glass-plated front of the building burst.

“It just sucked everybody out,” said Simonds, who was able to duck into a closet-sized room with a few other people, including a man in a wheelchair. When he left to find help, he had to pick his way past decapitated bodies. And there was no help to be found. All the buildings he passed were destroyed. “I didn’t think anybody was alive,” Simonds said.

Behind the nursing home, a Catholic church was destroyed. All that stood was a cross.

All around Joplin, families reunited gratefully, still puzzling over their fates and those of others less fortunate.

Melissa Clark and Richard Slimp, both 26, took refuge with their four children and six neighbors in the basement of their white clapboard house. When the tornado passed, they emerged to find their house seemingly intact, except for the windows.

But Monday afternoon, torrential rain poured through the roof. “Every one of our walls look like we mowed in here,” waving at the layers of mud and grass and debris surreally caking the vertical surfaces.

Joplin’s Walmart, on a slight incline overlooking the city’s business district, was half-collapsed, with ceiling supports sticking out like an exposed ribcage. In the parking lot, dozens of destroyed cars were stacked in pyramids.

“I hope to God nobody was in these cars,” said Cavin Cowan, 45. He pointed to a ravaged Buick. “Somebody was probably still in that one,” he said. “The lights are on, the radio’s still going.”

David Utter, 26, was driving with his wife, and two children when the tornado touched down. “The rain started going sideways, and it lifted us up and pushed us into the oncoming lane,” Utter said. They were unharmed; their van was untouched. Monday, he was standing in front of a brick home, looking for a friend and the friend’s 3-year-old son.

“I’ve lived here my whole life,” he said, “and I no longer recognize where I am.”