Early Monday afternoon at the Shear Perfection hair salon, a stylist named Lisa Lentz decided to outrace it. Her one o’clock, a cut-and-color, was done, but two other clients had just canceled, and the ominous tone of Gary England, the meteorological oracle on News 9, commanded attention.

Ms. Lentz, 47, left the other stylists, who would soon be praying in the lemon-scented bathroom, plastic baskets over their heads like combat helmets. She hustled to her old minivan, redolent of pampered dogs and countless family journeys, and looked to the western sky: like smoldering charcoal, and all too familiar.

Not 10 miles to the southwest, in a two-story brick house in the small city of Newcastle, Shelly Codner, 50, was making the same reacquaintance, only from a closer angle. Two televisions were shouting a duet of warning, and an alert on her iPhone was saying the thing was five miles from the nearby Newcastle Casino.
Time to grab the emergency bucket, packed with flashlights and glow sticks, bottled water and food for dogs and baby. Time to head to the steel-encased storm shelter built into the garage floor last summer, with husband, teenage daughter, 11-month-old granddaughter, and all five dogs. Oh, and a couple of Fisher-Price distractions.

Here we go. Again.

In this breeding ground of Oklahoma tornadoes, people prepare for the season with the care that the defensive coordinator for their Sooners prepares for the inevitable autumn. They develop family plans, hang on the words of meteorologists, and, in places like Moore, become accustomed to the Saturday noontime testing of emergency sirens. At the same time there exists disbelief that the devastation visited upon neighbors could ever happen to them or, that is, could ever happen to them again.

Shelly Codner, 50, at what is left of her home in Newcastle.

Amid all the siren tests and awareness and false alarms, the warning can still be a half-hour, maybe a little more, maybe a little less. This means you must stop what you are doing, shake off the disbelief, track down loved ones and find shelter, all in the time it takes to watch a few rounds of “Jeopardy!”
A half-hour or so is not much, but the alert system saves lives. This monstrous tornado would kill 24 people, but in an earlier, low-tech time, the number of dead would certainly have been much higher.

As Ms. Lentz pulled out of the strip mall’s lot and turned right onto South Western Avenue, she saw to her right a pulled ashen curtain in the sky — the creation of the collision between cool, dry air down from Canada and warm, moist air up from the Gulf of Mexico, helped along by the strong jet-stream winds over the Oklahoma plains. This natural alchemy had created a whirling vortex, awesome, destructive, seemingly alive.
Despite the caprice of its path, she still found herself thinking as she drove on, “It’s not going to hit you.”

But tornadoes are not benign optical illusions. Thirteen miles to the southeast, on the University of Oklahoma campus in Norman, a shift of a half-dozen forecasters at the National Weather Service’s storm-prediction center was continuing the never-ending monitoring of heavy weather across the continental United States. The focus at the moment was their backyard.
They had spotted the trouble a week in advance. Synthesizing information from satellite and radar, computer models and ground-level observations, the center warned on Wednesday, May 15, of severe weather headed for the region, pointing to Monday, May 20, as having perhaps the “highest tornado potential.”

Thursday, Friday and Saturday provided no reason to alter the prediction. On Sunday, the center tightened the potential target area, warning of a “threat for very large hail and tornadoes” from central Oklahoma into west central Missouri.