Starting next month, the administration and its allies will conduct a nationwide campaign encouraging Americans to take advantage of new high-quality affordable insurance options. But those options will be unavailable to some of the neediest people in states like Texas, Florida, Kansas, Alabama, Louisiana, Mississippi and Georgia, which are refusing to expand Medicaid.

More than half of all people without health insurance live in states that are not planning to expand Medicaid.

People in those states who have incomes from the poverty level up to four times that amount ($11,490 to $45,960 a year for an individual) can get federal tax credits to subsidize the purchase of private health insurance. But many people below the poverty line will be unable to get tax credits, Medicaid or other help with health insurance.

Sandy Praeger, the insurance commissioner of Kansas, said she would help consumers understand their options. She said, however, that many of “the poorest of the poor” would fall into a gap in which no assistance is available.

The Kansas Medicaid program provides no coverage for able-bodied childless adults. And adults with dependent children are generally ineligible if their income exceeds 32 percent of the poverty level, Ms. Praeger said.

In most cases, she said, adults with incomes from 32 percent to 100 percent of the poverty level ($6,250 to $19,530 for a family of three) “will have no assistance.” They will see advertisements promoting new insurance options, but in most cases will not learn that they are ineligible until they apply.

Administration officials said they worried that frustrated consumers might blame President Obama rather than Republicans like Gov. Rick Perry of Texas and Gov. Bobby Jindal of Louisiana, who have resisted the expansion of Medicaid.

The Congressional Budget Office estimates that 25 million people will gain insurance under the new health care law. Researchers at the Urban Institute estimate that 5.7 million uninsured adults with incomes below the poverty level could also gain coverage except that they live in states that are not expanding Medicaid.

In approving the health care law in 2010, Congressional Democrats intended to expand Medicaid eligibility in every state.

But the Supreme Court ruled last year that the expansion was an option for states, not a requirement. At least 25 states — mainly those with Republican governors or Republican-controlled legislatures — have balked at expanding the program, in part because of concerns about long-term costs.

Several Republican governors, like Rick Scott in Florida, wanted to expand Medicaid, but met resistance from state legislators.

Mr. Obama and administration officials, including Kathleen Sebelius, the secretary of health and human services, plan to fly around the country this summer promoting the health care law to a public largely unaware of the new insurance options.

Bee Moorhead, the executive director of Texas Impact, an interfaith group that favors the expansion of coverage, said: “A lot of people will come in, file applications and find they are not eligible for help because they are too poor. We’ll have to tell them, ‘If only you had a little more money, you could get insurance subsidies, but because you are so poor, you cannot get anything.’

“That’s an odd message, a very strange message. And if people are sick, they will be really upset.”

In Atlanta, Amanda Ptashkin, the director of outreach and advocacy at Georgians for a Healthy Future, a consumer group, said: “Hundreds of thousands of people with incomes below the poverty level would be eligible for Medicaid if the state decided to move forward with the expansion of Medicaid. As things now stand, they will not be eligible for anything. What do we do for them? What do we tell them?”

Jonathan E. Chapman, the executive director of the Louisiana Primary Care Association, which represents more than two dozen community health centers, described the situation in his state this way: “If the breadwinner in a family of four works full time at a job that pays $14 an hour and the family has no other income, he or she will be eligible for insurance subsidies. But if they make $10 an hour, they will not be eligible for anything.”

Senator Frank R. Lautenberg, a New Jersey Democrat who is 89 and in ill health, traveled to Washington in April for the first time in two months to vote in favor of the gun bill, sponsored by Senators Joe Manchin III, Democrat of West Virginia, and Patrick J. Toomey, Republican of Pennsylvania.

Mr. Manchin was so grateful for Mr. Lautenberg’s appearance that he hugged him, almost lifting him out of his wheelchair. A few weeks later, when Mr. Lautenberg was looking for help in his so-far futile effort to rewrite the toothless Toxic Substances Control Act of 1976, Mr. Manchin, a conservative with close ties to the chemical industry, helped corral Republican and business support.

“This bipartisan bill represents a dramatic improvement to our broken chemical law, and I appreciate the role that my friend Senator Manchin played in helping us reach this point,” Mr. Lautenberg said in an e-mailed statement.

Mr. Manchin’s service as go-between helped bring along Senator David Vitter, a conservative Republican from Louisiana and a strong advocate for the powerful petrochemical industry. This week, Senators Lautenberg and Vitter surprised their colleagues and groups that have been pushing for a new chemical safety law by introducing a bipartisan measure that could lead to the first major update to the toxic chemical law, best known by its initials, TSCA (pronounced Tosca).

The act purports to regulate potentially harmful chemicals in industrial and consumer goods, like plastic bottles and children’s pajamas. But the law is better known for its failures than for its successes. Of roughly 85,000 chemicals registered for use in the United States, only 200 have been tested by the Environmental Protection Agency and fewer than a dozen — including polychlorinated biphenyls, dioxin and hexavalent chromium — have been restricted

After a federal appeals court denied the E.P.A. the authority to issue new limits on asbestos in 1991, the agency all but abandoned its efforts to enforce the law, even as evidence of health problems from exposure to a range of chemicals in consumer products has piled up.

Mr. Lautenberg, who will not seek re-election next year, has tried since 2005 to update the law. But Republicans and the powerful American Chemistry Council, which represents chemical and consumer products giants like DuPont, Exxon Mobil, Dow Chemical and Procter & Gamble, have stymied every effort. The council, led by Calvin M. Dooley, a former moderate Democratic representative from California, has pushed its own outline of a bill, which has never won Democratic support.

Mr. Lautenberg was determined to make one last effort at overhauling the chemical law, aides said. He introduced his bill again this year, with no Republican support. But a few weeks ago his staff began meeting with Mr. Vitter’s aides to see if a compromise could be fashioned.

What emerged was a bill that satisfied industry’s major concerns while allowing a number of Democrats, including Charles E. Schumer and Kirsten E. Gillibrand of New York and Richard J. Durbin of Illinois, to support it. Republican co-sponsors include Senators James M. Inhofe of Oklahoma, Marco Rubio of Florida and Susan Collins of Maine.

Some environmental and public health groups also signed on, although others have expressed reservations about provisions in the law or reject it outright.

Tellingly, Senator Barbara Boxer, the California Democrat who is chairwoman of the Environment and Public Works Committee, which has jurisdiction over the law, demurred. Ms. Boxer, who has supported Mr. Lautenberg’s previous bills, said this week that she would give the new proposal serious consideration but suggested that she had strong concerns about it. One of Ms. Boxer’s main considerations has always been whether any revision to the toxic substances law would pre-empt or undercut California’s stronger chemical safety laws.

The language on state pre-emption in the Lautenberg-Vitter proposal is ambiguous, allowing most state chemical safety measures to stand unless or until the E.P.A. takes action.

The bill would strengthen the current law, at least as enforced, by requiring safety evaluations for all chemicals, which would be labeled as either high or low priority based on the potential risk. New chemicals would have to be screened for safety, and the E.P.A. would gain the authority to prohibit or limit their use. The proposed measure would also require the E.P.A. to assess risks to vulnerable populations, but critics say this provision is weaker than in Mr. Lautenberg’s earlier bills.

Environmental and health groups gave the proposal measured praise. While calling the bipartisan bill a policy and political breakthrough, Richard Denison of the Environmental Defense Fund said it lacked enforceable deadlines and did not give the E.P.A. the authority it needed to protect infants, children and low-income communities exposed to high levels of toxic chemicals.

Two groups with a longstanding interest in chemical safety, the Natural Resources Defense Council and the Environmental Working Group, said the bill gave away too much to the chemical industry in exchange for modest improvements to the status quo. They said they could not support it as written.

“If you look at the bill Lautenberg was pushing last year, I don’t know if this is a retreat or a rout, but it’s somewhere in that range,” said Ken Cook, the president of the Environmental Working Group. “We will be much better off pushing for improvements at the state level than surrendering to the chemical industry’s preferences here and now.”

Mr. Cook said that under the proposed bill the E.P.A. would be required to analyze the costs and benefits of any proposed chemical regulation, a provision that he said would give an advantage to industry. He also said the bill would not assign priority status to chemicals that show up in the umbilical blood of newborns.

Mr. Dooley, of the chemical industry trade group, said that although he had some concerns about how the safety standards in the bill would be applied, he was “pretty comfortable with the balance that was struck.”

He said his group’s board decided in 2009 to try to find a compromise that would bring the law into the 21st century without jeopardizing the industry’s trade secrets or adding to its regulatory burdens. Only this year, he said, did the politics come together in Mr. Lautenberg’s impending retirement and desire for a legacy and Mr. Vitter’s ability to find Democrats like Mr. Manchin willing to support a bill acceptable to industry. “We’re delighted there’s a path forward, and we’re eager to capitalize on it,” Mr. Dooley said.

Jim Wilson/The New York Times

Workers at Sundance Berry Farms in Watsonville, Calif., are learning ways to enhance food safety, like spotting signs of contaminants or not wearing jewelry that might fall into boxes of berries.

Jim Wilson/The New York Times

Ernie Farley, right, of Andrew & Williamson, with farm workers who earn more for alerting bosses to produce contamination risks.

The initiative, unfolding along neatly planted rows of berries at the Andrew & Williamson Fresh Produce’s Sierra Farm here, is an effort to prevent the types of bacterial outbreaks of salmonella, listeria or E.coli that have sickened consumers who ate contaminated cantaloupes, spinach or other produce.

One of the workers, Valentin Esteban, is on the front lines of the new effort, having gone through a training program that helps him avoid practices that lead to possible bacterial contamination that could undermine the safety and quality of the strawberries he picks.

In exchange, Andrew & Williamson is providing Mr. Esteban better pay and working conditions than many migrant farmworkers receive, a base pay of $9.05 an hour versus the $8 average in the area.

“Sure, the money is important, but I also feel good because I am helping to improve quality and safety,” Mr. Esteban said. “Those things are important to my family, too.”

Last summer, more than 250 people in 24 states were sickened and three died after eating cantaloupes contaminated with salmonella. A year earlier, cantaloupe tainted with listeria killed 33 people.

The Food and Drug Administration laid the blame on conditions like stagnant pools of water and dirty surfaces in packing areas, problems that farm workers could help prevent.

“In those cases, the workers weren’t trained to address it or even recognize that those conditions might be problematic,” said Peter O’Driscoll, project director of Oxfam America’s Equitable Food Initiative. “Farm workers can be the eyes and ears of the farm, helping to improve food safety and pest contamination.”

Under the new program, with Andrew & Williamson the first grower to participate, berries sold under the label “Limited Edition,” would carry certification to inform consumers that food safety protocols had been followed and that the workers who harvested the crop were treated fairly.

With Andrew & Williamson paying higher wages than almost all its competitors, the participants in the program hope that the promise of better-quality, safer fruit and better conditions for workers will entice distributors, retailers and consumers to pay a little more, too.

Costco has agreed to play a major part and pay a little extra for the berries once they are certified.

“Who is it that’s delivering the result — safer, higher-quality berries? Those workers,” said Jeff Lyons, the company’s senior vice president for fresh foods. “So yes, I’m willing to pay more, so long as the certification really means something.”

Costco’s participation was critical, said Dan Glickman, an Oxfam America board member who was agriculture secretary during the Clinton administration. “This can’t be a field of dreams, where if we grow it this way, they will buy it no matter the cost,” Mr. Glickman said. “Having a retailer like Costco buy into it was key.”

Ernie Farley, a partner of Andrew & Williamson, pointed to the important role that farm workers play. “This program means that instead of one auditor coming around once in a while to check on things, we have 400 auditors on the job all the time.”

The company, which grows berries, tomatoes and cucumbers, pays a higher base wage than most growers, according to the United Farm Workers union. Andrew & Williamson also provides clean bathrooms, gloves to protect pickers’ hands, folding chairs to sit on at lunchtime and other seemingly small but much appreciated perks, like cups for water from coolers, a rare luxury for the workers.

In the program, the workers are trained in practices that enhance food safety — from not wearing jewelry that might fall into boxes of berries to spotting signs of contaminants and insects or other pests that might spoil crops. These practices can reduce the use of pesticides, something that the environmental groups participating in the project are pushing for.

Fred R. Conrad/The New York Times

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Hiroko Masuike/The New York Times

A bedroom/solarium, with marble fireplace and stone floor, opens onto a wraparound terrace. More Photos »

The asking price of the 11-room residence, PH16/17B at 1158 Fifth Avenue, is $17 million, and the monthly maintenance fee is $7,845.

With 90 feet of frontage on Fifth Avenue and an entrance on East 97th Street, the building, created as a 17-story co-op in 1924 and designed in the French Renaissance style by C. Howard Crane and Kenneth Franzheim, has an opulent marble lobby that retains its original Old World aesthetic. There are four duplex penthouses, with PH16/17B situated on the northwest corner of the building, a vantage point that provides commanding views of the Reservoir from the 16th-floor living room and the 47-foot wraparound terrace on the 17th floor, which is accessible by a curving staircase or an interior elevator.

The lush terrace, immortalized in several gardening books and featured on multiple magazine covers, was designed by the seller, Pamela M. Scurry, an interior and landscape designer, author and collector. The conservatory, created from a section of the terrace in 1984, was a passion project commissioned by her husband, Richard G. Scurry Jr., who retired as an investment manager and partner at the Jefferson Financial Group.

“I love being outside,” said Mr. Scurry, a Texan, “and sometimes a terrace can get cold, but with a conservatory you have it all. And in the summer, it’s air-conditioned.”

The conservatory’s 16-foot-high curved ceiling is reminiscent of the Crystal Palace in Paris and incorporates details from the Cooper-Hewitt and the San Francisco Conservatory; the marble bathtub, which includes an antique brass shower fixture, was made in San Francisco. There are period moldings and a floor of tiny white hexagonal tiles modeled after, Mrs. Scurry said, “the bathrooms in historic Paris hotels.”

Both sunlight and moonlight spill directly into their bedroom from the conservatory, where a flourishing lemon tree adds a touch of the tropics.

The couple said they fell in love with the apartment at first sight because it epitomized what each most craved in a home: he wanted an outdoor space with conservatory potential and indoor space with working fireplaces (it has two); she wanted a formal dining room that could accommodate the first piece of furniture she ever owned, an antique table with room for 20.

“It wasn’t just an apartment, it was like a home on top of a building,” she said. It still is, and thanks to Mrs. Scurry’s green thumb, it is a home surrounded by an English-style garden with tumbled bluestone underfoot, a vintage wall-mounted fountain, and a profusion of rose bushes, rhododendrons, lilacs, perennials and even a Mimosa tree.

They reluctantly decided to leave it, Mrs. Scurry added, because “It’s grand and big and really just too big for the two of us.” Their two children are grown, married with children of their own, and live in Texas and California, she said, so she and her husband are moving in a southwesterly direction: the historic district in Charleston, S.C., is their retirement home target.

The duplex’s entrance gallery has an Italian terra-cotta floor. To the west is the 31-foot living room with a fireplace and windows facing the park and the Palisades. To the east are the 25-foot formal dining room and the suite of rooms that make up the old-fashioned chef’s kitchen: both windows have built-in planters filled with herbs and the counters are copper.

Upstairs, where the landing is illuminated by a stain-glass window, the master bedroom opens onto the 30-foot conservatory, and a second bedroom with an en-suite bath has French doors that lead to the terrace. A third bedroom/solarium has a marble fireplace, an original stone floor with tile insets and a triple set of brass-and-glass doors that open onto the terrace. There are three rooms and a bath designated for staff.

For many years, Mrs. Scurry owned a wicker and antiques shop, Pamela Scurry’s Wicker Garden, right in the neighborhood at Madison Avenue and 93rd Street. She is, she said, taking her collection of Heywood-Wakefield wicker furniture with her to Charleston, but the garden is staying behind. “I’ll miss my memory garden,” she said.

Linda F. Stillwell and John Burger of Brown Harris Stevens are the listing brokers.

They met in Princeton, N.J., as graduate students at Princeton University, where she studied chemical engineering and he economics. Ms. Budijono, 28, now works in Princeton as a scientist, while Mr. Kagy, 31, works in Midtown East for an economic consulting firm.

The two toyed with the idea of living in New Jersey, but both prefer city life. As grad students, they often visited New York, sometimes just for dinner. When they returned to Princeton in the wee hours, “we imagined living in the city and being just 10 minutes away from home,” Mr. Kagy said.

And they never agreed with friends who cited the difficulty they might eventually face rearing children in the city.

The Upper East Side felt homey and family-oriented, filled with children on weekends. With a price range between $700,000 and $900,000 for a co-op, the couple initially hoped for a three-bedroom, but these turned out to be too pricey. So they went looking for a two-bedroom with a well-ventilated kitchen.

“For us, the home is really important,” said Ms. Budijono, who is from Indonesia. “It’s not only a place for us to sleep and shower, but a place for us to relax and hang out, and food is a very important part in our relationship. I like to cook and like to try out new recipes. People say New Yorkers don’t eat at home a lot, but I do. I need to eat at home to be able to enjoy eating out.”

They also wanted a real dining area with room for a “floating dining table,” she said, so nobody would face a wall while eating.

Last year, Mr. Kagy, who is from Montreal, went to his first open house, in Carnegie Hill, to suss out what kind of co-op their budget would allow. As he left, Sabrina Kleier-Morgenstern, an executive vice president of Kleier Residential, waved him into her open house on the same floor. Recognizing her from the HGTV show, “Selling New York,” he pocketed her card.

He started walking the cross streets of the Upper East Side, back and forth, becoming familiar with the neighborhood. “Maybe we can get more data points,” he thought, referring to such information as liquidity requirements for co-ops. He contacted Ms. Kleier-Morgenstern.

Sometimes couples disagree on what they want, she said, “but they really seemed to be in sync and have similar sorts of tastes.”

Often, places they saw were perfectly nice but had tiny second bedrooms or lacked dining space. A lovely prewar co-op in the Sutton Place neighborhood had a layout issue: with six steps up to the living room and down to the bedroom, the couple feared problems with reselling.

The location was a bit off, too: Though the apartment was across East 59th Street from the Bridgemarket, and though the view of the Roosevelt Island tramway was delightful, the couple didn’t relish walking on First Avenue beneath the dark and noisy Queensboro Bridge overpass. The apartment, listed for $875,000, with maintenance of around $2,300 a month, later sold for $872,500.

The couple decided they would rather be nearer Central Park and farther north. They hunted efficiently, kept to a schedule by Daniel Kerin, an agent at Kleier Residential. “I say it took a minute a block and two minutes an avenue, and we planned an itinerary,” Mr. Kerin said. “We are not time-wasters.”

The couple fell in love with a prewar co-op in the mid-80s near Park Avenue, listed at $832,000, with monthly maintenance in the $1,800s.

It was well located, near the 86th Street subway, and in a coveted school district. But they didn’t like the brick wall just outside the living room and master bedroom.

They returned again and again. “Each time we would go back and stare out the window and think, ‘Does it really make sense, because the view is close to zero?’ ” Mr. Kagy said. They turned the lights off and on, gauging the dimness or brightness.

“We thought maybe we could live with this, because we loved the apartment so much,” Ms. Budijono said. “We loved the kitchen, the lobby, the feel. Sometimes you cannot explain that.” They agonized, and decided against it. The place is now in contract.

In the end, they returned to an apartment they had seen earlier, in the low 90s near Park Avenue — a 1,000-square-foot two-bedroom, just at treetop level, with quaint row houses across the street.

The kitchen, though long and skinny, had a counter with seats by the window; the living room was large enough for a floating table. The bathroom had a tub as well as a separate shower stall. The master bedroom faces the street; the second bedroom looks over a ventilation system, but still gets sun and sky — which is better than having a brick wall a few feet away.

The unit was listed at $829,000, with monthly maintenance of around $1,900. The couple purchased it in March for $800,000.

“They were very, very thorough,” Ms. Kleier-Morgenstern said. “This was a thoughtful, precise decision for them.”

Though they thought their apartment might be a bit far from the subway, it doesn’t seem far because the walk is so engaging. The couple love Carnegie Hill’s small shops and its relatively quiet streets, as they knew they would.

Being so thorough, Mr. Kagy said, they had returned many times to make sure that right after they moved in, they wouldn’t be saying, “ ‘Oh, gosh, how come we didn’t see that and are stuck with it for the next five years?’

“This apartment came with no surprises. It is a two-bedroom. It is not as though we discovered a third bedroom.”

Illustration by Bob Scott

No umbrellas or wet boots in the hall.

No welcome mats or decorations on the front door.

No wearing flip-flops in the lobby.

These are but a few of the more extreme rules that apartment boards in New York City have imposed, or at least thought about imposing, on the residents of their buildings.

The average co-op or condominium has two dozen house rules. “Typically, they’re quality-of-life rules meant to benefit everyone in the closed community,” said Toni Hanson, a vice president and senior managing director of Douglas Elliman.

While there’s good sense behind many of these rules — don’t hang or shake things out the window; lay off the stereo before or after a certain hour — certain strictures can charitably be described as quirky, not to say capricious or overreaching. Your home is your castle? Think again.

It’s all, of course, in the interest of helping a building full of strong-minded New Yorkers coexist in (relative) harmony. Co-op boards have long issued directives about deportment and decorum, and condo boards are increasingly following suit. For the most part, they are well within their rights. Residents can either get with the program or get behind a co-op coup to remove the big-brother board members in their midst.

Generally, thanks to what’s known as the business judgment rule, boards have broad latitude in making, amending and rescinding house rules — the good, the bad and the decidedly wiggy. If board members think a situation needs to be addressed, they can address away without input from residents.

According to Steven D. Sladkus, a real estate litigator at Wolf Haldenstein Adler Freeman & Herz, one of the few exceptions is a stricture with a financial impact, for example, a proposal to institute a flip tax. “Then,” he said, “there has to be an amendment to the governing document, which requires a vote of the shareholders.”

Certain boards are more controlling than others, said Aaron Shmulewitz, who heads the co-op-condo practice at the law firm Belkin Burden Wenig & Goldman. “Some seek to regulate everything you do in a building, which I think would make it a less enjoyable building to live in,” he said. “But some residents like that, because they say it keeps inappropriate behavior out and keeps prices up.”

Rules tend to fall into several categories, including the use of shared spaces like the lobby or the elevator, pets and their comportment, and outward appearances — both of the apartment owner and the apartment itself. And then there’s the whole vetting process to even get into a building.

For one woman, an office coordinator in her late 20s who moved into a one-bedroom in the Clinton Hill section of Brooklyn last summer, it was the application request that took her by surprise.

In addition to the employment, asset, credit, reference and background checks a co-op board generally requires before scheduling The Interview, the officers of this particular building also demand that a security company check out the current residence of would-be buyers, a visit for which applicants must pick up the $50 tab.

“The lease on my rental was up, so I was staying with my parents on Long Island,” said the woman, who requested anonymity to avoid offending the co-op board. “I don’t think my childhood bedroom was going to give any clues about how I live, but if that was the policy, that was the policy. I wanted the apartment, so I was willing to do what I needed to do.”

The scrutinizing, which took 45 minutes, included questions about the number of beds in each room, the number of people who slept in the beds and the nature of the flooring. The inspector also took photos of the interior of the house, the applicant and her parents.

Reader, she passed muster — but remains puzzled. “I guess the board members wanted to make sure I was a regular person,” she said.

Nick Rutledge

Soon after it touched down Monday, a tornado plowed through Newcastle, Okla., as it made its way along a 17-mile path.

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.

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.

By early Monday morning, the “warning coordination meteorologist” for the local forecast office of the National Weather Service, Rick Smith, could not shake a bad feeling about the day. After checking the latest worrisome data, he sent an e-mail at 8:06 to 380 government, hospital and emergency-management officials in the region, signaling the strong likelihood of tornadoes in the early afternoon, right around dismissal time at the schools.