With a ferocious heat wave bearing down on New York City, a blizzard would seem to be the furthest thing from Mayor Zohran Mamdani’s mind.

But just five months ago, the deepest cold snap New York City had seen in years, accompanied by nearly a foot of snow, presented the mayor with one of the first crises of his fledgling administration. More than two dozen people died, most after exposure to cold outdoors but some in their homes, and Mr. Mamdani was criticized for letting people endanger their lives by not forcing them to come indoors.

Now, once again, Mr. Mamdani is preparing to prevent another wave of weather-related deaths as triple-digit temperatures are expected to arrive by Thursday, with parts of the city hitting 102 degrees. The heat index, a measure of how hot it feels based on temperature and humidity, is not expected to dip much below 90 on Wednesday, Thursday and Friday nights.

This week, in addition to activating about 600 cooling centers, the city is launching a new fleet of 15 outreach vans, staffed by nurses, to provide wellness checks both indoors and outdoors, medical care and emergency transportation to people at risk from the heat. The vans are modeled on ones the city sent out for the first time during the winter.

Joe Calvello, Mr. Mamdani’s press secretary, said that the lessons learned during the first cold wave — some of which were implemented during a second snowstorm in late February in which no deaths were recorded — were helping to guide the city.

“The proactive posture New Yorkers saw this winter, pre-positioning and moving earlier, deploying more, and going to our most vulnerable neighbors rather than waiting for them to come to us, is the same posture driving this week’s heat emergency plan,” Mr. Calvello said in a statement.

City parks are doubling the number of cooling centers they operate, and city-run community-health centers and overdose prevention centers are being used to provide respite from the weather, as they were during the winter.

The city is also expanding pop-up cooling stations for outdoor workers, including street vendors, delivery workers and day laborers, offering cold water and misting fans. Last week, the mayor issued an executive order aimed at protecting people who work outdoors.

Even people who are sleeping indoors are not out of harm’s way if they lack working air-conditioning.

Dave Giffen, the executive director of the Coalition for the Homeless, which monitors conditions at city shelters, said that during inspections several months ago, more than a quarter of shelters in the city’s main shelter system had no air-conditioning at all in the areas where people sleep.

When asked for options for New Yorkers to stay cool overnight when cooling centers are closed, the city recommended homeless drop-in centers, which are open round the clock.

The heat is particularly perilous for older people.

Allison Nickerson, the executive director at LiveOn NY, an advocacy group for older New Yorkers, said that beyond the emergency measures Mr. Mamdani is implementing, the city should help fill funding gaps in the federal government’s Home Energy Assistance Program, which helps people pay their electric bills.

“Many low-income people in their 90s will say, ‘Oh, a fan is sufficient, and I can’t afford my bills, so I’ll just keep the fan on,’” Ms. Nickerson said. “Well, if it’s 103 degrees in your apartment, you will die. You will not recognize that you’re dying, so having that blanket ability to help everybody who needs it is what the city should be looking at.”

During the first cold snap of Mr. Mamdani’s tenure, in late January and early February, at least 20 people died after exposure to the cold outdoors, and at least seven died of cold in their homes. Most of the people who died outside were homeless and many had shown signs of mental illness.

As the mercury fell, there were calls for the city to move aggressively to get people off the streets against their will in order to save them from freezing to death. But Mr. Mamdani, who had pledged to reduce the use of involuntary removal of homeless people from the streets if they were not actively threatening other people, held back.

During a three-week period that included the cold snap, the city conducted about two involuntary removals per day, less than during January and February of 2025 and about the same as that period in 2024.

The city stepped up efforts to persuade people to come indoors voluntarily, but many people simply refused.

At a City Council hearing on the administration’s response to the cold, the council’s speaker, Julie Menin, said the deaths were “not inevitable.”

“Every person who freezes to death in the city is a reminder that systems that are designed to protect human life are failing the people who need them most,” she said.

When a blizzard hit the city in late February, Mr. Mamdani had another chance to get the response right. This time, from the start of the storm, the city sent out ambulettes to deliver cold-weather supplies to people living outside, increased outreach, made new emergency shelter beds available and kept overdose prevention centers open all night. Nearly 20 inches of snow fell over two days, but no lives were lost, in part because temperatures did not get as cold.

In response to questions this week about whether the city would involuntarily take people off the streets if they refuse to get out of the heat, officials said only that the city would take people to hospitals if they fit the legal criteria for involuntary removal. If someone is mentally ill and behaving in a way that poses a danger to themselves or others, state law lets the police or medical providers order the person to be taken to a hospital for evaluation.

Mr. Giffen of the Coalition for the Homeless said that he worried that for some people, a round of visits from outreach workers would not be enough. “I’m very, very concerned that people are going to die because of this level of heat,” Mr. Giffen said on Tuesday.

“There needs to be repetition,” he said. “You can’t just check on somebody once and then move on. So it’s not a small job, but it’s a life-death one.”