Roughly one-third of Americans are seeing midsummer-like temperatures this weekend, as heat and humidity began to roast the Northeast and Mid-Atlantic States on Saturday, potentially setting hundreds of daily heat records. More than 38 million people were under a heat advisory Saturday afternoon.

In West Virginia and New Hampshire, public health officials urged people to look out for symptoms of heat exhaustion. In Washington, D.C., officials activated heat emergency plans, opening splash parks and cooling centers. A runner in the Brooklyn Half Marathon — where organizers had warned participants of potential heat concerns — died Saturday morning, although it was not immediately clear if temperatures had played a role.

Elsewhere in the country, the misery set in weeks ago. In drought-parched New Mexico, the largest wildfire in the state’s recorded history is burning months before peak fire season. Other blazes are driving evacuations and fears in Colorado, Arizona and Utah.

Parts of Texas, where heat-intensified wildfires burned 30 structures near Abilene this week, saw their earliest triple-digit temperatures on record this month. San Antonio has hit 100 degrees four times in May, more than it did in all of 2021. Dallas-Fort Worth reached 95 degrees for a fourth consecutive day Friday, making it the longest streak of such high temperatures recorded this early in the year.

And in a sign of just how strange things could get, Denver whiplashed from 90-degree weather this week to a late-spring snowfall overnight Friday into Saturday.

Memorial Day, the unofficial start of summer, is still more than a week away. But by the end of this weekend, more than half of all Americans will have experienced temperatures climbing to 90 degrees or higher from a blast of hot air that started in the Southwest, swept across the eastern third of the country, and will move this weekend through New England and even into Canada.

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Meteorologists warned that scores of heat records could be tied or broken in some 20 states.

In many places, temperatures could be 20 degrees or more above what residents are accustomed to this time of year. In Boston, for instance, the average temperature for the pre-Memorial Day weekend is typically in the high 60s; on Sunday, forecasts show a high of 95 degrees.

In Trenton, New Jersey, a predicted high of 94 on Saturday would match the record temperature set in 1934, according to the forecasting service AccuWeather. In Richmond, Virginia, on Saturday, temperatures climbed to the mid-90s as the city held its annual outdoor Boulder Bash, a competition that brought some of the nation’s top professional climbers together.

In Baltimore, temperatures peaking at 94 degrees were expected to drop to 90 in time for the horses running in the Preakness Stakes.

The predicted high of 96 degrees in Worcester, Massachusetts, on Sunday would exceed the monthly record of 94 degrees, set in 2010, the National Weather Service said.

The agency’s office in Gray, Maine, with a territory covering Maine and New Hampshire, noted that it has never issued a heat advisory during the month of May. “With the forecast continuing to hint at record breaking heat, and high humidity, this weekend,” the Weather Service said in a post on Twitter, “this streak may end.”

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The good news: The heat in much of the country is expected to pass relatively quickly — too quickly even to qualify in most places as an official heat wave, which in the Northeast is defined as three straight days in the 90s or above. A strong cold front reaching from the Great Lakes into the Southwest is predicted to gradually drift south and east over the weekend, according to a Weather Service forecast issued Friday.

Still, officials warned that even brief spells of intense heat carry the potential for grave danger, particularly for the most vulnerable populations.

Some cities opened splash parks earlier than scheduled, as well as cooling centers. But some cities, like Springfield, Massachusetts, have resisted opening cooling centers, balancing the risk of the heat against the dangers of exposure to the coronavirus.

Officials across a large swath of the country urged residents to take measures to avoid heat exhaustion and stroke, including staying indoors and in air-conditioned spaces as much as possible. But health officials in Westchester County, New York, cautioned against a danger that might seem unexpected: hypothermia.

Although temperatures could reach 90 degrees in the communities just north of New York City, the water at local beaches will still be frigid — as chilly as 50 degrees.

“Ocean temperatures remain cold,” Dr. Sherlita Amler, the health commissioner for Westchester County, said in a statement Friday, “so when a person is fully submerged, hypothermia can happen in as little as 10 minutes, greatly increasing the risk of drowning. It’s also important to take care in the heat. Drink lots of water and seek the shade or air-conditioned places.”