The last full day of spring will feel more like deep summer across much of the eastern half of the country.
Parts of the Midwest have been sweltering for days, and temperatures across the Northeast are expected to reach the mid- to upper-90s on Wednesday, just as they did in parts of the region on Tuesday. Humidity will make the air feel even hotter, and nighttime temperatures are expected to remain uncommonly high.
The springtime heat, which will linger through at least the weekend in part of the region, has already caught some cities off guard. Chicago was forced to open cooling centers earlier in the year than usual. And in New York City, the public pools are not open yet.
Inland New England and upstate New York began to swelter this week even before cities closer to the coast. In Rochester, N.Y., the heat index — an indication of how the air feels, accounting for humidity — soared higher on Tuesday than in San Antonio or St. Petersburg, Fla.
There were heat advisories in place all across New England late on Tuesday, along with some excessive-heat warnings that stretched as far north as Maine. More than 76 million people were under extreme heat advisories, watches or warnings, according to the National Weather Service. That is nearly double the population of the state of California.
Officials warned that even where temperatures would not break records, the length of this heat wave, coming so early in the year (spring officially ends with the summer solstice on Thursday evening), could make it especially hazardous.
“The early arrival of this magnitude of heat, the duration, abundant sunshine and lack of relief overnight will increase the danger of this heat wave beyond what the exact temperature values would suggest,” a National Weather Service update said on Tuesday.
The meteorological phenomenon driving the high temperatures is sometimes described as a heat dome. Pressure high in the atmosphere acts like a lid on a pot, keeping the heat from dissipating, said Frank Pereira, a meteorologist with the Weather Service’s prediction center.
In recent years, global warming has been making heat waves hotter, more frequent and longer lasting. Last year was the warmest on record, and global temperatures have continued breaking monthly records this year, too. The Weather Service has warned that this heat wave could be the longest one some places have experienced in decades.
Heat waves in mid-June are not unprecedented, but this one’s duration suggests that summer could be particularly brutal. And while the temperatures may soon ebb in a few places — including the northern reaches of New England — most people in the Northeast and mid-Atlantic regions can expect higher-than-usual temperatures at least through the end of this month, Mr. Pereira said.