Scorching Temperatures Sweep Across Eastern U.S.

The last full day of spring will feel more like deep summer across much of the eastern half of the United States.

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 Tuesday. Humidity will make the air feel even hotter, and nighttime temperatures are expected to remain uncommonly high.

Here are the details:

— Unexpected hazards : The heat, which will linger through at least the weekend in part of the region, has already caught some cities off guard. Chicago opened cooling centers earlier in the year than usual. And in New York City, the public pools are not open yet.

— Soaring heat index : Inland New England and upstate New York began to swelter this week even before cities closer to the coast. In Rochester, New York, the heat index — an indication of how the air feels, accounting for humidity — soared higher Tuesday than it did in San Antonio or St. Petersburg, Florida.

— Why it’s hot : 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 National Weather Service’s prediction center.

— Global warming’s role : 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.

— Summer warning signs : Heat waves in mid-June are not unprecedented, but the duration of this one 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, Pereira said.

This article originally appeared in The New York Times .