Sweltering heat gripped swaths of the U.S. as forecasters predicted the potential for record-breaking temperatures in the Pacific Northwest and as cooling stations were set up for residents to find some relief in cities across the nation.

More than 145 million Americans on Thursday lived in areas under excessive-heat advisories from the National Weather Service. Warnings spanned the Pacific Northwest, the Northeast and parts of Missouri and Illinois while advisories blanketed parts of the Northeast, Midwest and South. Over one million residents were in areas under various fire-weather advisories, which indicate hot, dry and windy conditions that could quickly spread wildfires, according to the National Weather Service.

It said triple-digit temperatures in the Pacific Northwest could break daily heat records late this week.

An unprecedented, record-breaking heat wave in the Pacific Northwest in June left more than 100 people dead. During most years, the current heat wave hitting cities including Portland and Seattle would be considered the most extreme event of the summer, said Colby Neuman, a meteorologist at the National Weather Service in Portland. But this set of sweltering temperatures isn’t as unusual as the record-breaking heat nearly two months ago.

“This event, three days of it in a row, is a very noteworthy event for this region and is relatively rare,” Mr. Neuman said. “But it’s not the three days of all-time record high temperatures that we had in late June. It’s hard to look at it in a positive light, but it’s at least not that bad.”

Temperatures in Portland hit 102 degrees, nearing the city’s daily record high for Aug. 12 of 104 degrees, set in 1977. It is possible temperatures could meet or exceed the record daily high of 102 degrees for Aug. 13, set in 2002, Mr. Neuman said. On Wednesday, temperatures recorded at Portland International Airport tied a 1977 record of 102 degrees.

Temperatures measured in Seattle on Thursday reached a high of 93 degrees, matching a previous record set in 1977, according to the National Weather Service. Bellingham, Wash., a city along the coast near the U.S.-Canadian border, recorded an all-time high temperature of 100 degrees. The previous record was 99 degrees, set during the heat wave in June.

Oregon Gov. Kate Brown declared a state of emergency earlier this week that extends through Aug. 20, activating emergency-response protocols and ordering state agencies to provide assistance. Ms. Brown urged Oregonians to check in with loved ones and urged residents without air conditioning at home to use available cooling centers.

One in five households in the Portland metro area doesn’t have air conditioning, according to Census Bureau figures.

“Please treat these hot temperatures seriously,” Ms. Brown, a Democrat, said Wednesday.

In Multnomah County, the Joint Office of Homeless Services said it would be handing out thousands of bottles of water and cooling supplies to homeless individuals in downtown Portland. Cooling centers across the county opened Wednesday, said Portland Mayor Ted Wheeler, who signed a local emergency declaration earlier this week.

In Seattle, the city opened cooling centers and shared information on places such as libraries, parks and pools where residents could get some relief from the heat. Meanwhile, smoke from fires in Canada affected air quality in several Washington counties Thursday.

Across the continental U.S., high temperatures baked the East Coast, with excessive-heat warnings in effect Thursday in cities including New York City, Philadelphia and Boston.

The National Weather Service in New York City said highs Thursday would reach the mid-90s but would feel as hot as 110 degrees. New York City Mayor Bill de Blasio warned of the potential burden on the electrical system and urged residents to reduce energy use when possible. The city has 369 cooling centers available, said John Scrivani, commissioner of the city’s department of emergency management.

—Talal Ansari, Randy Yeip and Max Rust contributed to this article.

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Emergency teams are battling wildfires in Turkey, southern Europe and U.S. states including California and Hawaii. Scientists say particularly hot temperatures in some places have exacerbated droughts and helped create the conditions for fires to spread this summer. Photo: AP The Wall Street Journal Interactive Edition

Write to Jennifer Calfas at Jennifer.Calfas@wsj.com