An intense heat dome is poised to build across much of the central and eastern United States, bringing triple-digit heat index values all the way into Canada as temperatures reach record levels. The National Weather Service is calling for “potentially dangerous and long duration heat.”

The heat will first build over the southern United States and Midwest this weekend before swelling over the Ohio Valley and East Coast next week. Some 82% of Americans will see highs over 90 degrees. Chicago, St. Louis, Nashville, Cincinnati, Pittsburgh, Atlanta, Washington and New York are among scores of population centers that could see multiple days with highs of at least 95 degrees.

People walk near a sign on Hanover Street in the North End, during a heat wave in July 2022, in Boston. Dangerously high temperatures Wednesday and Thursday threaten much of the Northeast. Boston is forecast to see highs of 95 degrees on both days. Charles Krupa/Associated Press, file

The heat dome is already gripping the Desert Southwest, allowing sunshine to bake the ground and heat the air. Heat advisories and excessive-heat warnings are in effect across southern Arizona and New Mexico, where temperatures of 105 to 113 degrees will be common.

On Saturday, the heat dome will shift into the southeastern United States, producing highs from 95 to 100 degrees from Little Rock to Atlanta.

By Sunday, the heat dome will expand northward and eastward, becoming centered first over the Ohio Valley into Monday before sliding over the Northeast on Tuesday. That’s around the time it will intensify markedly, leading to the hottest weather of the summer and potentially years in the Ohio Valley, Great Lakes, Northeast, Mid-Atlantic and southeast Canada.

The Weather Service will probably issue heat alerts by early next week. The agency’s 0-to-4 HeatRisk index shows top-tier Level 3 and 4 conditions spreading from the Midwest to Northeast as next week progresses. At Levels 3 and 4, the Weather Service says the heat will pose a threat to individuals without adequate cooling and hydration. The most vulnerable groups are typically outdoor workers, homeless people, older adults and anyone without access to air conditioning.

Temperature records will be tied or broken in widespread fashion beginning Monday across the Midwest and Ohio Valley. The record heat will then spread to the Mid-Atlantic and Northeast between Tuesday and Thursday, and could linger even beyond that.

Records will be broken for both hot daytime highs and unusually warm overnight conditions, which will “offer little to no relief to those without adequate or reliable cooling,” the Weather Service wrote.

Rising humidity will make the record-challenging heat feel even more oppressive, producing widespread heat indexes – a measure of how hot it feels factoring in the mugginess – over 100 degrees.

The extreme heat comes on the heels of Earth’s hottest May on record, which was the 12th consecutive month to set a corresponding record. Global temperatures averaged out some 2.74 degrees above the 1850 to 1900 average; detailed bookkeeping dates to 1850.

While heat will always be a staple of the summertime – and always has been – the frequency, intensity and duration of heat events is on the rise. Already-anomalous events are being pushed into record territory, with correspondingly more severe impacts.

HOW HIGH TEMPERATURES COULD GET

On Saturday, the southern tier of the United States will see the highest temperatures, with highs between 95 and 100 from Texas to Georgia. In the Desert Southwest, temperatures could spike as high as 110 to 120. By Sunday, highs in the mid-90s will spread as far north and east as Iowa and northern Illinois.

Next week, temperatures in the mid- to upper 90s will be common from the Corn Belt to the Eastern Seaboard, with some areas near and above 100. This heat will combine with humidity to send heat index values spiking above 100 or even 105 degrees.

On Monday, the core of the heat will be found over the Midwest and Mississippi Valley; Little Rock could hit 98 degrees, tying a record set in 1953. Chicago should hit at least 95 degrees, and St. Louis will flirt with 100.

Then on Tuesday, mid- to upper 90s will be widespread from the Midwest to the Northeast and Mid-Atlantic. Zanesville, Ohio, is forecast to hit 98 degrees, just 1 degree shy of a record. Kalamazoo, Mich., could hit 95, as could Cleveland – both within a degree of records. Pittsburgh, where records have been maintained since 1875, could tie a record at 97 degrees.

Washington Dulles International Airport is predicted to spike to 96, beating a record by 2 degrees, and Philadelphia might make it to 95. Hartford, Conn., and Manchester, N.H., will both climb into the mid-90s, too.

Wednesday is when the heat really ramps up. Detroit and Boston are forecast to see highs of 95 degrees, and Albany, N.Y. could set a record at 96.

Thursday could be even hotter. Pittsburgh could break a record at 97 degrees, and Hartford, Conn., could do so at 98 degrees. Boston is predicted to hit 95 again, and Manchester, N.H., could tie a record at 98 degrees.

A few 100 degree readings aren’t out of the question in the Merrimack Valley of northern Massachusetts and southern New Hampshire. Upper 90s could even make it to the Canadian border, with a record-setting high of 96 degrees predicted in Burlington, Vt.

Friday could be quite hot as well in the East. By the weekend, the heat dome may start to weaken and shift offshore but computer models continue to predict above normal temperatures for large parts of the United States into the following week.

A RECORD-SETTING HEAT DOME

Heat domes are ridges of high pressure that bring hot, dry, sinking air. That sinking air, or subsidence, squashes any cloud cover. It also shunts the jet stream to the north, meaning rain and storms are hard to come by.

Weather models are already bullish on the magnitude of the heat dome. Heat domes expand the air vertically because warm air expands. That means the halfway point of the lower atmosphere’s mass bulges upward. (Think of every column of atmosphere as its own balloon – if you heat the balloon, it will grow.)

Meteorologists call this halfway point the “500 millibar level.” And that’s pressure level might reach a key threshold next week – 600 dekameters (which is 6,000 meters), or 19,685 feet. The 500 millibar level has seldom, if ever, climbed above 600 dekameters anywhere east of Ohio. Some weather models indicate that could happen by next week. Even if it doesn’t, the heat dome should still be near record intensity.

Jason Samenow contributed to this report.