The National Weather Service is predicting a “prolonged period of dangerously hot conditions” with “intense heat and high humidity.”

“Heat related illnesses increase significantly during extreme heat and high humidity events,” the agency wrote, “particularly for those working or participating in outdoor activities.”

The heat is already building across the Midwest and South, with highs in the mid-90s expected Sunday in Chicago and Atlanta. Little Rock is forecast to hit 96 degrees and St. Louis might spike to 98. And that’s just a preview of what’s to come.

Highs in the upper 90s to around 100 could reach as far north as southern Canada, with cities like Boston, Pittsburgh and Washington flirting with 100 degrees as the workweek progresses. Heat index values — which take into account the humidity — will easily crest 100 degrees, and could do so several days in a row.

The heat will be exceptional in southeastern Canada and northern New England — setting not only calendar-day records, but also monthly and all-time marks, meaning unsurpassed on any day on record. Highs could exceed normal values by 15 to 25 degrees.

Fort Kent, one of the northernmost cities in Maine and the site of a Canadian border crossing, is predicted to hit 101 degrees Wednesday. Caribou, Maine, could jump to 99, blasting past the city’s all-time record of 96 degrees.

The Weather Service office in Caribou is considering issuing its first excessive heat watch, which can only be hoisted when heat index values are expected to reach 105 degrees on two consecutive days, and overnight temperatures aren’t anticipated to dip below 75 degrees.

The Weather Service office in Burlington, Vt., said the week could bring “the hottest 3-day stretch for some in 30 years.”

Excessive heat warnings already blanket the entirety of Ohio, much of central and southern Michigan, northeast Indiana and parts of West Virginia and western Pennsylvania because of the heat anticipated to intensify starting Monday. Watches have also been drawn up in the Connecticut River and Merrimack Valleys of southern New England. Upgrades to excessive heat warnings are likely in the days ahead.

The unusually long-duration heat wave will expand in coverage and intensify on Monday before dominating Tuesday through Friday. On Saturday, the core of the heat may shift south back into the Mid-Atlantic, including the nation’s capital, which could hit 100 for the first time since 2016. The lengthy spate of heat will itself be a problem, exacerbating heat stress.

Even more impressive is the strength of the instigating “heat dome,” which models indicate will reach a key atmospheric threshold never observed before.

Unsurprisingly, the National Weather Service is highlighting multiple consecutive days of “extreme” heat risk across the Ohio Valley, Mid-Atlantic and Northeast, representing a top-tier level 4 on their 0-to-4 scale.

“This level of rare and/or long-duration extreme heat with little to no overnight relief affects anyone without effective cooling and/or adequate hydration,” the Weather Service wrote.

On Monday, things will begin to heat up from roughly Lake Michigan east to Ohio and along the western slopes of the Appalachians. A few 90s may also spill toward the Eastern Seaboard.

• Chicago’s O’Hare International Airport is predicted to hit 95 degrees, falling just shy of the 96-degree record set in 1957. Records there date to 1872.
• Lansing, Mich., is predicted to hit 95 degrees, tying the record set in 1994. Records there date to 1863.
• Toledo is predicted to hit 98 degrees, exceeding the 97-degree record set in 1994. Records there date to 1873.
• Cleveland is predicted to hit 96 degrees, exceeding the 94-degree record set in 2018. Records there date to 1871.

The heat dome will reach a punishing crescendo Tuesday through Thursday, with temperatures maxing out near 100 degrees from Illinois to Maine. Interestingly, humidity may be greater in New England than in the Mid-Atlantic region to the south, meaning some of the highest heat index values could be in Massachusetts, New Hampshire, Vermont and Maine.

• Temperatures will reach the mid-90s or so in D.C.; while that will fall just shy of records, heat index values near 100 degrees could be dangerous to older adults and other vulnerable populations.
• In Newark, highs are forecast to reach 97, 99 and 99 degrees on Tuesday, Wednesday and Thursday, respectively. That would tie a record on Tuesday and break records by 5 degrees on Wednesday and 1 degree on Thursday.
• New York City won’t set records, as temperatures will stay in the lower 90s Tuesday and Wednesday, with mid-90s on Thursday. A subtle sea breeze will keep the Big Apple slightly cooler. In the unlikely event that the sea breeze doesn’t make it west to Central Park, the high could reach 96 degrees — which happens during the month of June only 20 percent of the time, according to research meteorologist Tomer Burg.
• Hartford, Conn., is predicted to hit 96 degrees on Tuesday, 99 on Wednesday and 101 on Thursday. All three days would break records. Similar temperatures are expected farther north in the Connecticut River Valley too, and could affect Springfield, Mass.
• Boston’s Logan International Airport probably won’t set any records because it’s located on the immediate coastline, and afternoon sea breezes will probably knock temperatures down by a few degrees. But at the Blue Hill Observatory in Milton, just a few miles inland, highs could reach 98 degrees by Thursday. That would beat the record of 94 set in 1953. Bookkeeping dates to 1893.
• Manchester and Concord, N.H., both in the Merrimack Valley, are predicted to hit 97 degrees on Tuesday and 98 or 99 on Wednesday and Thursday. That will tie or break several records.
• Mid- to upper 90s will be common across Lower Michigan, Ohio, Pennsylvania and New York each day, tying or breaking records in places like Toledo, Columbus and Pittsburgh.

By Friday, the heat will be focused most intensely from St. Louis and parts of the Midwest and northern Tennessee Valley up the Appalachians and into southern New England, with minor improvement in extreme northern areas. The Interstate 95 corridor in the Mid-Atlantic will be especially hot.

• Louisville is predicted to hit 98 degrees, tying a record set in 1988. Records there date to 1872.
• Pittsburgh is predicted to hit 97 degrees, exceeding the 95-degree record set in 1933. Records there date to 1875.
• D.C., is predicted to hit 98 degrees, just shy of the 99-degree record set in 2012. Records there date to 1872.

• Philadelphia is predicted to hit 99 degrees, tying the 99-degree record set in 1923. Records there date to 1873.
• Hartford is predicted to hit 97 degrees, exceeding the 96-degree record set in 2012. Records there date to 1905.

By Saturday, the greatest actual air temperatures, and chance of breaking records, will become centered on the Mid-Atlantic as the heat dome begins to break down some. But the greatest heat index values, and subsequently the most severe heat risk, may actually be in the Midwest, due to slightly greater moisture.

• Washington Dulles International Airport, west of D.C., is predicted to hit 99 degrees, tying a record set in 1988. Records there date to 1960. Some models are projecting triple-digit highs both weekend days around Washington.
• Georgetown, Del., on the Delmarva Peninsula, is predicted to hit 98 degrees, exceeding the 97-degree record set in 2012. Records there date to 1945.
• Philadelphia is predicted to hit 100 degrees, tying a record set in 1988. Records there date to 1873.

Spawning all the heat is a massive heat dome, or ridge of high pressure, that will stagnate for days across the northern U.S. Heat domes are akin to force fields in the atmosphere, deflecting inclement weather. Sinking air warms and dries out, squashing potential cloud cover. The result? Hot, sun-baked air that’s usually cloud-free, which allows for even more dramatic heating.

Warm air expands. If you warmed a balloon, its volume would grow. The same is happening with the atmosphere. The extreme heat is causing the atmosphere to bulge vertically — to a record extent.

Atmospheric scientists look to the “500 millibar level,” or halfway point of atmospheric mass, to see how much an air column has expanded. As far back as records go, that level has never reached 600 dekameters, which is 6,000 meters or 19,685 feet. But that’s probably in the cards this upcoming week.

The American GFS model is indicating that 500 millibar heights could reach 600 dekameters over New York City on Wednesday evening. There has never been a weather balloon launched from New York City/Long Island that has encountered a 500 millibar height greater than 598 dekameters.

There exists a firm link between human-induced climate change and the frequency, intensity and duration of heat events. Heat domes, like this one, are projected to continue becoming more frequent, stronger and more expansive as Earth’s climate warms.