Heat Wave to Envelope the Northeast Starting on Tuesday

A record-breaking heat wave with a combination of humidity and temperatures well into the 90s will descend on cities in the Midwest and Northeast this week in what is expected to be a dangerous weather system days before summer officially begins, forecasters said.

“The duration of this heat wave is notable and potentially the longest experienced in decades for some locations,” the National Weather Service said on social media on Sunday.

In Pennsylvania, temperatures in Philadelphia, Allentown and Reading are expected to reach 95 degrees or higher for five consecutive days, a streak that has not occurred since July 2022, August 1953 and July 2011, respectively, according to the Weather Service office in Mount Holly, N.J.

The severe heat was rapidly developing ahead of a low pressure system and spreading to the central Plains, Great Lakes region and Ohio Valley on Sunday, the Weather Service said.

High temperatures in Ohio on Sunday reached into the 80s, with humidity at fairly comfortable levels, but hot and humid conditions were expected through the week in Ohio and Pennsylvania, the Weather Service office in Cleveland said on social media.

The office said an excessive heat watch was in effect from Monday through Friday for areas that included Cleveland, Detroit, Pittsburgh and Erie, Pa., where temperatures could surge into the 90s during the day.

The heat wave was expected to push into the Northeast on Tuesday and continue through Saturday, the Weather Service office in New York said on social media on Sunday.

“The combo of heat and humidity will produce heat index values 95 to around 105,” the office said, referring to how hot it feels outside.

The highest values will be felt across New York City, New Jersey, the lower Hudson Valley and interior Connecticut, the office said.

An excessive heat watch was also in effect for parts of southern New England where the Boston office of the Weather Service warned of “dangerous heat” and humidity beginning Tuesday and lasting through Friday.

“We get these types of heat waves once or twice a summer,’’ Rob Megnia, a meteorologist with the Weather Service, said on Sunday.

Heat-related illnesses can range from minor to life-threatening. Mild heat illnesses can lead to heat rash, swelling in the hands and feet and muscle cramps while heat exhaustion can cause headache, nausea, vomiting and dizziness.

In New York City, about 350 people die prematurely from preventable heat illness each summer, according to New York City Emergency Management. The agency warned that older people and those with health conditions, like heart disease or asthma, are at higher risk. Those without air-conditioning are advised to find free cooling centers, such as local pools, libraries and malls.

Mr. Megnia said New England will start experiencing the high heat on Tuesday, with peaks on Wednesday and Thursday.

Typically at this time of year in the greater Boston area, high temperatures hover around 70 degrees, Mr. Megnia said. “Temperatures will be close to 20 degrees above normal,’’ he said.

By midday on Wednesday, temperatures were forecast to reach as high as 94 degrees in Boston, 99 degrees in Springfield, Mass., 96 degrees in Hartford, Conn., and 89 degrees in Providence, R.I.

Mr. Megnia said that coastal areas, such as Cape Cod, will be cooler, “more like in the 80s.”

High temperatures were expected to surge into the 90s as far north as Vermont and New Hampshire, forecasters said.

“Do what you can to stay cool,” Mr. Megnia said. For people who work outside, he suggested packing a cooler with drinks with electrolytes to stay hydrated.