A dangerous heat wave has locked much of the central and eastern United States in triple-digit temperatures, and forecasters expect the punishing conditions to hold through Friday and across the holiday weekend. For millions of people stretching from the Mid-Atlantic into the Northeast, ordinary summer weather likely won’t return until Monday.

A Nation Sweltering Under Warnings

Early Friday, roughly 180 million people found themselves under extreme heat warnings or advisories. Many had already endured several days of urgent alerts telling them to stay indoors during the hottest stretches of the afternoon.

The severity of this dangerous heat wave showed up plainly in the record books. On Thursday alone, city after city either flirted with or shattered daily temperature records.

• In New York, Kennedy International Airport soared to 102 degrees, beating the 101 recorded back in 1966.
• Central Park climbed to 100, matching its own mark from that same year, and it was the first time the Belvedere Castle station had touched triple digits since July 2012.
• Trenton, New Jersey, reached 101, edging out a record that had stood since 1901.
• Philadelphia hit 103, tying its own 1901 benchmark.

More Than Just High Numbers

The raw temperatures, though, only capture part of the disruption. The heat tangled up rail service along the busy Northeast Corridor. Even the New England coast, a summer escape famous for its mild weather and fresh seafood, ground to a slower pace as visitors fled indoors to seek out air conditioning.

In Washington, officials shut the public out of Thursday night’s dress rehearsal for the annual Independence Day concert on the Capitol lawn, citing the excessive heat.

The strain reached the power grid as well. Scattered outages hit tens of thousands of utility customers across New Jersey, New York, and Ohio on Thursday, according to tracking site PowerOutage.us. And the records aren’t finished falling. By Sunday, the Weather Prediction Center anticipates that more than 100 daily high records will have been broken at official weather stations.

Why It Feels So Brutal

Part of what makes this stretch so miserable is the humidity, which drives up the heat index, the measure of how hot the air actually feels against your skin. With overnight lows dipping only into the high 70s or low 80s, there’s little chance for the body to recover.

Forecasters have repeatedly warned about heat-related illness, and for good reason: heat is the deadliest form of weather in the United States. While pinning any single event on climate change takes careful study, scientists broadly agree that heat waves everywhere are growing hotter, striking more often, and lasting longer.

A Day-by-Day Look at the Forecast

Friday brought no reprieve. The Weather Service described “dangerous, record-breaking heat” stretching across a vast zone from Illinois to Maine and down to Georgia. Afternoon highs were expected in the mid-90s to low 100s, with the heat index potentially spiking to 115 degrees. Philadelphia was poised for 104, New York for 101, and Washington for 103.

The Fourth of July offered a sliver of relief, but only a sliver. The clearest cooling was forecast for the Great Lakes and northern New England, where readings could slide into the 80s. A few northeastern cities, including Philadelphia, New York, and Boston, were set to drop a couple of degrees. Still, nearly everywhere from Massachusetts to the Carolinas stayed hot, with highs again in the 90s to low 100s. The worst was expected around Washington, aiming for 103, and southward toward Raleigh, North Carolina, near 102.

By Sunday, the heat was projected to ease more widely, with scattered showers rolling in. Rain was likely from the Southeast through the Ohio Valley and into the Great Lakes and Mid-Atlantic, and that moisture was expected to help temperatures settle. Much of the Northeast and New England could see highs back in the 80s, though the southern Mid-Atlantic and Southeast would stay hot. The stubborn core of the heat was forecast to sit between Atlanta and North Carolina.

Knowing the Warning Signs

When the air turns this dangerous, prevention makes all the difference. Drink water steadily throughout the day rather than waiting until thirst hits, and stay out of the sun during peak hours, roughly late morning to late afternoon.

According to Dr. Christopher Bazzoli, an emergency medicine physician at Cleveland Clinic, one of the earliest red flags is heat edema, which is swelling in the ankles, hands, and feet. If you spot it, move to the coolest spot you can find, ideally somewhere air-conditioned. If you’re stuck outside, seek shade and try elevating your feet to ease the swelling.

Watch, too, for signs of heat exhaustion, a more serious stage that can bring:

• Nausea
• Faintness or dizziness
• Muscle cramps
• A racing heartbeat

Anyone feeling these symptoms should rest in a cool place and rehydrate right away.

The gravest danger is heat stroke, which can turn fatal. Its warning signs include a body temperature at or above 103 degrees, skin that’s hot and red and either dry or clammy, along with headache, dizziness, nausea, and a rapid pulse.

When Will It Finally Break?

Relief is on the horizon, at least for some. A return to more typical summer weather, with widespread afternoon highs in the 80s, was expected to reach the Great Lakes and Northeast by Monday. New York was projected to top out near 81 and Philadelphia around 83.

Not everyone will get the same break, though. Across the Carolinas, air conditioners may keep running hard. As one meteorologist put it, conditions will improve for most people next week, but the Southeast will linger warm and humid, even if it stops short of record-breaking.