Blistering Heat Expected to Persist Into the Fourth of July

A dangerous, long-lasting heat wave that has brought triple-digit temperatures across much of the central and eastern United States was expected to persist Friday and through the weekend. Typical warm summer weather is not likely to return to much of the Northeast and Mid-Atlantic until Monday.

Around 180 million people were under extreme heat warnings or heat advisories early on Friday, and many had already faced several days of severe warnings to avoid going outdoors in the hottest part of the day.

On Thursday alone, daily temperature records were challenged or broken in city after city.

In New York, Kennedy International Airport hit 102 degrees, topping the 101 mark set in 1966. Central Park reached 100, tying its record from that same year, and this was the first time the weather station at Belvedere Castle there had hit 100 since July 18, 2012. Down the Eastern Seaboard, Trenton, N.J., reached 101, one degree over its 1901 record, while Philadelphia hit 103, tying its 1901 record.

But the sweat-inducing numbers tell only part of the story.

On Friday, a Salute to Independence Semiquincentennial Parade in Philadelphia for the 250th birthday of the United States was canceled, with the event’s organizers citing “extreme heat” in social media posts. In the nation’s capital, organizers of the Salute to America Celebration adjusted the opening time of the Washington Monument grounds to 5 p.m. Eastern, part of an effort “to reduce prolonged exposure to heat.”

That followed conditions on Thursday when the heat snarled rail travel along the Northeast Corridor. Even the New England coast, a prime vacation destination known as much for its relatively temperate summers as its seafood, slowed under the weight of the humidity as patrons retreated indoors to the comfort of air conditioning.

The heat also put stress on power grids, contributing to scattered outages in New Jersey, New York and Ohio for tens of thousands of utility customers on Thursday, according to the tracking website PowerOutage.us.

More records are expected in coming days. By Sunday, the Weather Prediction Center expects that over 100 daily high records will have been shattered at official National Weather Service stations.

Key things to know about the heat wave

• Conditions are being made worse by high humidity, leading to oppressive heat index readings. This index is a measure of how hot it really feels outside.

• Temperatures dipping down only to the low 80s or high 70s are allowing for little relief and forecasters warned of heat-related illnesses. Heat is the leading cause of weather-related deaths in the United States. Here’s what it does to your body, and here are tips to stay cool.

• While tying a single heat wave to climate change requires analysis, scientists have no doubt that heat waves around the world are becoming hotter, more frequent and longer lasting.

The day-by-day forecast

Friday

“Dangerous, record-breaking heat” was expected to linger on Friday, across an area spread between Illinois and Maine and Georgia, the Weather Service said. Temperatures were predicted to be similar to Thursday, with afternoon highs in the mid-90s to low 100s. The heat index could reach 115 degrees.

Some of the highest temperatures are expected around Philadelphia, which could hit 104. New York was poised to reach 101 and Washington 103.

Saturday

The Fourth of July could bring some relief, but only a little. The most significant cooling was expected across the Great Lakes and northern New England, with temperatures likely to dip into the 80s.

Parts of the Northeast could cool slightly, with Philadelphia, New York City and Boston likely to drop a few degrees from Friday’s levels. But generally everywhere from Massachusetts to the Carolinas was forecast to stay hot, with afternoon highs in the 90s to low 100s yet again.

New York City was projected to reach 97 on Independence Day and Philadelphia 101.

The worst of the heat on Saturday was expected in the area around Washington, with a high of 103 expected, and southward toward Raleigh, N.C., where the high could hit 102.

Sunday

Temperatures were expected to ease more broadly on Sunday, with scattered rain possible across much of the region.

“There will be a chance of rain for a lot of the Southeast toward the Ohio Valley into the Great Lakes and the Mid-Atlantic,” said Marc Chenard, a meteorologist with the Weather Prediction Center. “The rain will help things cool down.”

Afternoon highs in the 80s were likely across much of the Northeast and New England, but the heat will remain across the southern portion of the Mid-Atlantic and into the Southeast.

Temperatures in the 90s are forecast for the area from Philadelphia to Washington, and triple-digit heat was expected across the Carolinas.

“The worst of it will be from Atlanta toward North Carolina,” Mr. Chenard said.

How to prevent heat-related illness

To stay safe in a heat wave, prevention is crucial. Drink water throughout the day and before you feel thirsty, and avoid the outdoors during the hottest part of the day, generally from late morning until late afternoon.

It is important to recognize the warning signs of extreme heat exposure, and one of the earliest ones is heat edema, or swelling in your ankles, hands and feet, said Dr. Christopher Bazzoli, an emergency medicine doctor at Cleveland Clinic. Mild, early symptoms can progress into full-blown heat stroke.

If you notice that type of swelling, head to the coolest place possible — ideally, somewhere air-conditioned. If you have to stay outside, find some shade. Elevating your feet can also help contain the swelling.

Symptoms of heat exhaustion, a moderate form of illness caused by heat, can include nausea, faintness or dizziness, muscle cramps and a racing heart rate. If you are experiencing any of these symptoms, get yourself to a cool place and rest and hydrate.

A more severe form of heat illness is heat stroke, which can be fatal. People suffering from heat stroke can develop a high body temperature (at or above 103 degrees) and hot and red skin that is either dry or damp. Symptoms also include headache, dizziness, nausea and a rapid heartbeat.

Here’s when the heat wave will snap

A return to typical warm summer weather with widespread afternoon highs in the 80s was expected from the Great Lakes into the Northeast by Monday.

New York City was projected to hit 81 and Philadelphia 83 on Monday.

But air-conditioners may still remain cranked up in some areas, particularly across the Carolinas.

“It improves for most people next week, but the Southeast will stay warm and humid, but nothing record-breaking,” Mr. Chenard said.

Reporting was contributed by Minho Kim, Claire Fahy, Jenna Russell, Dani Blum, Erin McCann and Nazaneen Ghaffar.