Warnings of blistering temperatures were expanded Friday to include parts of 26 states as a days-long "heat dome" scorched much of the nation, triggering dangerous thunderstorms.
From Austin to Boston, temperatures were forecast to push triple digits, with the heat index — a measure of how hot is feels — reaching 115 degrees in isolated spots. As many as 122 million Americans were under some sort of heat advisory, in an area that stretches 830,000 square miles.
And it could be even hotter on Saturday.
"Saturday looks to be the peak day for the heat on the East Coast," NBC meteorologist Bill Karins said. "The end of the heat wave appears to be with a cold front on Tuesday."
The intense weather is the result of an atmospheric phenomenon called a "heat dome" — a ridge, or high-pressure system, that traps hot air underneath it — creating unusually hot and humid conditions.
The heat dome is currently sitting over 100 million people, prompting preparations nationwide. In Chicago, workers were inspecting train tracks to make sure the warm weather wasn't causing any problems, and in New York City, cooling centers were being set up.
NBC New York reported a heat advisory was in effect for New York City and its surrounding counties from noon to 7 p.m. Friday. Forecasters said it was expected to feel like 100 degrees there.
On the National Mall in Washington, D.C., temperatures of 100 degrees or above were forecast, along with stifling humidity.
The intense heat was expected to bring strong to severe thunderstorms, with damaging wind gusts and large hail, to major Northeast cities Friday and Friday evening, including the I-95 corridor, before the storms shift back to the Upper Midwest for the weekend, according to the Weather Channel.
Torrential downpours may cause flash flooding in some areas, it said.
In the Dakotas, a few tornadoes are possible Friday, Karins said.
Much of the nation will start to cool off by Monday, with full relief forecast by Thursday, TODAY meteorologist Al Rokers said.
"But until then, it is going to be brutal," he said.
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Cal Fire engineer Clint Singleton looks out at a plume of smoke near Clearlake, Calif., Wednesday, Aug. 5, 2015. Thousands of firefighters battling an unruly Northern California wildfire were aided overnight by cooler temperatures and higher humidity, but the fire is still less than a quarter contained. (AP Photo/Jeff Chiu)
Corn damaged by heavy rains stands in a field in Sheridan, Ind., Wednesday, Aug. 12, 2015. The U.S. Department of Agriculture says farmers in 88 of Indiana's 92 counties are eligible for low-interest emergency loans because of heavy rains and flooding that have occurred since May 1. (AP Photo/Michael Conroy)
In a view from Whittier, Calif., the sun sets behind a hazy downtown Los Angeles after a week of high temperatures from the view Tuesday, Aug. 18, 2015. (AP Photo/Nick Ut)
A child plays in the sprinklers of Seward park, Monday, Aug. 17, 2015, in New York. Temperatures are expected to reach into the 90s in the New York metro area. (AP Photo/Mary Altaffer) (AP Photo/Mary Altaffer)
Beach goers crowd at Venice Beach, Calif., Sunday, Aug. 16, 2015. The Western heat wave began Thursday and was expected to continue through Sunday. Authorities warned people not to leave small children or pets in cars, where temperatures can quickly soar. Los Angeles and other cities were keeping libraries and other facilities open late to serve as cooling shelters for those without air conditioning.(AP Photo/Ringo H.W. Chiu)
Storm clouds build over the left field stands in Coors Field during the fifth inning of an inter league baseball game between the Seattle Mariners and the Colorado Rockies Monday, Aug. 3, 2015, in Denver. (AP Photo/David Zalubowski)
Debris rests on the ground after a garage was damaged by fallen tree during a severe thunderstorm in Traverse City, Mich., Sunday, Aug. 2, 2015. Authorities said gusts as high as 65 mph left thousands without power, damaged houses and left some roads impassable. (AP Photo/John Flesher)
A vehicle drives through a puddle after heavy rainfall, Tuesday, Aug. 11, 2015, in Albany, N.Y. Scattered rain and thunderstorms are expected in the region through Wednesday. (AP Photo/Mike Groll)
Firefighters walk under smoke from fires along Morgan Valley Road near Lower Lake, Calif., Friday, July 31, 2015. A series of wildfires were intensified by dry vegetation, triple-digit temperatures and gusting winds. (AP Photo/Jeff Chiu)
LAKE MEAD NATIONAL RECREATION AREA, NV - AUGUST 03: The ruins of the Hannig Ice Cream Parlor are shown in the ghost town of St. Thomas on August 3, 2015 in the Lake Mead National Recreation Area, Nevada. The town was founded in 1865 by Mormon pioneers at the site where the Muddy River flowed into the Colorado River and at one point had about 500 settlers. The town was abandoned in 1938 after the construction of the Hoover Dam caused the Colorado River to rise. The area was once submerged in 60 feet of water but became entirely exposed to the air as a severe drought in the Western United States over the last 15 years has caused Lake Mead to drop to historic low levels. (Photo by Ethan Miller/Getty Images)
UNITED STATES - JULY 30: A severe thunderstorm passes over the U.S. Capitol on Thursday, July 30, 2015. (Photo By Bill Clark/CQ Roll Call)
Clouds accompany hot, muggy weather over downtown Los Angeles and the Walt Disney Concert Hall, Thursday, July 30, 2015. July is wrapping up in California with more of the unusual weather that has marked the normally very dry month. Flash-flood watches are posted across the interior mountains and deserts of southern and eastern California as monsoonal moisture brings thunderstorms.(AP Photo/Nick Ut)
Motorists drive through heavy rain along Ninth Avenue, Thursday, July 30, 2015 in New York. More showers are predicted through the night, but skies are expected to clear by Friday morning. (AP Photo/Julie Jacobson)
Two youths play in the Swann Memorial Fountain Thursday, July 30, 2015, in Philadelphia. According to the National Weather Service temperatures are expected to reach 90-degrees. (AP Photo/Matt Rourke)
Young people cross a street during a rainstorm Thursday, July 30, 2015, in Philadelphia. According to the National Weather Service temperatures are expected to reach 90-degrees. (AP Photo/Matt Rourke)
Walter Swinehart cools off in the Swann Memorial Fountain, Thursday, July 30, 2015, in Philadelphia. According to the National Weather Service temperatures are expected to reach 90-degrees. (AP Photo/Matt Rourke)
A man cools off in the water sprinklers at Sara Delano Roosevelt Park, Wednesday, July 29, 2015, in New York. The National Weather Service has issued a heat advisory for New York City through 8 p.m. Thursday. (AP Photo/Mary Altaffer)
Clouds hang over City Hall Tuesday evening, July 28, 2015, in Commerce City, Colo. Forecasters predict continued warm weather for Colorado's Front Range communities in the week ahead. (AP Photo/David Zalubowski)
Storm clouds build over the Rocky Mountains in Colorado as the sun sets late Friday, July 24, 2015. Forecasters predict that the cool, stormy weather of Friday will move out for daytime highs hovering in the 90s for the weekend ahead. (AP Photo/David Zalubowski)
In this Tuesday, July 21, 2015, photo, Wendover Mayor Mike Crawford stands along the exposed mud track on the Bonneville Salt Flats, in Utah. Crawford, who owns an auto parts shop in town, said the decision by race organizers to cancel this yearâs event weeks away will be a bigger economic blow than last year, when a monsoon storm left standing water on the track on the eve of the race. Wet weather has forced the second-straight cancellation of an annual race at Utahâs world-famous Bonneville Salt Flats. (AP Photo/Rick Bowmer)
A teenager checks his cell phone as storm clouds pass Friday, July 17, 2015, in Zionsville, Ind. Scattered storms were in the forecast for most of Friday evening. (AP Photo/Darron Cummings)
Storm clouds hang over Great American Ball Park before the MLB All-Star baseball game, Tuesday, July 14, 2015, in Cincinnati. (AP Photo/Jeff Roberson)
Smoke drifting south from wildfires burning in Canada clouds the skyline Tuesday, July 7, 2015, in Denver. A smoke advisory was issued for the northeastern part of Colorado, Monday and expanded to all counties east of the Continental Divide Tuesday. (AP Photo/David Zalubowski)
Vehicles struggle to navigate through the intersection of Eighth Avenue and Grant Street south of downtown Denver as a severe thunderstorm swept over the metropolitan area late Thursday, June 25, 2015. Forecasters have issued a severe thunderstorm warning for communities south and east of Denver for Thursday night. (AP Photo/David Zalubowski)
NEW YORK, NY - AUGUST 17: People enjoy a hot afternoon at the Astoria Pool in the borough of Queens on August 17, 2015 in New York City. The main pool, the biggest in New York City and administered by the New York City Department of Parks and Recreation, sees over 3,000 people on a typical summer weekday. New York city is in the middle of a heat wave, with temperatures in the high nineties and with a heat factor making it feel over 100 degrees. (Photo by Spencer Platt/Getty Images)