Searing heat grips much of the U.S. with the worst of it still to come

The sweltering temperatures of recent days were just a warmup. The heat wave scorching two-thirds of the nation will hit its peak over the weekend.

Thinking about playing the dangerous conditions cool? Think again, health and government officials warn.

Officials from Wisconsin to Boston have declared heat emergencies, canceled classes and mobilized crews to prepare for the soaring temperatures through the weekend, when more than 100 local heat records are expected to fall.

Already early Friday morning, more than 173 million people were under excessive heat alerts and nearly 60 million more were under heat advisories, according to the weather service national map. The heat wave blanketed a swath of the nation from central Nebraska and parts of Oklahoma to Vermont and Massachusetts and northern South Carolina, according to the weather service.

Temperatures in New York City, where the mayor Thursday declared a heat emergency, are forecast to be as high as 96 degrees on Sunday, and with the humidity, it could feel like 110 degrees, the national weather service and emergency management officials said.

The New York City Triathlon, which expected to draw 4,000 participants from 33 countries and 45 U.S. states on Sunday, was called off because of severe heat warnings.

"After exhausting all options to mitigate athlete, volunteer, spectator and staff exposure alike, we are unable to provide either a safe event experience or an alternate race weekend,” triathlon organizers said in a statement Thursday.

In upstate New York, the state racing association canceled races Saturday at Saratoga Race Course, where the heat index could be as high as 110 degrees.

Mayor Lori Lightfoot of Chicago, where temperatures are forecast in the high 90s with a heat index topping 100 degrees through Saturday, declared a "heat emergency."

"It's all hands on deck," Lightfoot said at a news conference Thursday, where she pleaded with residents to take the heat seriously.

Related: 2019 weather across the U.S. 

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2019 spring weather across the US
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2019 spring weather across the US
A vehicle drives through Mississippi River flood water in downtown Alton, Il. on Monday, May 6, 2019. Flooding from the Mississippi River closed streets in downtown, forced the closure of Argosy Casino and flooded the basements of several businesses. The Mississippi River is expected to crest at 34.8 feet later on Monday, almost 14 feet above flood stage. The red painted line beneath the American flag on the grain silos denotes the height of flood water in 1993. (David Carson/St. Louis Post-Dispatch via AP)
Water from the swelling Mississippi River covers roadways and surrounds houses on Saturday, May 4, 2019 in Foley, Mo. he National Weather Service at St. Louis says rain in the coming days will determine whether Mississippi River levels will rise more than expected. A flood warning continues for areas on either side of the river from Minnesota all the way to Louisiana, where the river empties into the Gulf of Mexico.(Colter Peterson/St. Louis Post-Dispatch via AP)
A runner makes his way along South Grandview Ave. during a snowfall Saturday, April 27, 2019 in Dubuque, Iowa. (Dave Kettering/Telegraph Herald via AP)
John Love of Pacific Junction, Iowa, stands in flood water to wash the muck off of his golf clubs which were in a flooded shed Thursday, April 18, 2019. The mandatory evacuation of the city during the flooding from the Missouri River has been lifted Thursday and residents and owners were allowed to return to their property to determine the viability of their premises. (AP Photo/Nati Harnik)
Marissa Whitman, 20, wades in about 3 feet of floodwater from the swelling Mississippi River, while guiding a boat carrying her boyfriend Brendan Cameron and his mother, Tory Cameron, to their home along Pet Street, Sunday, May 5, 2019, in East Foley, Mo. "I just need to see if the water reached inside," said Tory. The family had to evacuate Saturday when the water rose suddenly. (Laurie Skrivan/St. Louis Post-Dispatch via AP)
A van stands in floodwaters as cornstalks cover its roof after a flood inundated Pacific Junction, Iowa, Thursday, April 18, 2019. The mandatory evacuation of the city during the flooding from the Missouri River has been lifted Thursday and residents and owners were allowed to return to their property to determine the viability of their premises. (AP Photo/Nati Harnik)
Floodwaters surround a home, Wednesday, May 29, 2019, in Ottumwa, Iowa, as rising waters from the Des Moines River has forced residents out of homes along the riverbank. (Matt Milner/The Ottumwa Courier via AP)
Residents load sandbags onto a truck Wednesday, May 29, 2019, in Ottumwa, Iowa, as floodwaters from the Des Moines River has forced residents out of homes along the riverbank. (Matt Milner/The Ottumwa Courier via AP)
In this Tuesday, May 22, 2019 photo, a late-spring snowstorm fell in Red Cliff, Colo. The unusually cold weather impacted other parts of the West, including California, that were hit by late spring storms. A storm dumped heavy, wet snow in Colorado and Wyoming, cancelling flights and snapping newly greened up tree limbs. (Chris Dillmann/Vail Daily via AP)
Frisco resident Dianne Stuhr walks with her dogs, Winston and Patty, Tuesday, May 21, 2019, along Tenmile Creek in Frisco, Colo. Nearby Breckenridge Ski Resort reported 9 inches of snow overnight. (Hugh Carey/Summit Daily News via AP)
This image from a Caltrans traffic camera shows traffic moving along Interstate 80 past new snow Thursday, May 16, 2019, at Donner Summit, Calif. Slopes of the Sierra Nevada sported fresh powder Thursday as a late-spring storm with a winter-like potency moved through California, adding to snowpack and rainfall accumulations that were already well above normal. (Caltrans via AP)
In this May 10, 2019 photo, flood waters from the Missouri River flow through a break in a levee, north of Hamburg, Iowa. Communities that were flooded when levees failed along the Missouri River earlier this spring will likely remain exposed to high water for months to come. More than 40 levees were damaged but only a handful of construction contracts to fix them have been issued. (AP Photo/Nati Harnik)
Repair and cleaning efforts begin on a neighborhood damaged by a tornado storm system that passed through the area, destroying homes and cutting off access to utilities, Wednesday, May 29, 2019, in Dayton, Ohio. Tens of thousands of Ohio residents were still without power or water Wednesday in the aftermath of strong tornadoes that spun through the Midwest. (AP Photo/John Minchillo)
Repair and cleaning efforts begin on a neighborhood damaged by a tornado storm system that passed through the area, destroying homes and cutting off access to utilities, Wednesday, May 29, 2019, in Dayton, Ohio. Tens of thousands of Ohio residents were still without power or water Wednesday in the aftermath of strong tornadoes that spun through the Midwest. (AP Photo/John Minchillo)
People watch from the Liberty Memorial as a severe storm that dropped several tornados earlier approaches downtown Kansas City, Mo., Tuesday, May 28, 2019. (AP Photo/Charlie Riedel)
In this aerial image, debris from damaged homes litters the properties on Fairground Road after a tornado storm system passed through the area the previous night, Tuesday, May 28, 2019, in Celina, Ohio. A rapid-fire line of apparent tornadoes tore across Indiana and Ohio overnight, packed so closely together that one crossed the path carved by another. At least half a dozen communities from eastern Indiana through central Ohio suffered damage, according to the National Weather Service. (AP Photo/John Minchillo)
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"No. 1, don’t leave your children in hot cars. ... It's a terrible mistake," she said. "If you stumble across something like that, by any means necessary get the kid out of the car and call 911."

In Appleton, Wisconsin, where a high of 90 was forecast Friday, the school district called off elementary and middle school classes because of the excessive heat.

Philadelphia, which is expected to see 100 degrees on Saturday and 99 on Sunday, declared a "heat health emergency," and extended it until 11 p.m. Monday.

Washington, D.C., also declared a heat emergency, and on Thursday, so did Boston Mayor Marty Walsh. Temperatures there are forecast to be in the high 90s on Saturday and Sunday, but will feel hotter once the humidity is factored in. Temps in Detroit will also climb to the mid-to-high 90s Friday and Saturday.

In Waterloo, Iowa, the Salvation Army was combatting the heat and trying to keep people safe by distributing water, popsicles and other treats in a mobile canteen. In the city of around 68,000 northwest of Cedar Rapids, temperatures were in the low 90s Thursday.

"No one should suffer through this. It can be dangerous and life-threatening if people don’t take it seriously,” Salvation Army Capt. Shannon Thies told NBC affiliate KWWL.

Humans aren't the only ones suffering. In New York City's Central Park on Thursday, Judith Kadi said that the recent heat has had her and her dog out walking earlier.

"The heat really affects him, so we definitely come out early, and I carry water with us constantly," Kadi said. "He's very, very thirsty; I definitely see a difference in him when it gets to this temperature."

So far, this summer "has not been bad, but this has been really unbelievable," she said. "Everyone’s affected by it." She was planning to leave the city Friday and go out to the country to beat the heat — the forecast high is 94 degrees — and return at night when it's cooler.

On Thursday, the National Oceanic and Atmospheric Administration released a monthly climate report that said that last month was the hottest June on record for the globe. The NOAA global dataset record goes back 140 years, to 1880. Nine of the 10 hottest Junes globally have occurred since 2010, the agency said.

It also said that for the second month in a row, warmth brought Antarctic sea-ice coverage to a new low for June.

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