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Americans braced for a second - and equally scorching - day of dangerously hot weather on Sunday, with daytime temperatures forecast to approach 100F across a number of major US cities. 

Temperatures were so high in Omaha, Nebraska, that the National Weather Service( NWS) baked a tray of biscuits on the dashboard of a parked car.

Within 45 minutes, NWS officials tweeted that the biscuits were rising. 

Once the biscuits had been in the car for an hour, the NWS tweeted: 'The pan has reached 175 degrees in 60 minutes and the tops of the biscuits are at 153. 

'This is a good time to remind everyone that your car does in fact get deadly hot. Look before you lock! On average 38 children die in hot cars each year. Don't be a statistic!' 

After nearly eight hours and with temperatures on the pan reaching 185F, the biscuits were almost fully cooked.

The oppressive heat wave stretching from the Midwestern plains to the Atlantic coast had nearly 150 million people struggling to stay cool amid stifling temperatures on Saturday. 

Authorities said at least six deaths have been blamed on the excessive heat. Four people died in Maryland, while one died in Arizona. In Arkansas, 32-year-old former NFL player Mitch Petrus died of heatstroke Thursday after working outside his family's shop. 

'The temperatures we're seeing in our city today and tomorrow could be the highest we've seen in years. Take it seriously,' New York Mayor Bill de Blasio said on Twitter.

Americans braced for a second - and equally scorching - day of dangerously hot weather on Sunday, with daytime temperatures forecast to approach 100F across a number of major US cities 

Temperatures were so high in Omaha, Nebraska, that the National Weather Service( NWS) baked a tray of biscuits on the dashboard of a parked car 

Within 45 minutes, NWS officials tweeted that the biscuits were rising. Once the biscuits had been in the car for an hour, the NWS tweeted: 'The pan has reached 175 degrees in 60 minutes and the tops of the biscuits are at 153.' The biscuits were somewhat edible after eight hours in the sun 

The oppressive heat wave stretching from the Midwestern plains to the Atlantic coast had nearly 150 million people struggling to stay cool amid stifling temperatures on Saturday. People enjoy the Astoria Pool on a hot afternoon in the borough of Queens, New York City on Saturday

As the heat wave descended upon New York, people sought refuge at the beach in Coney Island

The heat is expected to continue through late Sunday as a high-pressure system off the Atlantic coast ushered in steamy, subtropical air.

'Dangerously hot conditions will continue through Sunday,' the National Weather Service (NWS) warned for the Washington and Baltimore area, where forecasters predicted high temperatures ranging from 95 to 100 degrees for a second day.

On Saturday evening, Baltimore logged a staggering heat index of 122 degrees, and temperature records were smashed from Virginia to Maine. 

'High temperatures and humidity could quickly cause heat stress or heat stroke if precautions are not taken,' the NWS added.

People were urged to stay hydrated, watch out for the sick and elderly, stay inside as much as possible and not leave children or animals in cars.

Heat warnings have also been issued for parts of eastern Canada. In New York, de Blasio declared a heat emergency.

The New York City Triathlon, which had been scheduled for Sunday, was cancelled for the first time since its founding in 2001. Meanwhile the two-day OZY Fest - a food, comedy and music festival set for Central Park - was also called off.

Excessive heat alerts are in place from the Carolinas to Maine as well as parts of the Midwest and Ohio Valley

Heat indexes, or 'real feel' temperatures, have been scorching throughout the Midwest and East Coast this weekend

People cool off at a park in the Navy Yard neighborhood on Saturday in Washington, DC

People cool off at the fountain of the World War II memorial during an excessive heat wave on Saturday in Washington, DC

In Washington, a popular weekly outdoor summer jazz concert at the National Gallery of Art Sculpture Garden was cancelled.

New York City opened 500 cooling centers for residents.

At least three public defenders said on Twitter that inmates in New York's notorious Rikers Island jail complex were suffering with no air conditioning, and that some guards had turned off fans as punishment, resulting in 'deadly conditions'.

The Brooklyn Defender Services legal aid group said some inmates didn't have summer clothing, only long underwear provided by the group last winter.

Top officials from the city's Department of Corrections were at the facility monitoring the response to the heat wave to 'protect health and safety of everyone in the facility,' de Blasio wrote on Twitter.

The department said in a statement that extra staff were on hand to distribute summer clothing, and clinics were open around the clock to treat heat-related symptoms.

Those in units without air conditioning were given access to fans, ice, water and 'multiple cool showers'.

The city's electrical grid was so far handling the extra demand, which came just a week after a major outage, blamed on mechanical problems, left tens of thousands of Manhattan residents in the dark.

Triple-digit temperatures in the US northeast are unusual.

Forecasters said temperatures in Philadelphia would potentially reach 100F on Sunday. 

Chicago Cubs fans cool off before a baseball game between the San Diego Padres and the Chicago Cubs in Chicago, Saturday. Storms on Thursday postponed excessive heat warnings that went into effect at 10am Friday for Cook County

Fans use umbrella hats to protect themselves from the sun during the sixth inning of a baseball game between the New York Yankees and the Colorado Rockies Saturday in New York

San Diego Padres' Hunter Renfroe cools off in the dugout after hitting a solo home run against the Chicago Cubs during the fifth inning of a baseball game in Chicago on Saturday

Much of the East Coast is expected to start cooling down Sunday night (as depicted on this graphic)

Monday's forecast of rain will bring some much needed temperature drops to the US 

In Boston, where the weather service said that Saturday and Sunday would be 'major scorchers,' city officials scrapped entry fees at public pools.

Climate data showed June was the hottest month on record worldwide, with a heat wave across Europe smashing national records. 

Over three days in July 1995, more than 700 people died during a heat wave in Chicago as temperatures rose above 97 degrees. Many of the dead were poor, elderly and lived alone.

While the Midwest will get some relief Sunday as a cold front brings storms and lower temperatures, the East won't be so lucky until Monday, the weather service warned.

At ballparks across the country, fans and players struggled to cope with the sweltering heat on Saturday. 

At Wrigley Field, misters in the back of the bleachers tried to cool the crowd. At Yankee Stadium, only one player took batting practice on the field. In Cleveland, rules were relaxed on what fans could bring into the park. 

People enjoy the day playing in a water fountain as the Empire State Building is seen from Williamsburg section of Brooklyn on Saturday

Officers said in a tongue-in-cheek Facebook post: 'Folks. Due to the extreme heat, we are asking anyone thinking of doing criminal activity to hold off until Monday'

Boston City employees man a watering station for passers-by on the Boston Common in Boston, Massachusetts, on Friday

Hours before Baltimore played Boston at sweltering Camden Yards, Orioles manager Brandon Hyde pulled aside starting catcher Chance Sisco and issued a stiff directive.

'I told Chance, "Do not go outside until the game starts,"' Hyde said.

Sisco went all nine innings Friday night and was in the lineup again while backup Pedro Severino recovers from an illness.

'I told Chance, I don't want him hustling on and off the field. I want to see him walking,' Hyde said. 'When you're a catcher in these type of games, it's not easy. After the eighth inning, his face was beet red.'

No relief in sight, either. The temperature is forecast to top 100F for Sunday afternoon's series finale.

At Great American Ball Park in Cincinnati, where it was a sizzling 94 as the Reds played St. Louis, the stadium PA system got in the swing, sort of. The pregame songs? 'Let It Snow' and 'Tenth Avenue Freeze-Out,' among others with winter-sounding titles.

In Chicago, where it also was 94F wen the Cubs started against San Diego, there was a big ovation in the seventh inning - for the weather.

That's when the wind suddenly shifted and began blowing in. The temperature immediately dropped and many in the crowd of 40,314 cheered the wind in the Windy City. 

In Norwich, Connecticut, Larry Konecny watched as one of his workers a couple of stories up in a boom lift cleaned the outside of an office building. The pair had no choice but to work in 90-degree heat and stifling humidity because the job needed to be done when office workers were away, Konecny said.

'He's pressure-washing, so the water is splashing. So at least there's some degree of refreshment,' he said. 

The National Mall was nearly deserted on Friday as people sought shelter in the shade. The heat index in DC is forecast at 111F for Saturday, within two degrees of the forecast heat index for Death Valley, California

Children play in open sprinklers on the National Mall on Friday in Washington, DC, during the heat wave

Such is the extreme heat that police officers in Braintree, Massachusetts, asked residents 'to hold off' on all criminal activity until the extreme heat is over. 

Officers said in a tongue-in-cheek Facebook post: 'Folks. Due to the extreme heat, we are asking anyone thinking of doing criminal activity to hold off until Monday.

'It is straight up hot as soccer balls out there. Conducting criminal activity, in this extreme heat is next level henchmen status, and also very dangerous.' 

'Stay home, blast the AC, binge 'Stranger Things' Season 3, play with the face app, practice karate in your basement We will all meet again on Monday when it's cooler.' 

The heat index for DC was forecast at 111F for Saturday, within two degrees of the index expected for Death Valley.

In Sioux Falls, South Dakota, soaring temperatures of 93F with a heat index of 107F were reported when portions of Insterstate 229 buckled and cracked. 

On Friday night, a massive power outage was reported in the Detroit metro area, affecting some 80,000 customers of DTE Energy. 

Fearing potential fatalities, New York City declared a state of local emergency and canceled a number of major outdoor events.

'Hot weather is dangerous and can kill. People with chronic physical and mental health conditions should use air conditioning if they have it, and get to a cool, air conditioned place if they don't,' said New York City Health Commissioner Dr. Oxiris Barbot.  

A portion of Interstate 229 buckles from high temperatures and heat indexes, Friday in Sioux Falls, South Dakota

A girl cools off from the heat in water from an open fire hydrant in the Washington Heights section of upper Manhattan in NYC

People enjoy the beach at Coney Island Friday. Fearing fatalities, New York City declared a state of local emergency

The heat in New York City was compounded by a major disruption of service on seven subway lines Friday. The MTA blamed a 'network communications' issue for the suspension of service on the Nos. 1, 2, 3, 4, 5, 6, and S trains 

On Friday, de Blasio announced the cancellation of OZY Fest, a festival scheduled for the weekend in Central Park featuring soccer star Megan Rapinoe, musician John Legend and 'Daily Show' host Trevor Noah.

A de Blasio spokeswoman says a Times Square commemoration of the 1969 moon landing was also canceled.

Officials earlier announced the cancellation of the New York City Triathlon, which was scheduled for Sunday.

Temperatures in the high 90s are forecast for Saturday and Sunday in New York, with a heat index well over 100.

De Blasio has directed owners of office buildings over 100 feet tall to set thermostats to 78 degrees through Sunday to conserve energy.

Amid fears of massive blackouts, utilities in the eastern half of the US said they expect to have enough resources to meet power demand, but asked consumers to turn down air conditioners to avoid putting stress on the system.

In Chicago, residents cool off at 31st Street Beach as temperatures hit the mid 90's on Friday

Children cool off in Crown Fountain in the Loop in Chicago as amid heat index of around 115 degrees on Friday

Concertgoers cool off in a spray of water at Pitchfork Music Festival in Union Park, Chicago on Friday

A fan of the Chicago Cubs is sprayed with water to cool off before a baseball game between the Chicago Cubs and the San Diego Padres at Wrigley Field on Friday

'I'm very confident,' Consolidated Edison Inc President Tim Cawley said when asked at a news conference if the utility, which serves New York City, could quickly respond to any outages in the country's most populous city.

He said 4,000 employees were poised to work 12-hour shifts over the weekend.

Last weekend, parts of Manhattan lost power for hours, darkening Broadway theaters, halting subways and closing restaurants and shops in a partial blackout blamed on a faulty piece of equipment.

Early on Friday, as the heat intensified in downtown Madison, Wisconsin, 11,600 homes and businesses lost power after fires erupted at two substations near the state capitol.  

By mid-afternoon, power had been restored to all but about 3,500 customers, according to Madison Gas and Electric.

On the East Coast, temperatures on Saturday were forecast to reach near 100F in Washington, and 99F in Philadelphia and New York, where it would feel more like 108F with high humidity. Much the same was in the forecast for Sunday.

To keep cool during past heat waves, suburban children typically ran under lawn sprinklers and city kids frolicked in the spray of fire hydrants, but the New York City Fire Department warned that special spray caps that firehouses hand out should be used to avoid creating a hazard.

'If you open a fire hydrant without these caps, you endanger your neighbors because the water pressure drops and our firefighters are not able to fight fires,' FDNY Commissioner Daniel Nigro wrote on social media. 

In Madison, Wisconsin, a fire is seen at an electrical substation started during the start of the heat wave on Friday

Kids and adults cool off in a fountain on the Rose Kennedy Greenway on the first day of a summer heatwave on Friday in Boston

Children run through the water of the Boston Common Frog Pond in Boston, Massachusetts on Friday