Temperatures soar into the upper 90s again Monday in many parts of the country that experienced roasting heat over the weekend.

A high-pressure system was hovering over much of the country is expected to keep temperatures in the triple digits through Friday.

• Click here to check out FOXNews.com's 'extreme heat' photo essay.

From coast to coast in states like Missouri, Illinois and New York, residents took to beaches, pool and shade to get relief from the choking heat and humidity, which is expected to continue for the next few days, meteorologists said, as hot air is moving toward the East Coast.

The temperature in New York was expected to hit the middle to upper 90s later in the day, with a few spots hitting the century mark, said John Cristantello, a meteorologist with the weather service. Temperature-wise, New York City's record for the day was set in 1953, when Central Park recorded 100 degrees. The mercury reached 90 before noon Monday.

But the heat index — how it will actually feel on a person's skin when factors beyond weather are measured — was expected to climb to between 100 and 104 degrees, Cristantello said. He also said it's possible a second heat advisory would be issued for Tuesday.

People were wearing sunglasses, T-shirts, miniskirts and sunscreen and were choosing iced coffee instead of the heated brew. At least one man carried a towel to wipe his brow. A spot in the shade competed for a parking slot as the most valuable commodity in Manhattan.

Men and women made their way under narrow awnings, lounged under trees and took breaks under the large umbrellas of hot dog stands.

"It feels oppressive and sticky," said Laura Shaffer, a 30-year-old from Manhattan.

Shaffer, who studies how corporations deal with environmental issues, decided to buy her lunch at a fast-food restaurant before heading into work so she didn't have to come back outside.

"I think that it's a precursor of what we could be experiencing on a daily basis if we don't do something about global warming," she said near Pennsylvania Station.

But she didn't skimp on the spices for her rice and tomato burrito.

"They actually help keep you cool," she said.

In Cleveland, 22 outdoor pools that are normally closed on Mondays and Tuesdays were being opened as Monday marked the second 90-degree day of the year. Temperatures throughout Ohio are expected to hover around 90 the rest of the week.

Tony Godel, working on a remodeling project at a hotel in downtown Cleveland, was already sweating through his brown Corona Extra T-shirt by 10 a.m. He planned to drink a lot of water to cope.

"You get used to it after a while," Godel said. "You know what you're getting into. You're paid to deal with it."

The National Weather Service issued excessive heat warnings for Las Vegas, Chicago, St. Louis, Philadelphia, Tulsa, Okla., and parts of New Jersey.

Click here to read the heat warnings.

The combination of hot temperatures and high humidity will create potentially dangerous conditions, especially in the urbanized areas of Cook County, Ill., and Chicago, until 7 p.m. central time Monday. A cool front is forecast to pass through the region this evening, bringing a chance of thunderstorms and some relief from the heat.

Excessive heat warnings remain in effect until 9 p.m. EDT Tuesday in places like Camden and Trenton, N.J., Wilmington, Del., and the Philadelphia area. Monday is expected to be the hotter of the two days, with some parts of the Philadelphia metropolitan region to reach 100 degrees.

Afternoon temperatures are also expected to reach 100 degrees from now until Friday in the St. Louis metropolitan area, with heat index readings between 105 and 100 degrees. It was so hot Sunday that some Cardinals fans had to leave the game early.

California temperatures forecast to rise above 100 degrees Monday from the Mexican border to as far north as Redding and near the coast. State highs are expected to be 115 degrees near Barstow and 112 near Parkfield, said meteorologist Will Pi.

More than 60 cooling centers were opening Monday in St. Louis. Area hospitals have not reported any heat-related deaths. But some treated heat cases Sunday.

Indiana environmental officials are extending warnings for at least 26 counties because of high ozone levels Monday. Officials at New York's Department of Environmental Conservation and Health Department issued an air quality health advisory for the entire state because of ground-level ozone, a major component of smog.

Still, people outside the state Capitol in Albany were taking the heat in stride.

"It's not bad in the shade," said construction worker Alan Reynolds. "We got our briefing in the morning to take more breaks and drink more liquids. You just have to pace yourself."

PJM Interconnection, which operates the electric grid for all or part of 13 states and the District of Columbia, asked people to use electricity prudently, especially between 3 p.m. and 7 p.m.

PJM suggested closing curtains and blinds to keep cooler air inside and limiting use of stoves and other appliances. Power grid managers also asked California residents to conserve electricity, predicting demand will spike for air conditioners.

Illinois Gov. Rod Blagojevich said the state would make more than 130 office buildings available as cooling centers beginning Monday.

Officials in Chicago, where a 1995 heat wave killed 700 people, opened 24-hour cooling centers and pleaded with people to check on elderly neighbors. Minnesota Gov. Tim Pawlenty had ordered the National Guard out to help firefighters as temperatures even in the normally cool northern part of the state pushed 100 degrees amid very dry conditions.

Officials with Middletown City Schools, halfway between Cincinnati and Dayton, Ohio, canceled the last week of elementary classes for the summer because of the heat. The two buildings where those classes are held lack air conditioning, district spokeswoman Debbie Alberico said.

The Philadelphia Corporation for Aging on Monday activated its "heatline" hot line, staffed with nurses to answer questions and give advice about coping with the heat.

The city Health Department also was sending outreach workers — as it does during excessively cold weather — to help the homeless and elderly. Managing Director Pedro Ramos said workers would help them get hydrated and out of the heat.

In South Bend, Indiana, a 3-year-old boy died after he apparently locked himself inside a car in 90-degree heat on Saturday, relatives and neighbors said.

Abraham Barlue was found in the car by his grandmother, Vester Zeon, and a neighbor tried to resuscitate the boy before he was taken to a hospital.

"Somehow he got into the car, shut the doors, and child safety locks, he couldn't get out," said neighbor Tammy Wallace. "I tried to revive the baby, I evidently got a heartbeat back, and on the way to the hospital he passed away."

The Associated Press contributed to this report.