The highest temperatures of the year left the U.S. Midwest sweltering today and headed into the Northeast, threatening to break records in New York City tomorrow.

The heat could reach 104 degrees Fahrenheit (40 degrees Celsius) tomorrow through Aug. 3 in New York, and feel like 113 degrees when combined with the humidity, said Bill Goodman, a National Weather Service meteorologist in Upton, New York. Night temperatures will fall only to about 80 degrees, with humidity of 70 percent to 80 percent.

``It means the nighttime will be pretty oppressive as well,'' Goodman said.

The hot air mass moving east contributed to a two-week heat wave in California that resulted in the deaths of at least 112 people, forecasters said.

Warnings of excessive heat or heat advisories were in place today for much of the Midwest and Plains states, including Chicago, where it felt like 104 degrees at noon local time, the weather service said.

Northeast of Chicago, American Electric Power Co. shut down one of two nuclear reactors in Bridgman, Michigan, yesterday after lake water, used to cool the facility, pushed readings in the containment building to 120 degrees, spokesman William Schalk said.

St. Louis hit 98 degrees, feeling like 105 with the humidity, about 1 p.m. local time. Less than two weeks ago, Missouri Governor Matt Blunt sent the National Guard into the city to rescue people from dangerously hot homes following a power failure.

New York Power Demand

New York, Boston, Philadelphia and Washington, D.C., are under excessive heat warnings or watches from tomorrow through Aug. 3.

In New York, temperatures of 98 to 104 degrees will threaten records, the weather service said. The records for Central Park tomorrow and Aug. 2 are 100, set in 1933 and 1955, respectively.

Consolidated Edison Inc. expects the heat will push power demand in the New York to a record, potentially threatening an electricity system already damaged during a heat wave July 18. The utility yesterday asked its 3.2 million customers to conserve power by turning off air conditioners in unused rooms and unplugging unused televisions and computers.

New York Mayor Michael Bloomberg declared a heat emergency, ordering city agencies to ``take whatever steps are necessary'' to protect people from the heat. City pools will be kept open an extra hour, until 8 p.m., on days that exceed 95 degrees.

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Boston Bakes

In Boston, where the temperature may reach 100 tomorrow and feel like 110, Mayor Thomas Menino also declared an emergency. The city will make automated phone calls asking residents to check on neighbors and, as New York did this weekend, officials will open air-conditioned ``cooling centers'' where residents can escape the heat.

Relief for the Northeast is on the horizon toward the end of the week, Goodman said. A cold front is expected to move in from the north. It could bring severe thunderstorms, though cooler temperatures after that, he said. For the weekend, New York temperatures are forecast to be in the mid-80s.

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To contact the reporter for this story: Shannon D. Harrington in New York at sharrington6@bloomberg.net