With the National Weather Service calling for temperatures to reach a record-setting 100 degrees Tuesday, the Philadelphia School District has scheduled early dismissal for summer-school students and utilities will pay extra attention to the power grid.

Jim Poirier, a meteorologist at the weather service's office in Mount Holly, said the "significant heat wave" that began Sunday would result in temperatures being well over 90 degrees until at least Wednesday. The service has posted an excessive heat warning for the region.

Monday's high of 98 in Philadelphia fell short of the 100-degree record set in 1999. If the mercury reaches 100 Tuesday, it would eclipse that day's record of 98. NBC10 predicted the temperature would reach 101 Tuesday.

"This is fairly unusual, but it has happened before," Poirier said. "We're around 1999 levels."

The good news is that humidity levels have not been high. "It's more like a Western heat," he added.

The forecast prompted the Philadelphia School District to announce that the 58,000 students participating in the district's expanded summer school program, and others enrolled in summer activities in district schools, will be sent home Tuesday at 12:30 p.m.

The summer school program normally ends at 2:45, district spokesman Fernando Gallard said Monday evening.

The early dismissal, he said, also affects children enrolled in the district's comprehensive learning centers, prekindergarten programs, city summer camps, and Freedom Schools based in district buildings. Gallard was not sure how many children participated in those programs.

Updates will be posted on the district's website, www.philasd.org

The Heatline operated by the Philadelphia Corp. for Aging and the city Health Department handled dozens of heat-related calls Monday.

"We're busy," said Heidi Gambino, Heatline supervisor. "I'm assuming it will only get worse."

She said her office handled 90 calls between noon and 10:30 p.m. "People are hot, and they're asking what to do," she said.

Gambino said one caller was worried about a 90-year-old neighbor and did not have a phone number, so the Heatline contacted 911. Medics checked on the woman and found her all right.

The staff is giving tips for keeping cool, counseling people to keep hydrated, and warning of the dangers of running fans in rooms with the windows closed. The center is not providing fans or air-conditioning units.

A prolonged heat wave is especially dangerous for the elderly, pregnant women, children, and those on medication for chronic health problems.

Gambino said a nurse was on duty to talk to callers with specific health concerns.

She said the Heatline (215-765-9040) would operate between 8:30 a.m. and midnight Tuesday.

Despite Monday's near-record temperatures, the holiday meant there was less demand on utilities because most businesses were closed.

"Obviously, with the heat coming down the pike in the next few days, we're really monitoring that closely," Peco Energy spokeswoman Karen Muldoon Geus said.

She said Peco would work with the regional power grid to ensure there was enough to meet the demand.

At 10 p.m. Monday, Peco was reporting scattered outages affecting 5,000 customers in the five-county region, down from 8,000 earlier in the evening.

Geus said the outages were heat-related, noting that increased usage caused some transformers to overload.

"We're deploying crews and working to get power restored as quickly as possible," she said.

Peco's peak usage of 8,932 megawatts was on Aug. 3, 2006. On Monday, she said, the five counties used 7,800 megawatts.

While some power outages were reported in North Jersey, a spokesman for Public Service Electric & Gas said an hour-long outage that affected traffic lights along Route 561 in Cherry Hill Monday was caused by an equipment failure.

Spokesman Ed Sullivan said, "We're not sure if it was the result of the heat."

Contact staff writer Martha Woodall at 215-854-2789 or martha.woodall@phillynews.com.