Meteorologists predict today will be the hottest day of the year so far, and public schools in the District closed early to beat the heat.

The National Weather Service issued a heat advisory for the entire Washington metropolitan area from noon until 6 p.m. today. Although temperatures will be in the mid-90s, the high humidity will push the heat index to more than 100 degrees, the weather service said in its advisory.

"We're looking for the hottest day of the year," said meteorologist Chris Strong of the National Weather Service.

District school officials closed the city's 147 public schools at 12:30 p.m. today. After-school care and other activities were also cancelled, officials said. Yesterday, nine D.C. schools were closed because their air-conditioning wasn't functioning. About half of D.C. public schools have only window air-conditioning units.

Schools in the city of Baltimore are also closing more than two hours early. And Rappahannock County Public Schools also closed at 1 p.m. today because of the heat.

The heat advisory put out by the weather service means that weather conditions will "create a situation in which heat illnesses are possible."

"Minimize your time outside, limit your time in the sun, stay in air-conditioning if you can, and keep hydrated," Strong advised. "If you have family or friends who are elderly, sick or weak, check up on them and be sure they're keeping cool."

Glenn Druckenbrod, medical director of the Emergency Department at Inova Fairfax Hospital, advised people to avoid caffeine and alcohol -- they accelerate water loss -- and to stay well-hydrated by drinking lots of water if they're outdoors. He also advised people to avoid strenuous activity outside during the hottest part of the day.

People at greatest risk of heat-related illnesses are the young and the old. Infants and young children are unable to recognize when they become dehydrated. And it's the frail elderly who often are hurt by heat.

The Maryland Department of Health announced today that two state residents have died from complications of hyperthermia this year.

One, a 71-year-old woman from Prince George's County, died June 10, with heart disease as a complicating factor. The other, a 26-year-old Harford County man, died earlier in the year.

"It is important for everyone to take precautions," department Secretary S. Anthony McCann said in a written statement. "We are especially concerned about elderly people, young children, and those who are overweight. Heart disease, diabetes, respiratory illnesses and other chronic health conditions also increase an individual's risk."

Druckenbrod warned, however, that "even people in great shape can get into trouble with heat like this." No age group is exempt from the risks of heat illness.

Druckenbrod said that symptoms of heat exhaustion and its more serious complication, heat stroke, include: confusion, rising body temperature and an inability to sweat.

"People can get heat stroke without doing anything strenuous, while sitting in a chair," the doctor advised. He said the elderly, the sick, and people without air-conditioning were the most susceptible to heat-related illnesses.

For those who don't have air conditioning at home, the best way to protect against the ravages of high temperatures and humidity is to try and get to a place that is cool for at least several hours.

Meteorologist Strong said thunderstorms could develop in the area later this evening.

Experts predict temperatures will retreat Wednesday and Thursday, and humidity will lessen as well. And by Friday, they expect a high only in the low 70s.

Staff writers Anne Bartlett and Sally Squires contributed to this report.

The District Department of Parks and Recreation opened five outdoor pools and two indoor pools today because of the heat, city officials said. The city also opened two cooling centers in the lobbies of government buildings where the public can go cool off in the air conditioning and drink a glass of water.

The outdoor pools opened especially today are:

Anacostia and Oxon Run pools in Southeast; Banneker pool in Northwest; Kelly Miller and Harry Thomas pools in Northeast.

The indoor pools opened today are the Takoma pool in Northwest and the William Rumsey Sr. Aquatic Center in Southeast.

The city has also opened the lobbies of two government buildings to the public today where people can go cool off and drink some cold water. They are the Reeves Municipal Center at 2000 14th St. NW and One Judiciary Square at 441 4th St. NW.

City spokesperson Jo'Ellen Countee said a white city van was also cruising the streets of the city with cold water for the homeless.