Extreme hot weather and the threat of power outages drove many area residents indoors or to swimming pools Tuesday, while some communities announced plans to set up shelters today for overheated seniors and other residents without electric power or sufficient cooling at home.

The announcements came as the temperature shot to a sizzling 99 degrees shortly after 3 p.m. Tuesday, the Attleboro water department reported, breaking the previous record for the date, 98 degrees.

And its going to be worse today, officials said.

The National Weather Service issued extreme hot weather warnings for all of Norfolk and Northern Bristol Counties, forecasting a high temperature of 100 degrees or more today.

The state Department of Environmental Protection posted an unhealthful air advisory for portions of Bristol County and Cape Cod. Regional electric power officials said extreme weather conditions could drive energy use to record levels and urged customers to conserve.

Attleboro Mayor Kevin Dumas said rolling electrical brownouts could occur today and that the city would open shelters beginning at 8 a.m. at the Hyman Fine and Hill Roberts Schools for those who lose power or otherwise without access to sufficient cooling.

City officials also urged residents to check on senior citizens in their neighborhoods and keep close tabs on small children to ensure they do not become overheated.

Dumas said shelters would be stocked with food and attended by a police officer and emergency medical technician. But he urged residents who use the shelters to bring their own folding chair, medicine and clothing as well as small games or toys for children. Those who are unable to obtain transportation should call the fire department, the mayor said.

The city recreation department also announced it would extend hours at municipal pools to 8 p.m. It also postponed a popular horseshoe tournament scheduled for today to Sept. 6.

In addition, the Norton Senior Center on Route 123 will be open until midnight tonight for residents seeking relief from the heat.

And the Seekonk Public Library will hold its Wednesday concert performance at 6:30 p.m. today indoors in its multi-purpose room because of the sweltering heat.

ISO New England, operator of the region's bulk power system, warned that power supplies could be `` tight'' today.

`` We are asking customers to voluntarily conserve electricity as a precautionary step because high demand for electricity during severe temperatures can stress the region's power system,'' said Stephen Whitley, ISO New England's senior vice president and chief operating officer.

New England's electricity usage is forecast to reach record-breaking levels today, with demand potentially reaching 28,030 megawatts. New England's record for electricity usage is 27,395 megawatts, set on July 18. Both figures are higher than last year's summer demand peak of 26,885 megawatts. A megawatt of electricity is enough to power 750 to 1,000 households.

Throughout the Attleboro area Tuesday, residents did their best to cope with uncomfortable temperatures and humidity.

Scott Smith and Duane German, who religiously have jogged during their lunch hours for years, did their best to ignore the swelter as they set out on a jaunt from the downtown YMCA.

`` You don't want to skip a day and get out of the habit,'' said Smith, who added he's used to the heat because of several days already this summer with temperatures in the 90s.

Others were less reticent about taking the day off: Normally Smith and German can count on two or three companions accompanying them.

City recreation and parks officials reported healthy use of the city's pools and the zoo at Capron Park, where children romped through a `` mist tent'' that sprayed a continuous cloud of water droplets.

Michelle Fuchs of Coral Gables, Fla., and her 3-year-old daughter, Brielle, arranged to meet friends from New Hampshire and elsewhere in the Northeast at the park.

Mothers and children enjoyed the mist and strolled off to view the animals, which were also given their own showers with sprinklers installed in their cages by park personnel.

Others out in the heat Tuesday weren't necessarily there by choice.

Tom Marino of Peabody, an employee of Pavement Maintenance Services of Woburn, had his crew out patching and paving a trench dug by gas company workers on Locust Street.

Marino and his crew had the job of scraping, melting and rolling pavement to make the road passable over the repair.

`` It's a hot job,'' said Marino, whose melting machine produces temperatures of about 1,800 degrees.

On Oakdale Avenue, landscaper and brick paving specialist Keith Blais and assistant John Lima were painstakingly laying a herringbone-pattern walkway for a client as the midday sun raised the air to a broiling 95 degrees.

`` There's not much you can do about the weather,'' said Blais of Blais Landscaping and Construction. `` We take turns laying brick, and we brought along a 5-gallon cooler of water.''

Cameron Roofing, which provides roofing services throughout the Attleboro area, advised supervisors and workers to drink extra fluids and to wear hats to ward off sunstroke, said owner Chip Cameron, who added crews would probably knock off work earlier than usual.

At the city's public works department, Superintendent John Clover said crews would probably ease up on the heaviest construction and maintenance assignments today because of the heat, and some workers probably would be moved indoors.

While temperatures were expected to reach 100 degrees today, the National Weather service forecasts slightly less heat on Thursday, tapering off to the low 80s through the weekend.