With temperatures today and tomorrow expected to reach the mid-90s and the city blanketed with stifling humidity, the corporation that controls the state’s power grid planned precautionary measures to avoid a blackout like the one in 2003 that left 50 million people in America without lights and air-conditioning.

Although they do not anticipate a blackout today, officials of the New York Independent System Operator, which has been described as the air traffic controller for the state’s energy utilities, will be in closer contact with utility companies across the state and with the operators of neighboring power grids, according to a spokeswoman, Carol Murphy.

Yesterday, for only the second time this year, the not-for-profit corporation asked more than 1,500 large commercial and industrial customers to be on alert for a phone call or e-mail message today asking them to lower their use of electricity immediately.

The tactic is part of the corporation’s demand-response program, in which large users of electricity are paid $500 per megawatt-hour to lower their consumption on days when the state is deemed vulnerable to brownouts or blackouts.

“This is the time when you really need to have operators at their top performance,” Ms. Murphy said. “We have put our demand-response programs on notice. They may be called tomorrow.”

While officials said they have made important progress toward preventing a recurrence of the blackout of August 14, 2003, the largest in history, a combination of high summer demand for electricity and failure to enact federal legislation to institute uniform and better-enforced reliability standards for power utilities across the country, makes another blackout possible.

The chairman of the New York Affordable Reliable Electricity Alliance, an umbrella organization that acts as a spokesman for downstate energy consumer interests, said he had confidence that the state’s utilities can keep the lights on, even during the high energy demand caused by a heat wave. The chairman, Arthur “Jerry” Kremer, a former state legislator from Long Island, said the real threat of another blackout is posed by a possible breakdown of less regulated power grids in the Midwest.

“We are going to deal with excessive power usage but can’t protect ourselves from utility companies far away from New York who haven’t taken the necessary steps to prevent another blackout,” Mr. Kremer said in a telephone interview. “Could it happen again? No question.”

Last Tuesday, New York State set its all-time record for peak electricity demand at 31,741 megawatts, breaking the record of 30,982 set in August 2001. Con Edison, servicing about 9 million residents of New York City and Westchester County, also set its all-time demand record, at 12,361 megawatts.

Isolated blackouts affecting hundreds of residents and businesses in Staten Island and Brooklyn in the last month has some residents worried about another citywide crisis, but the outages were the result of equipment failures unrelated to the health of the city’s power grid, according to a spokesman for Con Ed, Chris Olert.

Mr. Olert explained that a heat wave, which he characterized as three of four consecutive days with temperatures above 90 degrees, and the resulting high demand for air conditioning, can lead to a burned-out power line – the cause of last month’s blackouts in Staten Island and Brooklyn. But he said those isolated equipment failures would not cause a power outage across the city.

The 2003 blackout was a result of the human error of a grid operator in Ohio, which caused a cascading power failure across the Northeast. The weather was considered normal for summertime, and the state’s electricity usage was not significantly high. The blackout cost New York City an estimated $1 billion.