A cool front preceded by a string of thunderstorms pushed toward the Washington area last night, promising very limited relief today from the last three weeks of suffocating heat.

National Weather Service forecasters said the cool front should break the grip of stagnant, pollution-filled air over the area and push it toward the Atlantic, replacing it with fresh air from the west.

Although forecasters did not expect the cool front to arrive before about 4 a.m. today, the thunderstorms sent evening temperatures plunging from 93 degrees at 6 p.m. to 75 degrees at 8 p.m.

The storms also brought winds estimated at as high as 70 miles an hour in parts of Montgomery and northern Prince George's counties. Scattered power outages were reported throughout the area.

High temperatures today should range from 85 to 90 degrees, which is warm but near the normal level for this time of year and a good 10 degrees below recent readings.

But more important, forecasters said, is that the air will be cleaner and less humid. It is the high humidity, or excessive moisture in the air, combined with eye-stinging pollution that has made the weather more than usually miserable.

Both temperature and humidity will probably start creeping back up Saturday and Sunday, the Weather Service said, but another surge of cooler, drier air is expected to arrive from the west Monday night, restoring seasonally normal conditions.

In New York City is 2:40 p.m., the mercury rose to 104 degrees, the highest temperature recorded there in 41 years.

Washington area residents also endured another scorching day. The official high temperature at National Airport was 95 degrees.

The Potomac Electric Power Co., which services the District and portions of Montgomery and Prince George's Counties and a small section of Arlington, set another "peak load" record yesterday of about 3.853 megawatts between 3 and 4 p.m. as its generators strained to meet unprecedented demands for air conditioning.

This surpassed Pepco's peak load record of 3,835 megawatts set Tuesday.

"We had no real problems meeting demand," said Pepco spokesman John Grasser. "Things were tight, but we were ok."

The Virginia Electric and Power Co., in suburban Virginia, which also set peak load records earlier this week, has had to buy 25 per cent of its power from adjacent reserves. Vepco district manager W.R. Black said late yesterday it was still not known if a new consumption record was set yesterday.

For the seventh consecutive day, the Metropolitan Washington Council of Governments also extended its pollution alert through 3 p.m. today, after recording an air quality index measurement of 115 at 3 p.m. in Suitland. A reading 100 or higher is considered "very unhealthy" by COG.

The current pollution alert is the fourth declared this year by COG. The alerts have included a total of 17 days so far, compared to 18 days for all of 1976. The greatest number of alert days in one year - 25 - occured in 1973.

At 3 p.m. yesterday, a Cheseapeake & Potomac Telephone Co. reported message giving weather details erroneously reported that the air quality index for the city had jumped to 315 - an unprecedented "dangerous" level necessitating warnings to the general public to curtail activity and stay inside.

"That's when the tilt sign goes on," said COG spokesman William H. Gilbert yesterday. he said COG officials quickly learned of the error and had the recorded message corrected to "115" within minutes. The highest official AQI reading ever recorded was 180 on Aug. 1, 1975.

The mass of cool air heading toward the Washington area has already brought relief to much of the Midwest and Great Lakes states. Heavy rains also drenched many parched farm areas throughout the Midwest.

Virginia Gov. Mills Godwin said in Richmond yesterday that crop losses in 42 drought-stricken Virginia counties have exceeded $152 million. He said he has added another 15 countries to a list of 27 for which he is asking a disaster declaration by President Carter.

Carter has already declared seven counties in Pennsylvania a disaster area in the wake of severe flooding in the Johnstown area where at least 37 bodies have been recovered and untold millions of dollars in damage caused.

The Potomac River here has been flowing at well below it normal level for several weeks, but hydrologists from the U.S. Geological Survey say they see no immediate threat to the area's water supply.

Washington and its surrounding suburbs are diverting about 340 million gallons a day from the river, said Survey spokesman Frank Forrester, but the river's total flow is averaging about 1.5 billion gallons a day. Normal flow for this time of year is about 2.8 billion gallons a day.

Although total rainfall for the year so far is also below normal, scattered thundershowers in recent weeks have kept the river "fairly steady," Forrester said.

"The bad thing is that the thundershowers have happened mostly in the lower part of the Potomac basin," he said. "We need morein the upper part to replenish the tributaries."