The hottest and muggiest spell of the season apparently was on its way out of the Washington area last night as a new front pushed its way in from the west with the promise of lower temperatures and humidity, all making for a sunny and pleasant weekend.

Yesterday's low temperature was 80 degrees, recorded at 6 a.m. by the National Weather Service after more than 24 hours of record high minimum temperatures that never dipped below 82 degrees. Yesterday's high of 93 degrees at 3:10 p.m. was just six degrees above normal for this date, and a good deal lower than this season's highof 98 degrees, recorded again Thursday. g

Virginia Electric and Power Co. reported an unusually heavy demand for power, and appealed to its residential, industrial and commercial customers in Northern Virginia to reduce their consumption of energy for such appliances as air conditioners and clothes dryers.

Officials of the Potomac Electric Power Company reported no problems in meeting its customer demands despite a record demand level of 4152 megawatts reached at 4 p.m. Thursday whent he temperature read 98 degrees. Pepco's previous demand record of 4142 megawatts was set last July 16. Pepco officials said they expected to continue service routinely "barring unforeseen equipment failures."

Forecasters at the National Weather Service said a high pressure system of air was expected to move into the Washington area last night and cool temperatures to 65 in the suburbs and 75 in the city. That system is expected to bring dry air into the area and lead to more tolerable temperatures Saturday in the 88- to 93-degree range.

Sunday is expected to be sunny with temperatures in the low to mid 90s. The dry air mass is expected to move off the East Coast early Sunday and make way for the return of more typically humid Washington summer days at least through Tuesday. The sunshine, however, may be interrupted by the first precipitation since July 6 as afternoon and evening thunderstorms are expected Monday.

For the last two days, pollution levels measured by the air quality index have been stable in the moderate range of 60 to 65 for ozone and are expected to remain so.

Area hospitals reported scattered cased of heat-related illnesses and said it "usually takes three to four days of continually high heat" before they see a marked increase in the number of such cases.