A blast of cool air that started out in the Canadian Pacific late last week reached the Washington area last night and was expected to push temperatures in the suburbs into the 50s by dawn today.

Highs aren't expected to reach 90 again until early next week, and area crops and lawns should get a drenching from showers and thundershowers throughout the day tomorrow, according to Frank Rosenstein, a forecaster for the National Weather Service.

The weekend is expected to be sunny and pleasant and nighttime low temperatures through Sunday should be in the 60s, he said.

While predicting a respite from the above-90 temperatures that have gripped the area for the past 15 days, Rosenstein acknowledged that the weather is anything but predictable at this time of year.

"The jetstream, which has been up along the Canadian border all summer, took a dip across the Great Lakes and into New York, allowing some cool air to slip through."

During the summer months, the jetstream is something of an undulating line, he said, "and what causes these little kinks is really hard to tell."

"About three or four days ago it looked like the cool air might come through, but we don't believe that enough to forecast more than two days in advance," he added. "The cool front could just as easily have stalled north of us."

Forecasters explained that while the cool front passed through the area yesterday afternoon, its greatest effects wouldn't be felt until early today. "The cool air is always behind the actual front, and this front is slow-moving," a forecaster said. The temperature at midnight last night was 78 degrees, compared with 83 degrees 24 hours earlier.

Though daytime temperatures today are forecast to be in the middle 80s, Rosenstein said it will feel even cooler because breezes will help blow away the polluted and stagnant air that has built up over the area for the past several days.

Behind the front is a low-pressure system that will help to change the skies to cloudy by tonight and should bring more than a sprinkling of rain to the area throughout tomorrow.

So far this month, the Washington area has had .91 inches of rain, about a third of an inch below average. The average rainfall for August is 4.40 inches. In July, 1.78 inches of rain fell here, compared with the normal for the month of 3.88 inches.

Virginia Agriculture Commissioner Mason Carbaugh said yesterday that the lack of rain in recent weeks already has cut expected yields 50 percent for Eastern Shore potato farmers and that some corn-producing areas of the state are experiencing losses as high as 60 percent.

The high temperature yesterday reached 96 degrees at 2:17 p.m., according to the weather service. The highest temperature in the latest hot spell was 97 degrees on July 31.