Washington will have a warmer than normal fall and a colder than normal winter - but not a record cold spell like last year - visitors to the World Weather Building open house in Camp Springs, Md., were told last weekend.

It was a warm, wet day, exactly what National Weahter Service forecasters had predicted for Sept. 26 in their 3-5 day forecast, which helped demonstrate to 1,500 visitors the increasing accuracy of the forecasts produced round the clock by one of the world's most advanced weather centers.

And what the National Oceanic and Atmospheric Administration (NOAA) is doin for the air around the earth at its World Weather Building it is also doing for the earth's waters in Rockville at its National Ocean Survey building, which is holding open house this week for those wishing to see how the ocean's tides and currents are charted and predicted.

NOAA moved into the World Weather Building two years ago because it needed space for the growing number of computers to analyze the 52,000 weather reports a day from ground stations around the world, 1,200 detailed cloud photographs from satellites and the endless stream of reports from ships at sea, balloons and rockets in the air.

Not to mention the 400 Washington-area volunteers who have official rain guages and thermometers in their back yards and record for local forecasters the aberrations of Washington weather - such as August 8 when rain guages in Bethesda filled to the brim in a deluge that dropped 3 inches of rain in a few hours, while not a drop fell 10 miles away in parched Fairfax County.

The end results of all these measurements are forecasts that some meteorologists say are as much as 25 per cent more accurate that they were only 10 years ago. It also has meant that conservative meteorologists are hazarding longer-range forecasts than they ever made before.

In November the National Weather Service regularly will begin issuing 6-10 day forecasts, to supplement the present 3-5 day extended forecasts. The 6-10 will predict only general weather conditions, not give a day-by-day forecast as is done in 3-5 day predictions (which were first started in 1940 but only became detailed, giving day-by-day predictions in 1970).

The Weather Service has been making long-range forecasts since World War II but they were not made public until the Arab oil boycott when 30 and 90-day forecasts began to be publicized to help sections of the country prepare for severe winter weather and fuel demands. Last fall the long-range prediction group predicted a colder than normal winter, though not the record cold spell that occurred.

Prior to 1973 the public had little advance knowledge about the weather other than the 3-5 day forecasts of the Weather Service and the amusing long-range predictions of the farmers' almanacs, many of which talked about the length of the fuzz on a caterpillar's back as a guide to coming winter weather (about as reliable as the ground hog's shadow in predicting the arrival of spring). Most almanacs base their fuzzy predictions largely on averages of previous years' weather, statisfics which the Weather Service also feeds into its computers.

The increasingly long-range weather forecasts of meteorologists are becoming more accurate largely because of the tremendous increase in weather information being fed into the analyzed by computers, and the resulting jump in forecast accuracy. "We think our forecasting now is as good at 48 hours as it was at 36 hours 10 years ago," says Roy McCarter, chief of the aviation weather branch, "primarily because of more and better pictures from satellites which help us identify initial conditions."

Washington area residents wishing to be more closely involved in the weather were greeted outside the World Weather Building on Saturday by several of the 400 members of the Metropolitan Washington Volunteer Weather Network, which helps gather detailed rain, snow, hail and temperature information for the Weather Service here. The network needs several hundred more volunteers to fill gaps in Washington-area reporting. For information call Thomas Blackburn (427-7884 or 589-5870).

Area residents who prefer just to watch NOAA in action can do so today and Friday, between 9 a.m. and 3 p.m., at the National Ocean Survey open house at its headquarters at 6001 Executive Blvd, off Old Georgetown Road in Rockville.