Custom Forecasts; Watching The World's Weather From Bedford, Mass.

In a large two-story house nestled comfortably in the Massachusetts town of Bedford an important bit of news is being analyzed:

It is snowing in Missouri.

In a large room downstairs four men in sweaters and T-shirts sit at desks arranged in a cross. By touching a few buttons they call up on the computer terminals before which they are seated information ranging from the latest barometric pressure in Los Angeles to road conditions in Pittsburgh.

What concerns them now, though, is the storm brewing 5,000 feet above Missouri. From computer-derived weather maps and their own calculations, they chart the storm's likely path. Snow will fall on the East Coast beginning tomorrow morning, they predict - an inch in Boston to perhaps six inches in northern New Jersey.

From information and estimates such as this, corporations, public utilities, small-town highway departments, and even airlines make the decisions that affect millions of people - how much home heating oil to supply, how clear the roads will be, and how much food will cost.

The source of such weather information is a company called Weather Services Corporation, which operates from the Bedford, Mass., house. It is one of about 60 private weather forecasting firms in the United States, none of which compete with the US government's National Weather Service. The private companies tailor the weather information to the special needs of their clients, and play watchdog in case the climate takes an unexpected turn.

Weather Services Corporation, or WSC, is one of the largest of the private weather predicting firms. To enter its offices is to have the world's weather at one's fingertips.

For the moment, at least, the seas off the Brazilian coast are calm. That's good news for the convoy of five barges carrying equipment for an oil project in Brazil. Because the barges cannot withstand rough seas, they would have to put into port if the weather turned bad.

Half a hemisphere away, WSC's William Saulnier plots on a large mapthe slow passage down the South American coast. He plays weather watchman for the convoy , warning the bargemen of any storms coming their way. So far, though, only a few squalls near the mouth of the Amazon River have hampered the convoy's two-month journey from Houston.

Down the hall from Mr. Saulnier, in a room filled with stacks of papers, WSC meteorologist Mike Palmerino gets a phone call near noon.

The caller is a meteorologist from Cargill Inc., one of the world's largest agribusinesses. He is checking on the latest weather reports and outlook for the Ukraine and lower Volga valley. To many, a Soviet weather forecast would make a TV test pattern look interesting. But a two-day cold snap in the Soviet Union's major wheat-producing region may have severely damaged the important winter wheat crop. A big loss in the Soviet crop could drive up wheat prices worldwide.

Although Mr. Palmerino reports temperatures low enough for potential damage, he concludes from satellite weather photos and precipitation reports that crop damage was isolated: A four- to eight-inch snow cover has insulated most of the wheat. His forecast: a warming trend.

This kind of up-to-the-minute information can mean the difference between profits and losses to grain dealers and farmers. According to Peter Leavitt, director of agricultural services at WSC, many US soybean farmers delayed selling their crops in January, anticipating that a drought in Brazil's soybean region would push up market prices. Indeed, soybean dealers bid prices up to about $6.65 a bushel before a February rain broke the Brazilian drought. Soybean prices quickly fell by 40 cents a bushel.

Twenty-four hours a day, seven days a week, WSC spews out information to its more than 400 customers all over the world. The staff of 90 works in shifts to ensure that clients are kept abreast of changes in the weather. ''What we're selling,'' says WSC vice-president and meteorologist John Murphy, ''is the ability to get a professional meteorologist at 2 in the morning.''

By 3:30 p.m. the snow in Missouri has moved into Indiana and Kentucky. The superintendent of highways for Needham, Mass., calls Mr. Murphy for the latest forecast. About an inch of snow beginning tomorrow morning, Murphy tells him. On that information the superintendent orders his spreaders loaded with salt and sand, ready to go in the morning. ''They're very good about telling us a storm is coming,'' the superintendent, Robert Lanigan, says of WSC. ''I haven't been in a storm situation - in the last six or seven years, anyway - (where) I haven't been ready.'' (No one - not even WSC - predicted the once-in-a-century intensity of the Northeast's great blizzard of 1978. But WSC did give Mr. Lanigan enough warning to get his snow-removal equipment ready, and kept him up-to-date on developments.)

The service costs small towns about $900 for the winter. But with employee costs, Mr. Murphy says, ''all you have to do is save them two hours for one storm during the whole winter and they've paid for the service.''

Dozens of towns and cities as far away as Raleigh, N.C., as well as state transportation departments and turnpike authorities (including the New York Thruway) are WSC clients.

''We've seen periods of rain and drizzle . . .'' begins Bonnie Cameron Saulnier's live weather report to an Atlanta radio station.

She sits at a desk before two microphones in a small soundproof chamber in the WSC building. Several times a day she does live and taped weather reports for radio stations all over the country and includes specific temperatures, sky conditions, and outlook for the area. ''Sunday will be gorgeous,'' she promises her Atlanta listeners. Few perhaps realize the weather reports are broadcast from 1,000 miles away.

Altogether, nine WSC meteorologists serve as local forecasters for 44 radio stations nationwide. Most local stations can't afford their own meteorologist. But, once one station subscribes to a private weather service, others in the area probably will hire a competing weather firm, says George Stamos, WSC vice-president.

Many local television stations, as well as NBC's ''Today,'' ABC's ''Good Morning, America,'' and CBS's ''Morning'' use data supplied by Weather Services International (WSI), a sister company of WSC. While WSC supplies forecasts, WSI simply sells weather data, leaving the television meteorologists to make their own forecasts.

Scores of clipboards, lining the walls of WSC's operations division, hold weather data from which many utilities will determine their energy requirements for the coming days.

Public Service Electric & Gas Company of New Jersey, for example, gets average temperature, wind and snow conditions, and other weather data transferred directly from WSC by computer. The utility then calculates how much heat its customers will need.

Citibank of New York calls WSC to learn the day's maximum and minimum temperatures and when they will occur. The information is fed into computers, which then adjust the building's heating and cooling.

The WSC operations room is where long-range forecasts are made. A meteorologist asks the computer for a section of the country, and a chunk of states outlined in blips and dots appears on the screen. He presses a button and dozens of temperature readings in the area superimpose themselves on the map. Another button brings barometric pressure readings; a third, sky conditions, and so on. The information is updated every hour.

The computerization technology of the 1970s spurred great growth for the private weather forecast industry in general, and WSC in particular. The firm tripled gross sales to a little under $2 million and quadrupled its employees during the past decade.

As facsimile machines downstairs print out National Weather Service maps -- predicting weather up to 84 hours ahead -- upstairs John Wallace, WSC president, leans back in his chair.

''In the old days, we didn't even have time to do these calculations,'' he reminisces. ''We used to spend most of our time tearing paper (from the weather bureau's teletypes). Forecasting then was part science, part artistic activity.'' Behind him, through the window, WSC's satellite dish points skyward. It receives photos directly from two National Oceanic and Atmospheric Administration satellites orbiting 22,000 miles above the equator.

''Before the war,'' Mr. Wallace continues, ''so much of the forecasting was broad-brush forecasting: 'It's going to be fair.' 'It's going to snow.' They never told you when. They probably didn't know when.''

But the military campaigns of World War II changed all that, he says, because they depended on expert forecasting. Wallace was among the many meteorologists trained by the armed services. After the war, he and a handful of others set up shop as private weather forecasters.

In 1947 Mr. Wallace started operating from a small room in Boston with one employee. ''You'd sleep by the phone,'' he recalls of the long shifts. Although his was one of the first private forecast firms, he faced a problem common to the infant industry: Most businesses didn't see a need for the service.

''There weren't too many prospects,'' Wallace remembers. But ''we knew the highway departments (and) public works departments had a need for the information -- the best they could get. Then we started after the gas companies.''

Gradually, more companies began to see the value of accurate, localized forecasting. And many began to save substantial amounts of money.

But perhaps for Wallace, WSC's shining moment came during Hurricane Hazel in 1954. Predicted by many to stay out to sea as it traveled up the East Coast, the hurricane unexpectedly veered inland into North Carolina. Wallace, who stayed up all night to track the storm, managed to get word to a team of meteorologists stationed on a beach directly in the hurricane's path. Ironically, they were working on a cloud-seeding project.

Before the storm hit about four hours later, the meteorologists not only got themselves to safety, they were able to warn nearby residents to evacuate as well.

Not every day is that exciting in weather forecasting, Wallace says. ''You can't get excited about a summer day at 80 degrees.'' But when your business is tracking the whims of wily Mother Nature, he says, ''something is going on all the time.''