STILL IN A FOG about your plans for the weekend? Not to put high-pressure on you, but this Saturday, the National Weather Service is predicting a 100 percent chance of Open House at the World Weather Building, a local phenomenon occurring only once every two years. This is an indoor program, weather or not. The Camp Springs complex is right off the Beltway, so traveler's advisories are in effect.

The Weather Service is calling for a stimulating atmosphere. They have prepared eight floors of exhibits geared to almost all age groups, a self-guided tour that allows you to visit what interests you most. Scientists and meteorologists will be waiting on each floor to show you what they do.

"We are trying to acquaint the public with the sort of functions we do with their money," says Dr. Ron McPherson, chief of the meteorological operations division of the National Meteorological Center here. "It is their tax money, after all, that supports this process, and we would like it to be as well understood as possible."

You can watch meteorologists preparing Washington's local and regional forecasts -- the basis for most of tonight's spiel by the TV weather gurus. Or you might prefer to take in a movie: perhaps a National Geographic film that explains how forces in the atmosphere create a rainy or sunny day, or "Terrible Tuesday," a chilling 23-minute footage of a 1979 tornado touchdown at Wichita Falls, Texas.

Unlike that maddeningly cheerful voice on the C&P Weatherline, these real weatherpeople will respond to your questions. Maybe you'll see Valerie Thompson, who is operating an astonishing computer that animates satellite imagery. She taps a few keys to zoom in on areas of interest, in this case, flash floods in Texas, minutes ago. Or Jim Kemper, who at this moment has responsibility for virtually the whole North Atlantic. To the ships on that treacherous sea he's the most important person on earth.

The meteorologists a few floors down are responsible for predicting visibility, takeoff and landing conditions at 14 air terminals -- and wind shear probability. You can witness the life-and-death implications of forecasting. "We serve as the standard in the industry," says Earl Laws of the Washington Forecast Office, as he pulls still-wet satellite photos out of a blue UPI Unifax machine. "Everybody checks against us."

Gordon Barnes? Does he check against you?

"He wouldn't admit to it, probably."

Meanwhile, in a barren field in Sterling, Virginia, meteorologists are preparing to launch a hydrogen-filled weather balloon, one of the most fundamental observations to the weather service function. An attached radio transmitter dangling from a parachute will send back crucial data on atmospheric conditions. When the balloon pops at an altitude of 20 miles, the radio will fall harmlessly into the countryside. Usually. Once, a stray fell on the White House grounds, causing some excitement. Another one scared a cow and dried up her milk, prompting the angry dairy farmer to sue the Weather Service.

This evening, the western skies are full of high, dark, clouds. The balloon launch can't wait -- hundreds of them are released around the world at precisely the same moment. We quiz Richard Stone, chief of the test section here: Will it be raining at launch time?

"You're asking me?" he says, good naturedly. "I'm just a meteorologist. I have no ideeeah! I need my maps, I need . . ."

It's time. The dome of sky is thickly blanketed in gray. Like Van Gogh brushstrokes, a swirl of cloud pops open where the sun is, low in the sky. For just a moment, grassy fields shimmer in the red glow. We watch the balloon slip the surly bonds of earth and soar skyward, a thousand feet a minute. When it's out of sight, the clouds click shut, and we reach cover as rain pelts the field.

You can't tell us these weather people don't have clout. "Let's hope so," says Stone.

"Thirty years ago," McPherson says, "meteorology was perhaps an art. Today, it is a science, but a science that involves approximation."

Something to remember the next time you're shoveling 20 inches of "partly cloudy" off the front walk. COME RAIN OR SHINE

WORLD WEATHER BUILDING OPEN HOUSE --

Saturday, 10 to 3, 5200 Auth Rd., Camp Springs. Take the Beltway to exit 7B, Branch Avenue (Route 5 north). The World Weather Building is to your right just beyond the exit. For more information, call Carolyn Hodge at 763-4702.

NATIONAL WEATHER SERVICE PROGRAMS --

The National Weather Service School Visitation Program will send speakers to your school or civic group to give film/video presentations on any weather-related topic. Meteorologists are available to speak at career fairs, and tours can be arranged of the World Weather Building in Camp Springs. Twice-daily weather balloon launchings at the test and evaluation site in Sterling can be viewed with permission. Call Jo Ann Harding, 427-7718, or Don Witten (for Howard County only), 427-7622, to arrange any of the above, or write to them at NOAA/ National Weather Service, Silver Spring MD 20910.

NOAA WEATHER RADIO --

Provides continuous broadcasts at 162.55 MHz of the latest weather information directly from the National Weather Service Offices. Receivers available from radio/TV electronics stores (Radio Shack has a basic model for about $18).

SPECIAL FORECASTS --

Public Service Computer, 899-3244 or 3249; Marine, 899-3210; Extended, 471-1741, 899-3240; Ski (winter), 521-7777; Air Quality, 682-0677; Hurricane (fee), 900/410-NOAA.