Ollie North isn't the only Marine who's been keeping secrets.

For decades, at the Commandant's House at the Marine Barracks, successive commandants, their commanding ladies, master sergeants and one or two others with high-level clearances and oaths signed in blood have kept a secret Washington hostesses would kill for: how to prevent it from raining on the weekly garden party and parade.

The sundown party in Washington has always been a favorite for those held prisoner during the late summer by Congress. It's also a great time for hostesses to give garden dinner parties of sizes exceeding their dining salons. (In her heyday, the late Lorraine Cooper gave an annual dinner for Congress in her Georgetown garden -- carefully informing the neighbors by letter, lest they call the police.)

But the worthy who slave over a stove to feed the multitudes tremble at the thought of packing a party scaled for the great outdoors into a dining room that will seat only 12 very close friends.

Not the Marines.

Jan Gray -- whose husband, Lt. Col. Alfred M. Gray, has just assumed command -- hasn't even moved into the Commandant'sHouse, yet she's already received a top-level briefing on the closely held Marine secret from Master Sgt. Edwin (Top) Smallwood.

She said she didn't know all the intricacies of the rite yet. "But I do know it involves a drink for the god in the garden."

The seal of secrecy thus breached, Sgt. Smallwood said he wasn't sure how it started, only that the magic formula was revealed unto him 12 or so years ago, when he first came to the Commandant's.

"There's this Mickey Mouse glass that somebody gave the house. We fill it full of gin and put it just behind the bodhisattva statue in the garden," he said. "If it doesn't rain, after the parade as a reward, we pour it out on the ground by the statue."

The libation works, Sgt. Smallwood says.

The Marines aren't the only ones with their own private methods for warding off rain.

Bess Abell, when she was Lady Bird Johnson's social secretary and in times of need ever since, called Irving Kirck, a California meteorologist who picked the date for the World War II invasion of Europe.

"He likes to have plenty of choice of dates. Ideally, he wants you to call and say, 'What night in 1988 would be best for my dinner party?' One day, in deepest winter, he turned up on our doorstep for a visit, without a coat. I said, 'You're lucky in the weather.' He said, 'I planned my trip accordingly.' "

Gretchen Poston, formerly Rosalynn Carter's social secretary and now a partner in Washington Inc., has a simple formula: "If you plan an outdoor event, you must have a rain plan and a tent. I've had good luck. When we had a big party for NATO in the Rose Garden, great huge clouds started to roll in, but I stood my ground and it cleared."

Linda Faulkner, the reigning social secretary, says: "Somebody calls the Weather Bureau. And I pray."

Caroline (Mrs. John Farr) Simmons remembers that Marjorie Merriweather Post had weather to match her middle name, by consulting the Farmer's Almanac before her annual spring garden parties.

Deborah (Mrs. John) Toll, who gives a number of outdoor dinner parties at the University of Maryland's president's residence, says, "We have a leakproof tent. And we find that if they don't get too wet, people don't mind a few raindrops."

Lady (Eve) Cotton, wife of the onetime Australian ambassador, claims an antipodal belief that if you throw your unmentionables over the roof, it won't rain on your tea party.

Channel 5 weather caster Sue Palka says: "I maybe get a call a week from someone planning a party. One lady said she was giving a big party at the Washington Hilton, and should she put up a tent? I told her if her party started at 8 p.m., instead of the 7:30 she'd planned, the thunderstorm would be over." Palka calls forecaster Scott Prosise at the Weather Bureau.

Prosise, a man with a sunny disposition, says, "We don't get too many calls for party predictions, maybe two or three a week, but a good many for official ceremonies. Sometimes we have calls for long-range forecasts, and then we have to tell them the usual values {temperatures, chance of rain/wind} for that date. It's during the thunderstorm watches that we get the most calls. Yes, these can be hysterical."

Not everybody knows that anyone can call one of three numbers that will give you not only a recording, but also, if you stay on the line, a real live forecaster who'll be glad to give advice on your outdoor feast. The numbers to call, 8 a.m. to 4 p.m., Monday through Friday, are: 471-1741, 899-3244 and 899-3249.

Have a nice day!