"I always plan for the worst," says event organizer Anne Fleming. "I close my eyes and envision horror, then work backwards from there. Professionally, it has served me very well."

With the spring social season about to kick into high gear, that means factoring Mother Nature and her fickle moods into the mix, whether you're planning your own party or simply attending one as a guest.

"In the spring, I always assume it's going to rain in late afternoon," Fleming says. "Even if the sky is perfectly clear at 3 p.m., I'm convinced that we'll have a thunderstorm by 5:30." Cockeyed optimists resist this kind of approach in the belief that negative thinking begets negative outcomes. And because pessimism costs money.

A few years back, a dinner for 200 was scheduled for the gardens at Dumbarton Oaks, one of the loveliest spots in the city. Fleming had to beg organizers to get a tent, then beg some more to rent plastic sides in case it rained.

The weather the day of the party was flawless, and guests sipped cocktails under a balmy sky. Then, at exactly 6:30 p.m., the thickest, blackest clouds anyone had ever seen battered the tent with rain, wind and lightning. Pessimism -- or prudence -- had triumphed.

Fortunately, there are a few rules of thumb: February is the most likely month for nasty weather in Washington. May and June always produce late-afternoon storms, where the sky turns a sick shade of green and dumps a torrent of rain for 30 minutes. July and August will wilt native Washingtonians and fell out-of-towners.

Forewarned is forearmed, but try telling that to a dewy-eyed bride convinced that her wedding day will be perfect. There was the woman who got married at her aunt's grand estate in Potomac. The October ceremony was scheduled to take place outdoors. The bride opted not to have a tent for her 400 guests. It rained so hard for so long that the priest added "in sunshine and in rain" to the vows. The $5,000 saved on a tent went to regrading the lawn: It was so wet that all the cars parked on grass sank into the turf and had to be towed out.

Or consider the couple who opted for a tent but balked when told they should add flooring because it was placed at the foot of a hill. It rained, of course, and the grass flooded, forcing all the guests to take off their shoes. The story became "Everyone had a great time." Yeah, sure.

Last week the White House erected a tent on the South Lawn for the annual St. Patrick's Day bash for 1,000.

"We want to make sure a guest at the White House is comfortable," says social secretary Capricia Marshall. "What we've implemented are two walk-throughs: nice weather vs. bad weather. We always have a backup plan."

That means always renting a tent (the White House doesn't own one) and figuring out who pays for it each time. Now it also means a tent floor, so guests' high heels don't have to sink into the grass. It means extra umbrellas at the entrance gate and plans for a canopy at the visitors' entrance. And it means getting up at 4:30 on the morning of a state visit for a conference call to discuss weather at the arrival ceremony and official dinner: January's welcome for Argentine President Carlos Menem was so cold that the honor guard couldn't blow its horns.

"We sit around and try to think of the worst situation," Marshall says. "Everyone here is pretty good about thinking about disasters."

The best and only real defense is attire. Dressing defensively means wraps for chilly rooms or too-effective air conditioning, and clothes that can cope with wilting heat. And a small umbrella in the back seat is always smart because there should be attendants with big umbrellas to greet you, but you can never count on it. Assume it will be too hot or too wet. "Washington is a tropical city," Fleming says. "If you go with that, then you're okay."

The rest is up to the host. There are only two ways to combat the elements: Hold a party indoors or rent a tent.

"Nobody really wants a tent," says Michael Graves of HDO Productions, which provides tenting for events at the White House. "They want balmy weather and blue skies. But if God can't do that, they come to the tent guy."

But tents are expensive: One that seats 150 people for dinner and dancing costs $2,000 to $2,500. Flooring can double the price. Heating tents is relatively easy, but air-conditioning a tent is very, very expensive.

And a tent doesn't always work. Event planner Susan O'Neill once planned an exclusive fund-raiser at the F Street Club. The underwriters planned the February dinner for 100 people in a heated tent on the terrace. O'Neill tried to talk them out of it.

On the day of the dinner, the temperature dropped sharply and a storm hit as guests were arriving. Even though the heaters were running at full blast, the tent was covered with ice. "They could have put a furnace in this tent and it wouldn't have been warm," she says. "It was so cold everyone had their coats and gloves on. It was like eating in an igloo."

The moral of this story . . . well, of any weather story in Washington: Never invite more people than you can manage to move inside.