The pentagon, the Department of Transportation and the Secret Service have drawn up an elaborate plan to seek out and destroy the snow, which the weatherman tells them may fall on Jimmy Carter's parade on Thursday.

After conducting a dress rehearsal parade early yesterday, Armed Forces Inaugural Committee logistics experts concluded that snow would probably be the biggest threat to a trouble-free celebration.

Hence, they revealed at three-pronged combat attack plan to handle a snow-flake invasion. It will involve soldiers armed with long-handled shovels and sure-grip boots and heavy duty diesel-powered Macks with hydraulic ice-crushing wedges and optional flam throwers.

"We're definitely prepared," said an enthusiastic corporal in the public affairs section of the Armed Forces Inaugural logistics committee, "We can beat this thing."

"Our job," said a spokesman for the Secret Service after yesterday dress rehearsal, "will be subdue any stray flakes."

Four areas have been targeted by the military as priority combat zones - where visitors objectives, logistics spokesmen say. The Capitol grounds, the parade routes - from Fourth Street and Constitution Avenue N.W. to the White House - and parade assembly and dispersal areas and small streets around them will be under constant surveillance through Thursday.

"We have direct contact with the Mayor's command post," said Inaugural Committee chairman Bardyl Tirana. "We should know when any problem develops as it develops.We feel things will go smoothly - if not down right icily, according to current weather forecasts.

The thee-phase combat plan begins if one inch of snow or less falls during the inaugural. Then, D.C. and Transportation Department snow crews hit the streets with a barrage of sand and salt.

Depending on what the weather forecast is on Wednesday, city and government employees will be sent home, early to avoid what happened during the John F. Kennedy inaugural.

"It's not that we didn't have the equipment or the manpower then," said a Carter inaugural spokesman, "but a lot of people had to abandon their cars the day before the inaugural and the snow trucks couldn't clear the streets."

The spokesman said an agreement has been worked out with the D.C. and federal governments to release employees early Wednesday if a full fledged storm is expected.

"If bad weather is impending," he said, "the employees will be released."

If more than an inch of snow falls Thursday, heavier equipment such as dump trucks and power shovels will be brought to areas near the inaugural stand. As the snow continues, less attention will be paid to smaller access roads and other routes to be used by incoming visitors.

"If we have a blizzard, all resources will be brought to bear on the inaugural area," a spokesman said, leaving the counties and outlying areas of the District to fend for themselves.

A brief snow fell early yesterday as those marching and riding with the presidential motorcade conducted their mock parade for 1.3 miles along pennsylvania Avenue.

About 75 pintos and Hornets - representing each car to ride in the presidential motorcade - and about 500 members of the U.S. Armed Forces made the 7 a.m. journey to get an idea of how long the parade would last and how much gasoline it takes to drive it.

Although few motorists were on the road early yesterday, a parade spokesman said "a couple of Volkswagens cut in line," but were quickly waved aside.

Perhaps the most talked about mishap during the test run occurred when snow flurries fell inside a Secret Service vehicle that had its sunroof open.

"Things were all right," Tirana said, "That's how we expect it will go on Thursday."