The scenario was military; the enemy was snow. More tan 140 trucks, each loaded with 7 1/2 tons of salt, sat poised in Fairfax County garages, waiting for a command. In Montgomery and Prince George's counties, 190 Maryland state trucks, filled the night before, waited for the first flakes to cover the roads.

In the District, 35 of the city's 86 salting trucks were ready to go at 8:45 a.m., when the snow started to come down. Emergency drivers for the others, however, were scattered citywide, behind the wheels of the garbage trucks and Metrobuses they usually drive. Those vehicles had gone on the road before city officials were sure when the snow would arrive.

Despite the services of two separate wether forecasting services, despite the care fully drafted contingency plans and the continual phone calls from 6 a.m. on among the District officials concerned with snow emergencies, the everyone expected.

By 10 a.m. it was trickling down just hard enough to clog up the tail end of the morning rush hour, catching the salting trucks among the morass of sliding commuters, and tying up the garbage-truck and Metrobus drivers who were needed on the salting trucks.

"At 8:30 the weather service told us they had snow around Lorton - that hit us kind of suddenly," explained Jack Hartley, the District's assistant director of transporation. "It moved up on us fast."

"A daytime storm is a heck of a lot more difficult," he added. "The trucks are delayed in traffic. Some of them didn't have chains. We had to call some back in to put on chains."

"There's a judgement factor," Hartley added. "The best forecaster can tell you that the snow is going to start at 5 a.m., and you trucks can be sitting there till 10 with no snow and you're paying o vertime. At 8:30 or 8:35 this morning, that's when we made the move."

By that time, 35 garbage men and 20 Metrobus drivers were already on their routes. It was 9 a.m. before Hartley's office called Metro, to tell the agency to get its men out of buses 8:35 this morning that's when we and into trucks. The snow was already on the ground.

Two hours earlier, at 6:50, Hartley had received a call from the Joel Myers forecasting service of State College, Pa., a private weather-forecast firm which the District pays $3,500 annually for its reports.

Myers' information; the snow would start between 8 and 10 a.m., becoming heavy at 9 a.m. and dropping 2 to 5 inches on the city before 1 p.m. By the time the Myers call came, the bus and garbage truck drivers were already on their routes.

It took until 11:15 to get the full force of 86 trucks out on the 48 miles of District streets that the salt trucks cover. By 11:45, some 2,500 tons of salt - at $20 a ton - was down on the streets. But by that time, slick roads and traffic pile-ups had turned 20-minute commuting trips into two-hour ordeals.

In the District, the problems were worst in the Northwest. The first trucks were dispatched to the hilly Northeast and the far Southeast sections: most of the Northwest and Southwest had to wait for those driving the garbage trucks to switch to the salt trucks.

Even if they had all been ready. Hartley added, the salting could not begin before a quarter inch of snow was on the ground or the chemical would just scatter uselessly into the gutters.

While Hartley tried to switch his drivers in the right kinds of vehicles. Slade Caltrider, the Maryland state engineer charged with overseeing Montgomery and Prince George's main arteries, had to maneuver his trucks around traffic tie-ups on the interstate highways.

"We just drive over the median if we have to. We sure don't do any good sitting in traffic for hours. We get up ahead of the tie-up where we can do some good. In any case, we have 40 different routes, and even without traffic it takes as much as an hour to cover a route," Caltrider said.

Caltrider was alerted to the impending emergency at 6 a.m. yesterday by Northeast Weather Inc. of Boston the private service used by the Maryland and Virginia highway departments.

But that forecast was for snow at midday. Up until 8:30 a.m. less than half an hour before the first stakes began to fall, the latest bulleting predicted a noontime storm.

Still, by 9 a.m. Caltrider had his firt units on the road. "The build-up was very rapid thereafter until they all were in service at 11 or so."

In Montgomery County, the 51 county trucks that plow the secondary roads were on alert by 8:30 a.m., when Transportation Director Richard Lynch heard from the Joel Myers service, the service used by the county, that the storm would start at 9 a.m.

"As soon as it started to cover, trucks were sent out" over 480 miles of the county's 1,600-mile system, covering the 51 separate routes by 11 a.m., he said.

Montgomery County's Civil Defense Department also did its part in the emergency by declaring a "Condition Yellow" - a readiness alert used for everything from snowstorms to nuclear attack - at 10:57 a.m.

At the Park Police headquarters in Southwest yesterday, the continuing snow fall transformed maintenance worker Miles Hooper at 10:45 a.m. into the service's official "snow coordinator," complete with a radio console and authoritative voice.

"I love it," he said of the small white flakes that changed his job, "Everything went pretty smooth," he added.

In Virginia, which is accustomed to getting at least six hours notice from the North East Weather Service official had only a two-hour jump on yesterday's storm. Still, said Fairfax's resident maintenance engineer Roger McCleilan, "we knew it was coming." Fairfax had 140 trucks on the road by about 8:30, as the storm swept through.