At 6:30 a.m. yesterday, minutes after he arrived at his downtown Rockville office, John DiGiovanni, assistant Montgomery County highway maintenance chief, got a phone call from a private weather service warning him about an approaching snowstorm.

In the hours that followed, a dozen county employes were dispatched in pickup trucks to survey county roads, 184 dump truck operators were warned to stand by with sand and salt, and DiGiovanni began making plans to spend the night in downtown Rockville for an all-night snow watch.

Fairfax County was making similar arrangements for its roads, according to Andy Bailey, an assistant resident engineer who drove on Rte. 50 yesterday afternoon to look at a contruction site and check roads for icy snow.

In the District, Anne Hoey, chief of the Public Works Department's snow-fighting operations, took a change of clothes with her to a downtown command center where she spent the afternoon answering phones, listening to radio communications and monitoring weather reports.

Hoey's department was criticized Friday for reacting too slowly to an overnight storm of ice and snow that caused massive traffic jams in the city. This time, she said, the department is ready.

Hoey, as well as her suburban counterparts, didn't know how much snow to prepare for.

The National Weather Service predicted between two and four inches would fall during the night and continue throughout this morning. The low temperature last night was expected to be 30 degrees, according to the weather service, with a high today reaching 34 degrees.

Private weather service agencies, however, predicted considerably more snow.

Accu-Weather Inc., the Pennsylvania-based firm that provides Montgomery County with forecasting services, was anticipating "three to six inches with the potential for eight," DiGiovanni said.

In the District, Hoey said she received predictions as high as 12 inches. Fairfax County had forecasts of one to four inches.

"If [the snowstorm] materializes, we're ready," said Hoey. "It's a tricky storm to predict and if it moves 50 miles either way we could have none or lots of snow."

Hoey showed up at the snow emergency center at 14th and U streets NW about 9:25 a.m. yesterday. By early afternoon, she had ordered 29 trucks on standby in case it became necessary to begin de-icing streets. In addition, six patrol cars roamed the city streets checking for icy conditions.

At 7 p.m., she said, 81 trucks were on standby.

"You are predicting the future and hoping you are right and trying to cover all the bases," Hoey said.

Transportation officials said they must monitor closely conditions such as low temperature and rates of snowfall, rain and sleet in order to decide when to begin spreading sand and salt.

In Fairfax County, Bailey spent most of the afternoon checking with other state transportation offices for snowfall reports and peering out his office window, looking for signs of the on-again, off-again snow that had begun falling earlier in the day.

He said his department had 290 of its own trucks and an additional 200 that it had hired from contractors ready to begin clearing streets. A second shift of 150 workers was scheduled to work through the night from midnight until noon today.

"We have everybody ready, on alert," he said.

In Montgomery County, 92 trucks at six field offices were ready to go by 5 p.m., DiGiovanni said.

DiGiovanni said he planned to spend the night at his office, manning phone lines, checking weather reports and communicating with street patrols.

"It's easy for me to sit in an office," he said.

"The hard part is for the drivers . . . . They started working at 7:30 a.m. yesterday and they could work all night and all day [today]."