A subtle combination of imperfect weather forecasting, delayed salt trucks and incompetent and lawless driving by motorists--all striking during the afternoon rush hour--produced Tuesday's nightmare of endless and sometimes motionless commutes, transportation officials said.

Hoping to prevent an encore of chaos during this morning's commute, armies of plows, sanders and traffic officers were bracing overnight for three to six inches of snow that forecasters predicted would start falling after midnight and end by mid-afternoon.

In the District, public works employees started reporting to work at 10 p.m. Soon after, 100 six- and 10-wheel dump trucks headed onto their routes, where they were to spread an anti-icing solution on bridges and overpasses even before the first snowflakes fell. Dozens of plows also were ready for duty.

"It looks like the drive to work might not be fun," said Mark Tobin, a meteorologist with AccuWeather Inc.

Less fun than driving home Tuesday was hard to imagine.

Dubbing Tuesday's stormlet the "Snow Apocalypse" and the "Snow Flurry From Hell," motorists yesterday told war stories of needing five hours to drive up 16th Street NW (now known by some as the "Longest Street in D.C."); of facing hundreds of dollars in penalties from day-care agencies after arriving hours late to pick up children; of staying late into the night at work, or in restaurants and bars, and finding the roads still jammed with cars proceeding at the speed of a slow walk.

"I tell you, if aliens ever come looking for intelligent life on this planet, I hope they don't arrive in the D.C. area during a light snow at rush hour," said Keith Pew, who spent two hours driving from Reston to Bethesda.

In Washington, where grace under pressure from snow is a significant political test, Mayor Anthony A. Williams (D) said the episode revealed problems in the city's preparedness. "We can and will do better," he said of the city's traffic control efforts. "We've learned that in each of the critical intersections, we need to make sure the traffic is moving."

As for treating the slippery roads, Vanessa Dale Burns, the city's director of public works, said: "I think given the circumstances, we have done fairly well. There's always room for improvement."

Although this morning's accumulation was expected to be more than the barely measurable 0.4 inch of snow that paralyzed Washington and some suburbs Tuesday, area public works managers expressed hope that the region would be better prepared today. For one thing, the season's first snow, however light, is often the worst, they said. A helpful residue of sand and salt has not built up on the highways, and driving skills are rusty.

"Everybody forgot what they learned last year about how to drive in it," said Emil Wolanin, chief of transportation systems management for Montgomery County. This morning, he predicted, drivers and public works departments would be ready, and "you'll see a much different story."

The District was the scene of much of the traffic horror, with forward motion on the major arteries leading out of the city stalled for hours, and officials vowed to be better prepared for this morning's foul weather. A snow emergency was declared beginning at 7 a.m. today, meaning that drivers may not park in snow lanes along the city's major corridors.

Executive Assistant Police Chief Terrance W. Gainer said the D.C. police shifts would be extended, so 100 officers would be available direct traffic at 100 key intersections and bridge crossings. Tuesday night, 40 or 50 officers covered about 50 intersections, and many commuters complained that fellow motorists illegally blocked intersections, creating gridlock.

"That simply wasn't adequate," Gainer said of Tuesday's deployment. "People were frustrated, and they vented their frustration by saying, 'The hell with the law.' "

In Montgomery, John Thompson, chief of the division of highway services for the county's transportation and public works department, said a full complement of road crews was to go on snow duty last night at 10 p.m. In contrast, on Tuesday, three-fourths of the county's 150 maintenance trucks were on other duty when the snow started.

Amid preparations for today, officials offered their post-mortems on why Tuesday was so horrific.

The chain of events that ended with critical corridors brought to a standstill began with the weather forecasts. They weren't off by much--but they were off by enough.

Public works chiefs said they had been carefully tracking forecasts since early Tuesday morning, calling for harmless flurries. Relying on updates from AccuWeather, Burns said she kept expecting that the "flurries" would end at 2 p.m., then 3 p.m., then 4 p.m. "When it didn't stop at 4, we ratcheted up our response," she said.

Until then, 20 trucks had been treating bridges and major roads with magnesium chloride, a liquid ice-melting solution. In the next few hours, scores more trucks rolled in the District and the suburbs. It was too late.

Meteorologists from AccuWeather conceded that the behavior of the moving weather pattern surprised them. When it crossed Washington, it suddenly picked up moisture and "enhanced itself a little bit," said senior meteorologist Ken Reeves.

The temperature was so low--near 20 degrees--that snow that had melted from the friction of tires or contact with warm pavement later froze, creating hazardous conditions, especially in the District, southern Montgomery and parts of Prince George's County. Somehow, Mother Nature excused most drivers on the Virginia side of the Potomac River from the worst.

Vehicles started to fill the roads as the snow continued to fall, so the scrambling salt trucks couldn't reach their destinations. "As soon as everybody started seeing the precipitation, they all came to the conclusion at the same time: Let's all go home," Wolanin said. The result was what he called highway "compression" that clogged roads.

Still, those explanations didn't satisfy many. Usually, in Washington-style backups, you eventually come upon the cause--an accident, a parked car in a lane. The most mystifying thing about Tuesday's drive home was, you could be blocked for hours on a road, and as you finally moved forward, you would never see the cause of the blockage. This made everything seem irrational, surreal.

"I was looking for a reason for this backup," said Melinda Ulloa, 37, who was stuck on North Capitol Street, Georgia Avenue NW and New Hampshire Avenue NW trying to get home to Takoma Park. "Where's the ice, the car crash, the mounds of snow--something! But there was nothing."

She and her husband, Victor, were two hours late picking up their two young children at a day-care center. They face a bill of "hundreds of dollars" that the center still has not calculated, she said. For three more hours, while they inched home, the children slept, woke and screamed intermittently. The family members were "trapped in our own personal hell," Ulloa said.

The best explanation of traffic experts is that minimal disruptions far down the road in front of you--if they occur at a critical moment, like rush hour of the season's first snowstorm--can ripple into tremendous tie-ups. You may be home before you ever travel far enough to see the cause. For example, the hills of New Hampshire Avenue in Maryland near Takoma Park and Langley Park were icy, and until well into the night, cars took turns creeping up and down.

Many motorists believe much of the fault lies with their fellow drivers. Besides clogging intersections for as long as 10 light changes, these fishtailing road hogs didn't seem capable of handling a teensy bit of snow.

Jessica Woodruff, 22, spent 4 1/2 hours driving a rented Ford compact car from Dupont Circle to College Park to retrieve her Toyota Corolla from a garage. The garage was still open at 7:30 p.m. because the mechanics couldn't get home. Then she spent another hour driving across College Park, to a friend's house, where she spent the night. She never made it home to Gaithersburg.

Throughout her odyssey, she didn't see much snow, only plenty of sliding cars. But her cars never had a problem. "Hasn't anyone in this town ever heard of putting their [automatic transmission] in '2' or 'L'?" she wrote in an e-mail to The Washington Post. "And people in stick-shift cars really have no excuse."

Staff writers Joel Achenbach, Sari Horwitz, Phuong Ly, Alice Reid and Michael D. Shear contributed to this report.

CAPTION: CHAOTIC COMMUTE (This graphic was not available)