The world's best computer weather models, located in Bowie, had digested reams of data from weather balloons, ocean buoys, observation posts, satellites and ships.

Supported by a new IBM supercomputer firing up enhanced software for the first time, the models calculated the highs and lows, the ridges and troughs, the moisture and motion of the atmosphere. On Monday at 3:30 p.m., based on the results, the National Weather Service issued this prediction for the Washington area yesterday:

"A 40 percent chance of light snow. . . . Total accumulation less than one inch."

In the cold and snowy light of yesterday morning, with the aid of 20-20 hindsight, Dewey Walston, a meteorologist with the Weather Service, had something to get off his chest about those computer models. He typed up a post-mortem addressed to other forecasters.

"Wow," Walston wrote. "Eating a lot of humble pie this morning. . . . Too bad the [models] can't answer all the phone calls."

Walston went on to observe that the Monday morning computer weather assessment, predicting conditions for yesterday's rush hour, "were the most horrible I have seen in my 10 [years] in the NWS." The models, he added, "did a pitiful job."

The models are run several times a day, and data from a new computer run became available in the early evening, predicting the storm much more accurately. After analyzing the results, federal forecasters issued the first weather alert for Washington before 10 p.m. More alerts quickly followed for cities up the Eastern Seabord. Far from "less than one inch," this was the worst snowstorm since 1996, a "bomb"--to use a weather term. It was the sort of winter northeaster that has dumped snow over such a large area only 28 times in the last half-century.

In a day of meteorological hand-wringing and second-guessing, federal forecasters admitted they and their computers had been late in calling it right. But they denied that they had been anything less than thorough in tracking the storm.

"I wouldn't characterize it as catching us off guard," said Louis Uccellini, director of the National Centers for Environmental Prediction, a branch of the Weather Service. He noted that the medium-range forecast pointed to the possibility of a significant East Coast storm by Wednesday. "We were watching this thing like a hawk."

But Patrick Michaels, the state climatologist for Virginia, suggested that a subtle bit of human psychology had foiled the forecasters. He said that one of the computer models, called Eta (pronounced AY-tah) after the Greek letter, is so good (it "nailed" the storm of January 1996 by more than a day), that forecasters are loath to contradict it. So they didn't, even though they could see radar evidence Monday of heavy late-afternoon snow in Raleigh, N.C., and other contrary data.

"Here we have the best model we know of insisting that the main precipitation shield is to the south and east of Washington," Michaels said. "And here we have our eyeballs looking at the precipitation shield advancing north and west."

The forecasters--including Michaels--didn't believe their eyeballs.

The moral of this snowstorm, Michaels suggested, should be: "The computer Eta'ed my forecast."

Uccellini, whom Michaels praised as knowing "more about East Coast snowstorms than any living human being," pointed out that the Weather Service issued the more accurate forecast hours before people had to get up and face the day. The only alterations through the night were to add a few inches to the total predicted accumulation.

But the Weather Service's revision of the analysis still came too late to make the early evening broadcast news, the last time many people tune in to the weather. The bad news came later.

Sue Palka, weathercaster at WTTG-TV (Channel 5), went on the air for Monday's 10 p.m. news predicting eight inches and then spent yesterday doing live updates every half-hour. "It's been insane today. My back is killing me," she said late yesterday. "I just realized an hour ago that I hadn't eaten all day."

Sometimes the computer models disagree with one another. That's when a meteorologist is put to the test, sorting out the conflicting data, said Andy Woodcock, a meteorologist with the Weather Service. But on Monday, forecasters had three key models in essential agreement, all predicting the storm would pass far to the east of the major East Coast cities. To reject a unanimous chorus of computer models is "the kind of stuff that gets you awards if you're right," Woodcock said. "You're a gutsy dude if you do it."

No gutsy dudes were on the case Monday.

So why didn't the models catch this earlier?

"I'd love to have two or three years to do a research project on that," Uccellini told a group of reporters during a telephone news conference.

The models had predicted that a weather system over the Great Lakes would help push the East Coast storm out to sea, but that did not happen.

Chris Davis, a meteorologist with the National Center for Atmospheric Research, a consortium of universities and other institutions, said he suspects that the raw data going into the computer models from balloons, aircraft and satellites may have been insufficient.

Surprisingly strong snow in Raleigh and other evidence independent of the computers raised some concern among the Weather Service's forecasters in Sterling as well as meteorologists at AccuWeather, the private service based in State College, Pa., that supplies many Washington clients, including the D.C. government.

Uccellini said the new supercomputer, recently unveiled with fanfare, helped matters. Its first run of enhanced software happened to be Monday morning. The supercomputer detected some possible problems with the original forecast, he said, and by evening, its superior number-crunching ability helped pull together the revised forecast more quickly.

Another problem in the prediction was that the standard ratio of water to snowflakes did not apply. Usually, when forecasters know how much water is coming down, they use a ratio of 1 inch of water for 10 inches of snow to predict how much snow will fall. But this storm was more intense, 1 to 20.

"It's what makes snow forecasting so much fun," Uccellini said.

This storm was a northeaster, with winds that were strong but not blizzard-strength, except for brief gusts. It is also sometimes called a "bomb," Uccellini said. That signifies the explosive development of a storm off the coast accompanied by a quick drop in central pressure.

Such storms that dump snow on all the major cities of the East Coast tend to be episodic, Uccellini said.

"They come in groups," he said. "You can go without them for four or five years, and then you can have two or three happen in one or two years."

Storm Shifts, Gathers Strength

Yesterday's snow storm came earlier and with more force than forecasters had predicted. The storm's strength and position were altered by other weather systems, bringing snowfall farther inland than expected.

* The strength of the coastal storm also may have been intensified by the North Atlantic Oscillation, which affects the jet stream. Under normal conditions, the jet stream travels over the northern part of the country. But, during what is called the negative phase of the North Atlantic Oscillation, the jet stream dips, bringing with it cold, polar air.

* The negative phase allows for more frequent storms and rapid changes in the weather. Under these conditions, the region's weather shifted from relatively warm temperatures to bitterly cold temperatures last week. Experts say that the negative phase has not been this strong in three years.

Storm conditions

Yesterday's storm brought bands of heavy snow, burying some outlying suburbs in as much as 18 inches of snow. With winds gusting to at least 30 mph, even well-plowed roads became hazardous.

What's ahead

Flurries possible today; temperatures in the 30s for the remainder of the week will limit melting; strong winds may bring dramatically colder wind chills.

Worst D.C. Snowstorms

Inches of snow at Reagan National Airport*

1922 28.0

1899 20.5

1979 18.7

1996 17.1

1983 16.6

Jan. 2000 9.5

As of 7 p.m.

*1899 and 1922 figures measured elsewhere

SOURCES: Dr. Jim Hoke, director, and Dave Roth, forecaster, Hydrometeorological Prediction Center; Curtis Carey, National Weather Service

CAPTION: Phil Alperson digs his car out in front of his Silver Spring home. He got the word late that his employer, the federal government, was closed.

CAPTION: Jim Alexander travels Route 6 near Zekiah Swamp in Charles County--on horseback--for a surprise visit to friends.

CAPTION: National Airport workers de-ice a plane during the storm. The snow forced the airport to shut down in the afternoon.

CAPTION: Brady Hayek, of Wyoming, flies through the air off a jump in the new snow at Battery Kemble Park off MacArthur Boulevard NW in the District.