A 50- to 100-mile miscalculation in tracking the path of a low pressure weather system off the Atlantic coast was the main culprit causing forecasters to miss predicting yesterday's blockbuster snowstorm, National Weather Service meteorologists said.

Forecasters relying on computerized weather models failed to predict not only the path of the low pressure system, but also the storm's intensity. As a result, weather forecasts continued Tuesday evening and early yesterday to predict only an inch of snow for the Washington area, instead of the 12 inches that fell at National Airport.

Such miscalculations, which can have costly and annoying consequences for governments, school systems, transportation officials, parents and commuters, are partly the fault of nature. Forecasters say minor but sudden shifts in weather systems hundreds and even thousands of miles away can affect conditions in Washington, sometimes making it difficult to formulate accurate predictions.

"We knew it was going to rain," said Washington forecaster Bob Oszajca, "but not snow like this."

"A one- or two-degree change in temperature is all it takes to make what we're having now," said Amet Figueroa, looking out the snow-filled windows of a National Weather Service observation post at Baltimore-Washington International Airport yesterday.

A mammoth Weather Service computer complex in Suitland designed to digest millions of bits of meteorological information from around the world provided the data base for yesterday's miscalculation, Oszajca said.

A low pressure system with moist counterclockwise winds was slowly churning north along the Atlantic coast with its center near Cape Hatteras on Tuesday, Oszajca said. A computer "model," or mathematical simulation of atmospheric conditions, suggested that the system would continue along the coast in a northeasterly direction, he said, keeping colder air over Pennsylvania and New York but out of the Washington area.

In fact, he said, the system was some 50 to 100 miles farther east than the computer model suggested, well out over the Atlantic. Also, it had intensified "like a spring in a watch," Oszajca said.

This in turn created a vacuum over a broad area from northern Maryland to North Carolina, drawing in the cold air from the north, he said. Moist air from the counterclockwise winds of the low pressure system over Atlantic then mixed with the cold air and -- presto -- snow.

Computer models are "generally reliable," said forecaster Figueroa, ". . . but obviously this time there wasn't quite enough information."

The models are generated by two massive computer systems at the National Meteorological Center in Suitland and relayed to the World Weather Building in Camp Springs. Working around the clock, an operations staff of more than 100 analysts and other specialists "massage incoming data" on temperatures, humidity, air pressure, wind speeds and other phenomena gleaned from satellites, weather balloons and surface instruments throughout the world, according to Gale Haggard, senior duty officer at the meteorological center.

The raw data are fed into an IBM 4341 computer system, then transferred to a Cyber 205 computer system, which constructs the actual statistical models on which local forecasts are based.

The meteorological center circulates two complete computer models every 24 hours, one at about 4 p.m. and the other at 4 a.m., forecasters said. Based on Tuesday's 4 p.m. model, the National Weather Service was calling only for rain Wednesday. By the time the 4 a.m. model arrived, the forecast was modified to call for one inch of snow for the Washington area.

Many private forecasters, including television weathermen and companies such as Accu-Weather, use the government models to develop their own forecasts. And local governments often contract with private forecasters who tailor weather forecasts for specific areas, helping guide decisions on school closings, plowing and public transportation.

In Prince George's County, for example, authorities heard 5 a.m. forecasts from the National Weather Service and Accu-Weather that called for rain and light snow. Based on that forecast, officials decided to open schools two hours late. Again relying on forecasts that predicted a tapering off of snow by day's end, school officials decided to close schools only an hour early -- a decision that ultimately resulted in chaos for school personnel, students and parents.

Predicting weather, even with computers, is an imperfect science, said Haggard.

"It's a human-machine mix," said Haggard. "The models are used to develop forecasts . . . but forecasters also use their experience and intuition."