The snowstorm that pummeled Maryland yesterday was first spotted by weather forecasters late last week when it came in off the Pacific Ocean and produced weekend showers in Los Angeles.

With plenty of time to watch the conditions develop - and close agreement among the computer models they use to predict weather behavior - forecasters were on the money this week as the season's first big winter storm struck the Northeast.

"They had a choice: to go with the models or go out on their own. The consistency of the models gave them high confidence, and they went in the right direction," said James Hoke, director of the federal Hydrometeorological Prediction Center in Camp Springs.

The long lead time on this storm was a big break for forecasters.

A typical nor'easter develops quickly off the Carolinas and leaves little time for forecasters to predict its effects on the big Northeastern cities.

"This one has been around for a couple of days," Hoke said.

After coming ashore in Southern California, the disturbance produced snow showers in the Sierra Nevada before moving into Arizona and New Mexico.

"There was some indication, even as far back as the weekend, that ... by the time it got to the East Coast it would strengthen," said Jeff Warner, a meteorologist with Penn State Weather Communications.

"There was also strong evidence that there was going to be good cold air in place already in the Northeast," he said. "So there was the potential for a significant winter storm."

By Monday, the cold air was moving south with a large high-pressure system from central Canada. It brought clear skies and nighttime temperatures in the teens and single digits in many areas and sent a deep chill as far south as the Gulf states.

To have such cold air in place so early in the season is "a little unusual," Warner said. "It's much more common in January or early February."

But it shouldn't, by itself, lead Marylanders to fear an especially snowy winter. "It's generally not wise to take one event and forecast an entire winter," he said.

Such cold air is dense, heavy and hard to move. So when the storm approached from the Southwest with warmer, lighter air, it was forced to run up over the cold. The moisture it carried condensed and fell through the cold air, producing snow, sleet or freezing rain.

Predicting what would fall where, and when, was the job of complex forecasting models. These are huge software programs running on supercomputers. They crunch vast amounts of weather data, filter it all through mathematical formulas that approximate real weather physics and spit out regular forecast updates.

Each model is different, and when they disagree, forecasters are left to use their own expertise and experience.

But this week they all seemed to agree. "Even when the models were run with slightly different initial conditions, they still produced the same forecasts," Hoke said. That gave forecasters confidence in the guidance.

It's not clear why the models fell into line. "There were some improvements made to the models over the last year or so," Hoke said.

One new feature allows the computers to consider finer details in the condition of the atmosphere at different altitudes - a factor critical in determining whether precipitation falls as rain, sleet, freezing rain or snow.

Forecasters may also have been aided this week by the first test of an experimental consultation process. More than 30 local forecast offices were brought together twice a day to confer via email and telephone on the winter storm forecasts.

"Just getting people to talk often and earlier in process has been beneficial," Hoke said.

Forecasters were able to stay ahead of events as snow, sleet and rain - freezing at times - began falling Tuesday and Wednesday in Oklahoma and Texas, and eastward across Tennessee, Kentucky and the Carolinas.

Yesterday, secondary lows formed over the Florida Panhandle and off Cape Hatteras, N.C. Together, they added to the rain in the South, pushed icy precipitation as far north as Richmond, Va., and the lower Eastern Shore, and triggered snow from Virginia to New England.

The wetter weather that has prevailed in Maryland since Oct. 1 is also continuing to roll back the lingering effects of 13 months of drought.

Streams and water tables have returned to nearly normal levels throughout Maryland and Delaware.

And Baltimore's reservoirs continue to rise.

The three man-made lakes stood yesterday at about 51 percent of their combined capacity.