The three-day Presidents Day weekend storm in Maryland had its origins in a Pacific rainstorm that soaked the Los Angeles area with torrential rains, then crossed the country and collided with stubborn arctic air entrenched across the Northeast.

Its legacy will likely be the biggest snowstorm in Baltimore in more than seven years, weather experts said, with total accumulations that might rival some of the deepest snowfalls in the city's history.

"I would say we're destined here for the top five, and we've got a good shot at the top three," said Todd Miner, a meteorologist with Penn State Weather Communications. He said the writing was on the wall late last week when more than 6 inches of rain fell on parts of Southern California. The loosely organized system then headed east across the continent, powered by the southern jet stream.

Drawing warm, moist air northward from the Gulf of Mexico, the storm produced 8 inches of snow from Iowa eastward across the Central Plains, with heavy rains, icing and even tornadoes across the South, and widespread flooding in the upper Tennessee River Valley.

"This storm is a juicy one that has a lot of moisture associated with it," Miner said. But El Nino - often a suspect when there is unusual weather to explain - can't be tagged for this one.

"El Nino right now is in a weakening phase. Much of the warm water that was present in the eastern Pacific last month has gone away. This is a storm system that could have occurred in the absence of El Nino," Miner said.

The real explanation for the heavy snow lies instead in its abundant moisture, the arctic cold that had invaded the Northeast before the storm, and the weather system's slow pace as it crossed the region.

As the storm approached the East Coast, a strong, cold, high pressure system was centered over southeastern Canada, sending arctic air into the Northeastern United States.

"Temperatures have been well below zero in interior parts of New England, with temperatures as low as minus 30 across northern New York," Miner said. "So you have this ... cold air meeting with a plume of moisture moving north through the eastern states. And that's helped to create the heaviest snow."

As warm, moist air rides up over a thick layer of dense, cold air, the moisture condenses and falls as snow.

After a 2.4-inch snowfall Saturday, the precipitation paused, resuming about 3 a.m. yesterday with light snow at Baltimore-Washington International Airport. It became heavy after 5 a.m.

At times yesterday the snow in Washington and Baltimore was falling at rates of 1 to 2 inches an hour. "That's considered to be pretty heavy snow," Miner said. Reports of thunder and lightning in southern portions of Baltimore and Harford counties signaled the intensity of the storm.

"Thunder snow" occurs as the warmer air several thousand feet above the cold surface bubbles up through the even-colder air above it. The phenomenon creates the same sort of electrical instabilities seen when warm air rises rapidly in summer thunderstorms. The result is a lightning discharge and accompanying thunder.

The storm was expected to re-form and consolidate off the Atlantic coast overnight, then head north and east into New England.

"Once the low develops off the coast, it will help carry more energy farther to the north, so there will be heavier snow for northeastern Pennsylvania, metropolitan New York and even to southern New England and metropolitan Boston," Miner said.

"It's likely there will still be snow flying in the Washington, D.C., and Baltimore area. ... That's a long period of time if it's snowing hard. It doesn't happen all that often," Miner said. "This is a real slow-moving storm."

When it is all over later today, the Presidents Day weekend storm is likely to rival the heaviest snowfalls in Baltimore history. The deepest on record in the city occurred Jan. 27-29, 1922, when 26.5 inches fell.

But more Marylanders will remember the city's second-biggest snowstorm - the one almost exactly 20 years ago, on Feb. 11-12, 1983, that dropped 22.8 inches at BWI.

The most recent top-ranking snowstorm, and the third-deepest in city history, blew in Jan. 7, 1996, and left 22.5 inches at the airport.

The average snowfall in February at BWI is 6.9 inches. More than 10 inches had fallen this month before yesterday's snow began. The snowiest February on record was in 1899, when 33.9 inches piled up in Baltimore.

The average for a full winter season at BWI is 18 inches, based on 30 years of observations between 1970 and 2000. This winter's total snowfall (since Dec. 1) had topped 25.3 inches before the big snow started falling yesterday.