For much of the week, the forecasts were shaky. The storm might drop 6 inches, perhaps 10, or even 14 on some parts of Massachusetts. It could bring mostly rain. No one was sure.
By day’s end Friday, some towns around Worcester were tucked under a blanket of snow two feet deep and Boston was buried in more than a foot. The snow had been falling for hours, with the system’s center located hundreds of miles offshore.
“It was unusual in that the storm is located more than 400 miles offshore, and typically when a storm moves that far offshore we don’t have any impact from it,” said Joe Dellicarpini, a meteorologist with the National Weather Service in Taunton. He said forecasters generally expect high snowfall totals when a weather system is closer to shore, usually located a little southeast of Nantucket.
Harvey Leonard, a meteorologist with WCVB-TV in Boston, said the system was large enough that Southern New England fell within the reach of its circulation.
“If you looked at the satellite picture and it was the summer or early fall, you’d say that’s a hurricane,” Leonard said.
The difference between the snowstorm that walloped Massachusetts Thursday and Friday and a tropical storm, Leonard said, is that a winter storm is fueled by the convergence of warm and cold air masses. A tropical storm is driven only by warm air, he said.
When snow was falling but not accumulating on Thursday, Leonard said, some people thought the storm was a bust. For his evening forecast, he said he predicted 5 to 10 inches in Boston, and at 11 p.m. he bumped up his prediction to 6 to 10 inches or more.
“The idea was correct, it was just really the magnitude was a little bit larger,” Leonard said. The snow tapered off in much of the state late Friday afternoon.
A number of factors combined to create the storm that defied predictions, Leonard said. Meteorologists had expected temperatures on the borderline of freezing, and they stayed just low enough to support snow throughout the storm.
Winds off the ocean extended high into the air, so the system carried a lot of moisture that it eventually dumped on the Bay State, Leonard said. Towns around Worcester, like Holden, received the most snow — about two feet — because hills in that area lift the air, allowing for more precipitation, he said.
David Epstein, a meteorologist who blogs on boston.com, said the upper left quadrant of a storm generally carries the most snowfall, and that section expanded over Massachusetts on Friday.
“It basically passed off the mid-Atlantic coast, intensified, and like a paddle wheel it spun all of that moisture hundreds of miles westward into Massachusetts,” he said.
Dellicarpini said a strong jet stream augmented the system. The southern jet stream moves from the Gulf Coast toward New England as spring nears, he said, creating an environment for hazardous weather.
“Usually February and March can be notorious for some of the stronger winter storms,” he said.
Still, the jet stream moved slowly, stalling the system and allowing the snow to fall for hours, Dellicarpini said. Epstein said a high pressure system to the north and another storm farther out to sea also prevented the storm from moving away.
But as is the way of fickle New England weather, the March storm will be followed by placidity.
“We’re going to have sunshine back tomorrow,” Dellicarpini said on Friday. “And temperatures on Saturday and Sunday in the 40s and probably in the 50s on Monday.”