Surprise! Biggest November snow in three decades snarls region.

The snow came in bursts late Thursday morning, more an ambush than a snowfall. And by the time it backed off and mixed with the sleet in the afternoon, whatever part of the region wasn't shut down was left in slow motion.

A potent coastal storm set off a surprisingly vigorous snowfall — 3 to 5 inches in Philadelphia and its neighboring counties, up to 8.5 in upper Bucks County — and a rash of early closings that led to a premature and often adventurous rush hour in which routine commutes were taking longer than normal drives to the Shore.

Motorists encountered roads that had been transformed into snow-and-ice rings. Spinouts and stopped vehicles impeded traffic.

Perhaps exacerbated by such a robust first encounter with snow, better than five weeks before the winter solstice, hundreds of accidents were reported — 250 in Montgomery County alone, along with 350 disabled vehicles.

SEPTA Regional Rail trains were delayed up to 90 minutes during the peak evening commute and into the evening, and several bus routes were suspended or shortened; departing flights at Philadelphia International Airport were delayed, some for more than an hour; and the PATCO High-Speed Line was shut down briefly for icing problems.

Even after the  snow let up in the evening, however, Regional Rail woes continued, with SEPTA announcing it had ceased service on its Paoli/Thorndale Line through the night, and had suspended the Airport, Chestnut Hill West, Media/Elwyn, Trenton, and Wilmington Lines due to weather-related conditions, including downed trees and loss of power. SEPTA was working late to bring some of the lines back, and by 10 p.m. service had resumed on the Trenton, Wilmington, Chestnut Hill West, and Airport lines.

Furthermore, some area schools and school districts — anticipating a messy Friday morning commute — announced that they would open two hours late. Among the districts opening later were the Abington, Cheltenham, Lower Moreland, Upper Moreland, and Upper Dublin districts. Philadelphia schools made no announcements.

It was the city's first official measurable snow for a November in a decade; at 3.6 inches, a record for the date, and the biggest November snow since 1989.

"You can go many a year without seeing anything like this in November," said David A. Robinson, the Rutgers University professor who is the New Jersey state climatologist. And it came with unexpected ferocity, the first time this season that the atmosphere outwitted the computers, and likely not the last.

The National Weather Service posted a "winter weather advisory," but its Thursday morning forecast had called for an inch or less in the city.

The first flakes at Philadelphia International were reported at 10 a.m., after the peak morning commute. Then came the surprise, and an abrupt end to the good fortune. Curtains of snow virtually spilled from the skies, reducing visibility to a quarter-mile. It came so fast that it overpowered the stored warmth of paved surfaces and the ground. Robinson estimated that the subsoil temperatures were in the 40s, but it didn't stop the snow from accumulating, eventually burying the remnants of the grasses.

By 1 p.m., 2.4 inches already had been reported at the official measuring station at the airport

Snow was supposed to be mixed with generous amounts of sleet and rain, but that process took longer than expected. It was clear by lunchtime that some places were going to get more snow than expected.

"This storm just dumped so much snow so fast," said PennDot spokesman Brad Rudolph.

"We thought it was going to be more mix," said Al Cope, science and operation officer at the National Weather Service office in Mount Holly. However, the temperatures at the surface and in the upper atmosphere stayed colder than expected, despite strong winds off the ocean where sea-surface temperatures were in the low 50s.

Sleet, which is snow that melts as it passes through warm layers of the atmosphere and then re-freezes, finally materialized in the early afternoon, "but snow-wise the damage [had] been done," said Dan Pydynowski, a meteorologist at AccuWeather Inc.

"The timing of the storm was as bad as you can get, because of schools being in session," said John Haynes, Chester County acting director of emergency services. As did a number of other school districts in the region, most schools in Chester County closed early, as did the county offices.

Delaware County offices closed in the afternoon as well, and the county 911 center was experiencing high call volume related to vehicle collisions, said spokeswoman Adrienne Marofsky.

The nor'easter — so named for the winds it generates from the east and northeast — deepened Thursday off the Carolina coast and moved north, blanketing the Washington area with snow along the way.

Those onshore winds were supposed to scour away the cold air at the surface and the upper atmosphere and turn the snow to sleet and rain during the day.

However, those winds encountered a cold barricade of resistance, said Robinson. They "just ran into a wall," he said; while areas right along the coast were experiencing rain, snow was covering roads just 5 miles inland.

The precipitation finally became all liquid in the Philadelphia area after dark, and was forecast to continue through the night and end Friday morning. With highs expected well into the 40s, Friday could turn out to be a good day for melting any of the residual slush that survives the night.

As for what the recent brisk storm traffic might mean for the coming winter, "it doesn't portend anything," Robinson said.

Right now the atmospheric signals for the seasonal outlook are basically a wintry mix.

Added Robinson: "I don't know how anyone in their right mind is going to call this winter."

Staff writers Angela Couloumbis and Justine McDaniel contributed to this article.