The region was well braced for an attack from nature, but what happened Wednesday qualified was an all-out ambush.

Snow started spreading across the region before daybreak, accumulating up to a half foot by lunchtime, surprising highway crews and meteorologists.

And that was the appetizer before the main event that got under way after dark. Several inches of fresh, wind-driven snow, incited by "thundersnow" downpours, fell upon parts of the region. On Thursday, some areas are going to be shoveling their way out of a foot of snow.

The night attack was expected. The morning was something else.

"Obviously, we're going to go back and look to see what happened," said Gary Szatkowski, the meteorologist in charge of the National Weather Service office in Mount Holly, which had called for light accumulations during the day.

He said that the morning round was juicier and whiter than what was foreseen in the computer models.

"Thundersnow," is literally a snow thunderstorm. Thunderstorms are caused by violently rising air that condenses into precipitation and sets off lightning. They are more common in spring and summer when more heating is available, but they can occur during winter storms and can deposit 2 to 5 inches in just an hour.

The snow blitz turned into a nightmare for the morning commuter rush, forcing some schools to announce they were closing or opening late at the last minute.

Others that opened on time, including Philadelphia's public and Catholic schools, sent children home at noon.

Some government offices, including Delaware County's, shut down early.

Even emergency crews had difficulty navigating the slippery roads in Chester County, where some of the heftiest snow totals were reported, and officials reported the usual spate of accidents, including one involving an ambulance.

A Gloucester Township police officer was injured when she was struck by a car while investigating a crash on an icy road.

Philadelphia and surrounding municipalities declared snow emergencies requiring vehicles to be removed from designated streets.

The morning snow came as the advance guard of a massive, moisture-rich winter storm that will affect the region all day. Generally, 4 to 6 inches fell in the Pennsylvania suburbs, with 2 to 4 in South Jersey. Officially, 2.9 was measured at Philadelphia International Airport, pushing the winter total past 25 inches.

Officially, Philadelphia already has exceeded its seasonal average. Typically, by now the region has had only about a third of its winter harvest, which is about 22 inches.

Forecasters had originally thought the storm would begin as a relatively harmless wintry mix before changing over to rain and continuing in the afternoon before changing back to snow at night.

But the snow that fell in large, elegant flakes persisted until late morning, and the inches kept piling up.

"It took longer to change over," said Paul Walker, a meteorologist with Accu-Weather Inc., which also had underestimated the morning accumulations.

He said that since the storm was so complex, the models had a devil of a time with it.

That snow that showed up this morning was the advance guard of a storm in northern Alabama. The snow wasn't supposed to become a big deal until the storm bombed out later off the coast.

But the moisture came in robustly, and the air got an extra lift from high-speed winds in the upper atmosphere. The rising air condensed into accumulating snowflakes that kept coming and coming.

The snow reduced visibility and stuck to untreated roadways, creating hazardous conditions as the morning rush hour got under way.

Even treated roadways became snow covered, and cars slowed to a crawl on highways such as I-76, the Blue Route and Route 422.

More than three dozen motor vehicle accidents were reported around the region.

In the Gloucester Township incident, Ptl. Jennifer Rauscher was struck while investigating a two car crash in the vicinity of College Drive between Blackwood-Clementon Road and Broadacres Drive. Officers were in the process of closing the roadway while awaiting a salt truck when the incident occurred. She suffered minor injuries.

In the Chester County ambulance crash, the Elverson Emergency Medical Services vehicle was en route to Brandywine Hospital with a patient, said Patty Mains, a spokeswoman for Chester County's Department of Emergency Services.

No one was hurt in the accident, which occurred on Chestnut Tree Road near Route 322 in West Nantmeal Township, and another ambulance came to pick up the patient.

The Pennsylvania Department of Transportation ordered more salt spreaders into action as the snow kept falling, at one point closing the ramp from westbound I-76 to the southbound Blue Route.

As Nick Martino, PennDot's regional maintenance chief observed, the snow came at exactly the wrong time.

SEPTA reported service disruptions on a number of bus lines because of slippery conditions. Later, riders were advised to expect delays of up to 15 minutes on regional rail lines.

And the snow forced the cancellation of 41 flights out of Philadelphia International Airport. In addition, flights bound for Philadelphia were being held at their departure airports and were delayed by about three hours.

The Delaware River Port Authority reduced the speed to 35 miles per hour on its four bridges and then reduced it to 25 miles per hour on three of the spans - the Walt Whitman, Commodore Barry and Betsy Ross Bridges

New Jersey activated its Emergency Operations Center in West Trenton at 6 a.m. to monitor conditions throughout the state.

Just the day before, New Jersey Gov. Christie formally requested federal assistance to help the state and local governments cover the cost of the snowstorm of Dec. 26-27.

Perhaps another declaration request will soon be in the works.

Inquirer staff writers Ed Colimore, Mari A. Schaefer and Kathleen Brady Shea contributed to this article.