The region was well-braced for an attack from nature, but what happened Wednesday qualified as 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, people in 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 accumulation during the day.

The morning round was juicier and whiter than foreseen in computer models, he said.

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

The snow was heavy enough Wednesday night that Philadelphia public schools and administrative offices will be closed Thursday, spokesman Fernando Gallard said.

Early Wednesday evening, the Pennsylvania Department of Transportation temporarily ordered speeds reduced to 45 m.p.h. on interstate highways in the area. The same limit was temporarily imposed on the Pennsylvania Turnpike, according to the Associated Press. The New Jersey Turnpike cut speeds to 35 m.p.h. on the southern part of the highway.

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

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 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 struck by a car and suffered minor injuries 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 into Thursday. Generally, four to six inches fell in the Pennsylvania suburbs, with two to four in South Jersey, through the day Wednesday. 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 originally thought the storm would begin as a relatively harmless wintry mix before changing 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 AccuWeather Inc., which also underestimated the morning accumulations.

Since the storm was so complex, the models had a devil of a time with it, he said.

The morning snow 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 snowflakes that kept coming and coming.

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

Snow covered even treated roads, and cars slowed to a crawl on highways such as I-76, the Blue Route, and Route 422. More than three dozen traffic accidents were reported around the region.

In Gloucester Township, Patrol Officer Jennifer Rauscher was struck while investigating two crashes in the vicinity of College Drive between Blackwood-Clementon Road and Broadacres Drive. Officers were closing the road while awaiting a salt truck when she was hit.

In Chester County, an Elverson Emergency Medical Services ambulance was en route to Brandywine Hospital with a patient when an accident occurred on Chestnut Tree Road near Route 322 in West Nantmeal Township, said Patty Mains, a spokeswoman for Chester County's Department of Emergency Services.

No one was hurt, and another ambulance picked up the patient.

PennDot 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.

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

The Delaware River Port Authority reduced the speed limit on its four bridges to 35 m.p.h., and then to 25 on the Walt Whitman, Commodore Barry, and Betsy Ross.