Three people died and dozens were injured Sunday - in big multicar crashes and countless smaller accidents - as rain splashing onto frozen asphalt coated highways with a thin, dangerous layer of ice.
One wreck on Interstate 76 involved 60 cars and trucks. Another, on I-476 in Marple Township, smashed up a dozen vehicles after a tractor-trailer jackknifed.
Hospitals across the region were busy treating people injured in wrecks - and pedestrians who broke bones when they fell on icy sidewalks.
The so-called flash freeze shut the four major bridges that link Philadelphia to South Jersey, closed interstate highways, threw trains and airplanes off schedule - and left people shaking their heads about just what the heck had hit them.
"This isn't like anything I've ever driven in," said Kaitlyn Maier of Philadelphia, whose trip to a niece's baptism ended behind the pileup that closed I-76. "I was going less than 10 miles per hour, but I had no control."
As temperatures rose slightly by midafternoon, conditions had eased, traffic was moving, and the bridges reopened. The early-evening temperature of 40 degrees seemed positively balmy. But as night fell, the weather threatened anew.
The National Weather Service warned that water on roads could refreeze, with temperatures expected to drop into the upper 20s to 30 overnight. Icy conditions were possible overnight and Monday morning.
A flood warning that included the Philadelphia region remained in effect Sunday night.
Jim Bunker of the National Weather Service in Mount Holly explained Sunday's debacle: A warm layer of rain fell but turned to ice when it met already frozen surfaces and cold air trapped at ground level.
In city, suburb, and hamlet, pedestrians twirled as they slid across sidewalks, and cars that seemed under control banged into curbs and went off roads.
In Radnor, an SUV skated off Matsonford Road, skidded across a yard, and stopped on the rocky lip of a pond outside a home. The driver, a woman in her late 30s, was able to walk to an ambulance.
"Everybody should stay home," said tow-truck operator Charles Stead of Tommy's Towing in Wayne, one of three operators trying to tug the vehicle from frozen muck.
At 6 a.m., he said, his company had six trucks responding to more than 30 accident calls.
Emergency dispatchers received hundreds of calls for ice-related crashes - including 100 in Bucks County and more than 75 in Montgomery County. The ice caused numerous accidents on Interstates 76, 95, and 476, Roosevelt Boulevard, and a multitude of other roads.
State police identified the two drivers killed on the Blue Route as Thomas Michael Brennan of Lansdale and Jason Edward Anderson of Dover, Del. A third driver, unidentified, was hospitalized with head trauma.
Police said cars driven by Brennan and Anderson slammed under the jackknifed tractor-trailer; both were pronounced dead at the scene. That crash caused a pileup of 11 more vehicles, police said, and left four people with non-life-threatening injuries.
A pileup on the Schuylkill Expressway killed a Philadelphia man and injured 30 others, 11 of whom were taken to Bryn Mawr Hospital. Police said Eric Alan Blau, 31, got out of his disabled Mazda Protege for unknown reasons, and was killed when another car struck him.
All the injured at Bryn Mawr Hospital were expected to be sent home Sunday, said hospital spokeswoman Bridget Therriault. Five patients arrived after suffering injuries by falling on the ice.
Thomas Jefferson University Hospital treated a few slip-and-fall injuries - a low number, said spokeswoman Gail Benner, because the weekend allowed people to stay home instead of fighting their way into work.
There were similar injuries from cars and sidewalks in South Jersey.
Flights at Philadelphia International Airport were delayed throughout the day - by ice in the morning and low cloud cover later on.
The suddenness of the freeze seemed to surprise everyone.
Nicole Rohrbach, 28, left her Pottstown home early Sunday morning to worship at Grace Church in University City, where she plays violin and harp during services.
By 7:30 a.m., she was stopped on I-76 East. Ambulances and tow trucks rushed by, but no other vehicle moved for three hours. She finally made it to church - with 15 minutes left in the service.
Some people never ventured out on Sunday, deciding it was too risky.
"They said, 'Stay in' - and I did," said Penny Wolk, 52, of Ardmore, who held off on running to the local Wawa until roads improved.
Others went out and regretted it.
Karen Lynn Haver left her Reading home at 6:30 a.m. to drive her husband, Steve, to the airport. A half-hour later, they were stuck on I-76 East.
They kept the heat running to stay warm, used social media to stay aware of traffic news, and texted with friends who were stuck on I-476.
"It was boring," Haver said.
By the time they got off the highway, her husband had missed his flight, scheduled to take him to a vacation with childhood friends in the Cayman Islands. They drove to the White Dog Cafe in Wayne, ate brunch, and waited for the ice to melt.
They planned to try to reach the airport - and a rescheduled flight - on Monday.
Meralys Hernandez faced a similar ordeal.
The 16-year-old was traveling with her family from Lancaster County back to their home in Gloucester City when their car began sliding across Route 202. They managed to pull into a parking lot to wait out the storm.
"We didn't know about the weather when we left," Hernandez said.
The Benjamin Franklin, Commodore Barry, Betsy Ross, and Walt Whitman Bridges all reopened Sunday afternoon.
The biggest crash, on I-76, began at 6:40 a.m., involving 60 cars and trucks and blocking traffic in both directions between Gulph Mills and the Blue Route for about seven hours, police said. A second accident near mile marker 337 involved seven vehicles.
Around the same time, the tractor-trailer jackknifed on the Blue Route near West Chester Pike in Broomall, Delaware County.
Interstate 95 also was closed for a time but reopened on Sunday.
Gene Blaum, a spokesman for the Pennsylvania Department of Transportation, said rain that freezes on pavement "is the most difficult weather event to deal with. We would prefer to have six to eight inches of snow."
The agency sent 146 trucks to spread salt on roads throughout the five-county region, beginning about 5 a.m.
"We will keep them out until there is no longer a threat of any further ice conditions," Blaum said Sunday.
From morning to midday, roads were treacherous, including along a hilly stretch of Route 309 south of Quakertown. Blaum said it was impossible to uniformly eliminate ice on the 11,000 miles of roads PennDot covers in the region.
Maier, the Philadelphia driver who with her boyfriend came upon the I-76 accident, said she was shocked by how quickly the weather deteriorated.
"Ten minutes before I was asking him, 'What is this? Rain, or what?' " she said. Maier said emergency crews told her and other drivers to turn around on the highway and drive to the next exit.
The couple decided to stop at a diner and wait until the roads were clear.
This article contains information from the Associated Press.