The heaviest rain fell in a narrow corridor, but for thousands of commuters, nature couldn't have targeted a worse place.
As much as five inches of rain fell in the vicinity of the Schuylkill Expressway near the Blue Route interchange, sending a cascade of water and mud down rocky slopes onto the eastbound lanes and shutting the entire highway for nine hours Monday, forcing thousands of motorists to find other ways to get into the city.
With rains intensifying, shortly after midnight the National Weather Service warned of a "dangerous situation unfolding," and by 1 a.m., PennDot decided to close the expressway (I-76). It would not reopen until after the morning commute.
One of the eastbound lanes reopened about 10 a.m. after being closed between the Blue Route (I-476) and the Belmont exit. Crews worked through the morning to clear the roadway of mud and debris, and shortly before 2 p.m. PennDot reported both lanes were open.
Other highways that experienced flooding included I-95 in Northeast Philadelphia and Route 38 at the railroad overpass in Cherry Hill.
The downpours were focused along a five- to 10-mile-wide corridor extending from the Main Line through Philadelphia to South Jersey, said Alex Staarmann, a National Weather Service meteorologist in Mount Holly.
"We were in the bull's-eye zone," said Max Vido, a meteorologist with AccuWeather Inc.
Several waterways, including the East Branch of Brandywine Creek, near Downingtown; the South Branch of Pennsauken Creek, at Cherry Hill; and the Frankford Creek — which hit a near-record crest of 13.6 feet — spilled out of their banks during the morning.
Among the areas with the heaviest rains was the Schuylkill Expressway near the I-476 intersection, with close to 4.5 inches reported near Conshohocken, according to the weather service. Rainwater cascaded down the rocky slopes, and the eastbound lanes became too dangerous to leave open by about 1 a.m., said PennDot spokesman Brad Rudolph.
With the water came mass quantities of mud and debris that blocked a major drainage pipe, he said.
Rather than try to find the clogged pipe and clear it, PennDot pumped water directly off the highway into the westbound lanes and down the nearby hill.
"Once the water receded, there's still a lot of mud and debris," he said before crews cleared the road.
Rudolph said the blocked pipe needs to be cleared before another rainfall.
"Basically, it was just a tremendous amount of rain last night," Rudolph said. "Now we have to go in and flush the pipe."
This was the first time in 2018 that the highway has had to be shut down due to flooding. In early March, the westbound lanes were closed after a tree fell on a SEPTA bus during a nor'easter.
Rudolph said PennDot has tried to mitigate the runoff problems by planting vegetation on the rocky slopes, but in this case the rain was overpowering.
"It's geography and geology," he said.
Areas outside the corridor of heaviest rains, including Philadelphia International, Airport, received lesser amounts.
Staarmann said that a stalled front set off the deluges.
Vido said the rain got an additional lift from a system higher up in the atmosphere and strong jet-stream winds. "This is a very tricky setup and pattern. They are hard to model."
No serious injuries were reported due to the flooding, though police had to deal with a number of motor vehicle accidents.
The drenching rain also uprooted some trees, including one on Kelly Drive along Boathouse Row.
Looking ahead, it will be sunny Tuesday and, after rainfall on seven consecutive weekends, Saturday looks dry, Vido said. But he is making no guarantees about Sunday.
"It's still early," he said.