About 1,200 National Guardsmen have been deployed across the state - a third of them to Wilkes-Barre, which sits like a teacup, bounded by the Pocono Mountains, the Endless Mountains, and the Lehigh Valley.
The city issued a mandatory evacuation order for 8 p.m. - then moved it up to 4 as the Susquehanna River's rapid rise startled officials.
"The river was rising at an alarming rate," said Maryanne Petrilla, chair of the Luzerne County Board of Commissioners. "Two feet every half-hour."
Officials braced for flooding not seen since the deluge of Hurricane Agnes in 1972. Guardsmen went door-to-door to enforce the evacuation. Thursday morning, inmates from the Luzerne County jail were sandbagging the Pierce Street Bridge.
"The water's probably going to come up over the dikes," said National Guardsman Adam Rutkowski, stationed at an intersection with his unit, "and start flooding us out."
Thursday night, engineers were trying to shore up the gaskets of floodgates at the Market Street Bridge, pushing tons of dirt against the steel barriers. Meanwhile, people piled into shelters opened at schools, firehouses, and churches.
"We're opening more," Petrilla said.
At 3:30 p.m., 30 minutes before the deadline, the streets around the Black Diamond Railroad Bridge in south Wilkes-Barre were still and empty.
National Guardsmen patrolled the riverbank and the levee walls, chasing away curiosity-seekers who wanted photos of the rising Susquehanna.
A block from the river, Bob Ritter, a truck driver originally from Roxborough, and his wife, Cheryl, originally from Northeast Philadelphia, worked to quickly pack up their house.
Ritter pointed to the levee. If the river crossed the barrier, he'd have water up to his second floor. That's what happened in 1972.
The Ritters packed their four children and four grandchildren, including 11-day-old Evelyn, along with two turtles, a cat, and a German shepherd puppy.
"It's an adventure," Cheryl kept telling the kids.
"We know we can't save everything," Bob said. The family photos were wrapped and tucked into the attic.
As a teenager in the aftermath of the 1972 flood, Cheryl Ritter came to Wilkes-Barre with a church group and helped in the cleanup.
"She knows what we're going back to," Bob said.
"A house full of mud and more mud," Cheryl said.
By nightfall, water had reached the steps of the Luzerne County Courthouse, flooded the Hollenback Cemetery, and was nearing the county correctional facility, which holds 500 prisoners.
Meanwhile, in another city perched on the Susquehanna's banks - Harrisburg - Corbett was declaring a Level One emergency in Pennsylvania.
"We expect this crisis to continue through the weekend," the governor said, adding that almost every town along the Susquehanna was flooding.
At least five deaths were blamed on the rising water.
A Level One emergency is the highest of four categories, dictating that top personnel from state agencies meet to plan a response.
"It's essentially an 'all hands on deck,' " said Pennsylvania Emergency Management Agency Director Glenn Cannon.
Even Corbett and his wife, Susan, were forced to evacuate, leaving the governor's mansion, which overlooks the roiling Susquehanna. Sandbags were lugged into place around the residence, and all furniture, paintings, and carpets were moved to higher levels from the basement and first floor. So was a grand piano.
"It is bizarre," said the governor, now staying at Lt. Gov. Jim Cawley's residence in Fort Indiantown Gap. "It is empty."
Floodwaters had swept over some municipal sewerage plants, including one near Hershey, putting them out of commission and creating a public health emergency, the governor said.
"Flood water is toxic and polluted," he said. "If you don't have to be in the water, stay out of the water."
The northern part of the state has been the hardest hit, he said, but "all this rain water is coming down into Harrisburg, into Lancaster and York Counties, and on down."
In Harrisburg, the Susquehanna was expected to crest at 29 feet, just 31/2 feet below its record. All nonessential state employees at the Capitol complex were ordered to stay home Thursday and Friday.
Mayor Linda Thompson declared a state of emergency for Harrisburg, a city already in desperate financial straits.
In East Cocalico Township, Lancaster County, an 8-year-old boy was playing with some friends on a flooded street around 4 p.m. Thursday when he was swept off his feet and carried to a storm drain, County Coroner Stephen Diamantoni said Thursday night.
"His head became trapped in the storm drain," Diamantoni said. The force of the rushing water kept the boy's head submerged for 15 minutes, and he drowned, Diamantoni said.
In Wilkes-Barre, Stephen Urban, chairman of the Luzerne County Flood Protection Authority, said engineers were confident that the 15 miles of levees would hold.
The barriers were upgraded and raised from 36 feet - their height in 1972 - to 41 feet, part of a $250 million project executed in stages in 2004 and 2006, Urban said.
Jim Brozena, executive director of the agency, cautioned that the river was exerting enormous pressure on the levees. The barrier "has performed as designed at this point," he said. "We still have a long way to go in this battle."
The earliest evacuations began Wednesday night.
By late Thursday afternoon, a 583-cot shelter set up in the Solomon Plains Township elementary school was filling.
The county set up more than 20 shelters, and many were reaching capacity, said Solomon shelter manager Linda Urban.
John Rajchel, 84, and his wife, Marie, 79, of Miners Mills, settled onto cots.
"Take it as it comes is what I always say," John said.
In a joint letter Thursday, Pennsylvania's lawmakers in Washington urged President Obama to grant Corbett's request to declare flooded parts of the state a federal disaster area.
In Harrisburg and Wilkes-Barre, the forecast calls for more rain.
Contact staff writer Jeff Gammage at 215-854-2415, email@example.com,
or @JeffGammage on Twitter.
Inquirer staff writers Kia Gregory, Amy Worden, and Robert Moran contributed to this article.