In addition to the warning, a flood watch was posted until daybreak Thursday for areas along the river in Philadelphia and portions of Bucks, Burlington, and Mercer Counties.
And the threat was likely to persist for areas downstream from the jam as ice continued to break, giving the backed-up water a chance to surge through, said Mitchell Gaines at the weather service office in Mount Holly. "There's potential," he said, "for more water than usual coming down."
The weather service warned people to keep clear of the river. "The ice jam is an extremely unstable situation, as the ice could break without a moment's notice," it said.
On Wednesday, flooding closed River Road between Lower Makefield Township and Morrisville, Bucks County, and it was not expected to reopen until midday Thursday.
Temperatures were forecast to crest just above freezing in the afternoon Thursday and to rise into the 40s on Friday.
After a record snowstorm and a run of cold temperatures followed by a rapid warm-up in January 1996, suddenly liberated ice jams set off major flooding along the Delaware and the Susquehanna River, leading to a presidential disaster declaration for parts of the state.
Melting snow is not an issue this time, but frigid temperatures have locked some rivers and streams in ice.
On Tuesday, the Philadelphia region shivered through its coldest day in 20 years, one during which the mercury fell to 4 degrees, with chill factors up to minus-20.
The forecast is for a January thaw to continue, with daytime temperatures rising into the 50s over the weekend.