The government's Storm Prediction Center in Oklahoma sees a "moderate risk" for "severe" thunderstorms, with gusts of at least 58 m.p.h.
The thunderstorms are due to get started right after daybreak and continue into the evening, with the heaviest rains late in the morning and later in the afternoon.
The weather service says the heftiest rain totals should fall north of the region.
That might be a welcome development for the people who have to drain the greens at Merion Golf Club, site of the U.S. Open. But downpours near the Delaware River headwaters in southeastern New York would increase the chances of flooding along the lower Delaware.
Also, the saturated ground will tend to promote rapid runoff.
The region has been soaked by about six inches of rain in the last week, but several factors have worked against major flooding.
For one thing, the rains haven't been as heavy in the headwaters of the Delaware and Schuylkill.
For another, the previous four months were relatively dry. More rain has fallen in Philadelphia since Friday than fell during the entire March 18-May 31 period.
The region, however, did get enough rain, and enough cool weather, to allow the ever-thirsty foliage to thrive this spring.
"This time of year, the vegetation is very good at sucking this stuff up," said Gary Szatkowski, meteorologist in charge of the Mount Holly office.
But the ground and the streams could be reaching their limits, he said.
"Even Mother Nature can drink only so much from that straw."
In its latest analysis, the weather service says the flood potential is particularly ripe in Philadelphia and the neighboring Pennsylvania counties.
It estimates that a mere 1.4 to 1.9 inches of rain in a 12-hour period would be enough to set off significant stream flooding.
Montgomery County is alerting residents via Facebook and Twitter, said county emergency-management spokesman John Corcoran. "It could be bad," he said.
Ed Truitt, Delaware County's veteran emergency director, said the county was ready. "Unfortunately, we've had a lot of practice," he said.
"I don't respond to Chicken Little. You respond to what is."
Contact Anthony R. Wood at 610-313-8210 or email@example.com.