For the first time since — well, Tuesday — the entire Philadelphia region is under a flash flood watch for yet another sequence of downpours, with a potential for 2 to 4 inches of rain in and around the city.
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Strong storms are possible Monday, said Sarah Johnson, warning coordination meteorologist for the National Weather Service office in Mounty Holly, “but the bigger threat we are concenred about is heavy rain and flash flooding.“
While the weather service’s flood watch was in effect from 8 p.m. Sunday until 11 p.m. Monday, she cautioned that with computer models having their differences, the forecast had elements of “uncertainty.” (And when was the last time that happened.)
Once again the atmosphere likely will be playing a version of rain roulette across the region, as it did on Tuesday, said Marc Chenard, senior branch forecaster at the federal Weather Prediction Center, in College Park, Md. Waterfall-like downpours turned the Delaware River into a shade of flowing mocha north of Philadelphia, while less than an inch fell upon the city.
Regardless of how it all plays out this time around, “It’s definitely going to affect the morning commute” Monday, said Bill Deger, senior meteorologist with AccuWeather Inc., with rainfall rates of 1 to 2 inches an hour possible. Even it’s not raining, he said, significant road flooding is possible.
He said it’s likely that Philly and the surrounding area might be in the 2-to-4-inch bull’s-eye zone.
When is likely timing for rain in the Philly region?
“This is another setup where we have very moist air from the South,” said Deger, and the first set of showers was expected to mine those plumes of moisture late Sunday afternoon or in the early evening hours.
More rains were likely Monday morning, and again later in the afternoon with the approach of a cold front.
That front could fire up strong thunderstorms, said Chenard, and it might take its good old time moving through the area.
He added that as so often happens, the downpours “won’t be all that widespread, but where they do happen you usually get a flash flood.”
The rains likely will shut off Monday night; however, the workweek is looking quite unsettled with shower chances every day.
Will this end the drought in the Philadelphia area?
While the drought emergencies continue in New Jersey and Chester County, conditions have improved measurably in areas around the city, which was no longer in “severe” or “moderate” drought, in the latest interagency U.S. Drought Monitor update.
Still, even in Philly conditions were listed as “abnormally dry” in the advisory issued Thursday, despite a July that was officially the wettest month in Philadelphia since March 2024.
Philadelphia’s total, 7.03 inches, was 2.65 inches above normal. Over 8 inches was measured in Wilmington. However, testifying to the randomness of summer rains, precipitation totals were significantly below normal in Atlantic City and Allentown, the weather service reported.
And while downpours can trigger rapid rises on the local streams such as the Neshaminy Creek, in Bucks County, and the Frankford and Pennypack Creeks in Philly, they are less of a threat to the Delaware and Schuylkill Rivers, where water levels have remained relatively low.
Those levels are the result of the longer-term dryness: July broke a 16-month streak of below-normal rainfall in Philadelphia.
But Johnson said the region’s wetness trend has been showing some signs of staying power..
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Said Deger, “It’s been a long time coming.”