First Alert: Timing and Totals for Heavy Rain & Possible Flooding

Rain washed through most of the region Sunday, and the NBC10 First Alert Weather Team says a First Alert will remain in effect through 9 p.m. for Delaware and South Jersey. NBC10 First Alert Weather meteorologist Steve Sosna has the timing and expected rain totals.

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The NBC10 First Alert Weather Team says its First Alert for heavy rain will continue in South Jersey and southern Delaware until 9 p.m. An earlier alert for the Philadelphia area and northern Delaware has expired. Rounds of repetitive heavy rain and scattered thunderstorms swept through the area, producing dangerous travel conditions due to the possibility of flooded roadways. Small streams and rivers may also rise above flood stage.
The heaviest rain fell across northern Delaware, the northern and western suburbs, Philadelphia, and south Jersey. Widespread rainfall amounts of 2-4” are likely with localized totals up to 5”. This will be enough to trigger flooding in poor drainage areas, urban areas, and along some streams and rivers. Meanwhile areas up to the north in the Lehigh Valley, Berks County, and the Poconos will see less rainfall as well as areas along the immediate coast at the Jersey Shore and Delaware Beaches. There still could be localized flooding in the Lehigh Valley and Poconos due to the frozen ground and runoff from the rain.
In addition to heavy rainfall and flooding Sunday into areas along and near the I-95 corridor, dense fog will be an issue. Fog could be locally dense through most of the day Sunday.
This is a massive storm system that has prompted flood watches from Louisiana to Massachusetts. The plume of rain late Saturday evening extended over 1,300 miles. Since there is a tropical connection to the rain, that’s why the NBC10 First Alert Weather Team is concerned about potential flooding. Rainfall rates may exceed 1.5” per hour in some of the heavier rain bands. Flash flood guidance suggests that in Bucks, Montgomery, Chester, and Philadelphia counties it would only take 1” of rain in 3 hours to produce flooding. Our forecast exceeds that amount, so we’re most concerned about these areas.
The heaviest rain ended in the 4 p.m hour in most of the Philadelphia area, the suburbs and northern Delaware. Some stream flooding has already been reported, including at the Wissahickon Creek, which flooded and closed part of W. Valley Green Road in Montgomery County. The First Alert is extended until 9 p.m. for extreme South Jersey and southern Delaware, where very heavy storms are approaching and may continue on and off for several hours.
Finally, there will be a HUGE temperature change from south to north on Sunday. Areas of South Jersey, Delaware, and Philadelphia have a good chance of getting well into the 60s. Areas along the immediate coast will be cooler due to the chilly Atlantic Ocean. Meanwhile, the northern & western suburbs, Berks County, the Lehigh Valley, and the Poconos will stay very chilly Sunday. It will feel like two different seasons Sunday depending on where you’re located.
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