Tropical Storm Isaias could soak the Radnor and Haverford area with 1 to 2 inches of rain starting Sunday night, Oct. 11, as the storm's remnants push into southeastern Pennsylvania.
The National Weather Service office in Mount Holly, which covers Delaware County, said Wednesday, Oct. 7, that the remnants of Isaias will approach the region from the south and west on Sunday. Forecasters placed most of the Delaware Valley in a Marginal Risk for excessive rainfall, the lowest of four threat levels, according to the NWS forecast discussion.
The chance of rain climbs to 80% Sunday night and could continue into Monday, Oct. 12.
The storm formed Wednesday in the Gulf of Mexico and is forecast to strengthen into the first hurricane of the 2026 Atlantic season, the Associated Press reported. The National Hurricane Center (NHC) expects Isaias to make landfall along the northern Gulf Coast as a Category 2 hurricane late Friday or early Saturday before weakening rapidly inland.
Winds are not the main concern here.
NWS meteorologist Paul Fitzsimmons told the Delaware News Journal that the storm's intensity will drop quickly after landfall. By the time remnants reach the Philadelphia region, forecasters expect rain, not damaging winds or tornadoes.
"As long as the system is continuing to move and not stall out over us, then I think we'll be pretty okay," NWS State College meteorologist John Bowen told PennLive.
Bowen said the heaviest rain in Pennsylvania is expected along the Pennsylvania Turnpike corridor and a path along I-80 toward Williamsport and Scranton. NWS Mount Holly forecasters said precipitable water values could approach 2 inches, meaning some periods of heavy rainfall are possible across the Delaware Valley.
Wet ground raises flood risk
The rain arrives after a wet stretch in Delaware County. September's rainfall at Philadelphia International Airport reached 4.51 inches before a late-month nor'easter added more, the Delaware County Daily Times reported. That nor'easter knocked out power to nearly 2,000 PECO customers in the county on Sept. 26.
The NWS Mount Holly coverage area, which includes 10 southeastern Pennsylvania counties, had already issued a record 156 flash flood warnings in 2026 as of late September. The annual average is 63, and the previous record was 113 in 2018, PhillyVoice reported.
Delaware County has taken steps to improve flood monitoring. The county acquired 70 flood gauges in August using $332,119 in state and federal grants, according to a county announcement, though installation was still pending as of that announcement. The county's Planning Department has identified 230 flood problem areas countywide, with 130 classified as high priority.
What to watch
Forecasters cautioned that the storm's exact track and timing remain uncertain. NWS Mount Holly noted timing discrepancies among weather models but said there is strong agreement that the remnants will bring impacts Sunday into early Monday.
No statements from Radnor Township, Haverford Township or Delaware County emergency management about specific preparations for this storm were available as of Wednesday.
Residents can monitor updated forecasts at weather.gov by searching their ZIP code. The NWS Mount Holly office issues watches and warnings for Delaware County.
