Flash Flooding and Flood in Pennsylvania, August 26 to 27, 1999

During the evening of the 26th thunderstorms with torrential downpours and frequent lightning caused considerable urban and poor drainage flooding as well as flooding of some of the smaller creeks and streams and numerous power outages. The combination of frequent lightning and flooding from the heavy rain caused about 65,000 PECO Energy customers to lose power during the evening of the 26th in Southeast Pennsylvania. Hardest hit were Chester and Delaware Counties. Seven thousand customers were still without power the morning of the 27th. With respect to the flooding, the hardest hit areas were the southeast half of Chester County, the northeast tip of Delaware County and adjacent sections of Montgomery County. Storm totals in the hard hit areas averaged between 2.0 and 3.5 inches, most of which fell in two hours between 9 p.m. and 11 p.m. EDT. Heavy rain before dawn on the 26th left the region vulnerable to any additional heavy rain. In Radnor Township (Delaware County), the storm total was 4.85 inches. In Lafayette Hills (Montgomery County) 3 inches of rain fell within an hour from 945 p.m. to 1045 p.m. EDT. In Chester County, flooding of underground cables knocked out power to much of Coatesville. In Montgomery County, flooding of another underground cable forced the closure of the county courthouse on the 27th. In East Norriton Township, the rain flooded a swimming pool store and caused crystallized chlorine to liquify and leak out. Police in the area were rescuing persons in trapped vehicles and the build up of chlorine fumes forced them to close nearby roads until the morning of the 27th. In Delaware County the combination of the torrential rain, small stream flooding and debris clogged storm drains wreaked havoc in Newtown and Radnor Townships. Many vehicles were floating in flood waters or completely submerged. Several driveways were washed out and many basements were also flooded. In Philadelphia, the Frankford Creek at Castor Avenue crested at 9.2 feet before midnight on the 26th. Flood stage is estimated at 7.0 feet. No damage though was reported. Other storm totals included 2.77 inches in King of Prussia (Montgomery County), 2.39 inches in Valley Forge (Chester/Montgomery County border), 1.95 inches in West Chester (Chester County), 1.94 inches in Limerick (Montgomery County), 1.63 inches at the Franklin Institute within Philadelphia and 1.30 inches in Chadds Ford (Delaware County).