Flash Flood — Philadelphia, Pennsylvania

2011-09-08 · near (phl)philadelphia in, Philadelphia, Pennsylvania

Event narrative

A band of showers and thunderstorms with heavy rain caused flash flooding of smaller streams as well as poor drainage flooding in Philadelphia. It also caused the drowning death of a 27-year-old woman who was caught in flood waters in the East Germantown section of the city. She was discovered in the rear of her Sports Utility Vehicle at the intersection of Musgrave and Haines Streets on the morning of the 8th. The Wingohocking Creek, which is a creek that was buried underground during the 19th century had its waters back out of sewer grates and manhole covers in that section of the city. Godfrey Avenue and Wister Street also flooded in East Germantown. The wall of a church also collapsed. Most gaged creeks within Philadelphia flooded as well as the Schuylkill River.

Wider weather episode

The remnants of Tropical Storm Lee that interacted with a stalled frontal boundary produced several days with periods of heavy rain across Eastern Pennsylvania from the 5th into the 8th. Event precipitation totals averaged 4 to 9 inches, except in Berks County where 6 to 12 inches fell. One drowning occurred in Philadelphia. The heavy rain caused widespread poor drainage flooding as well as moderate flooding along the Delaware and Schuylkill Basins and minor to moderate flooding along the Lehigh River Basin. Crests in the upper Schuylkill, Lehigh and Delaware River Basins were higher than what occurred with Irene the previous month and the highest crests since late June of 2006 in most instances. The crests along the Delaware River were 2 to 4 feet higher than what occurred during Irene. Some of the smaller streams in southeastern Pennsylvania reached major flooding levels, but most of those crests were lower than what occurred with Irene. In addition to the freshwater flooding, moderate tidal flooding occurred along the lower Delaware River and its tributaries.

In the most recent available flooding assessment for Eastern Pennsylvania, there were 22 homes and businesses that were destroyed, 201 that suffered major damage, 672 that had minor damage and 1217 others that were affected. The preliminary damage estimate for the state of Pennsylvania was five hundred million dollars. Please look for possible updates in future monthly issues of stormdata.

The combination of fresh water runoff and a weak southerly flow produced moderate tidal flooding during the evening high tide cycles from the 7th through the 9th and minor tidal flooding with all of the other high tide cycles from the evening of the 6th through overnight on the 11th. The highest tides occurred during the evening high tide cycle on the 8th in Philadelphia and Delaware Counties and the evening high tide cycle on the 9th in Bucks County. The evening high tide on the 8th in Philadelphia reached 9.41 feet above mean lower low water. Moderate tidal flooding starts at 9.2 feet above mean lower low water. The high tide in Newbold Island (Bucks County) reached 12.11 feet above mean lower low water. There is no current categorical tide gage information for this site.

Tropical Storm Lee made landfall in Louisiana during the morning of the 4th and moved slowly northeast. An approaching cold front from the Mississippi Valley reached Lee by the morning of the 5th and transformed it to an extratropical system on the 5th in Mississippi. The same cold front acted as a conduit for Lee's moisture farther to the northeast. Showers and thunderstorms from that approaching cold front arrived during the evening and overnight on the 5th and set off the first round of flash flooding rains in Northeast Pennsylvania. The front then stalled just south of Delmarva on the 6th. A relative lull in the rain then occurred during the morning of the 6th. As the front started to back north as a warm front later in the day on the 6th, more bands of heavier rain returned that afternoon and particularly that evening. A blocking weather pattern then moved the frontal boundary little from the morning of the 7th into the morning of the 8th across southeast Pennsylvania. A couple of bands of flash flood producing rains started to form and move east into Eastern Pennsylvania during the late afternoon and evening of the 7th and continued through the night. The frontal boundary started to drift offshore later in the day on the 8th and most of the precipitation associated with Lee was done.

View location on OpenStreetMap → (39.8800, -75.2500)

Source: NOAA Storm Events Database, event_id 349590. Narrative written by the NWS forecast office that issued the report.