11 July 2021
Southwestern Pennsylvania has always been prone to flash floods but last week was exceptional with a flash flood every day, three days in a row.
At 3pm on Wednesday 7 Jul 2021 a heavy downpour in the Nine Mile Run watershed caused a flash flood recorded by Upstream Pittsburgh‘s stream cam (video below, blurry because it’s raining). The downpour was so localized to the East End that it did not register on Pittsburgh’s official weather gauges. Flood debris showed that if I’d been on the Nine Mile Run Trail the water would have been up to my ears! (photo at top taken at 40.4263341,-79.9068387).
On Thursday 8 July at 7pm a downpour over Pleasant Hills had devastating results as reported by CBS Pittsburgh.
And on Friday 9 July another localized thunderstorm let loose for half an hour in Squirrel Hill. I have no photos because I was driving down Braddock Avenue in the downpour, hoping the river on the road would not become a car-swallowing lake under the Parkway bridge. Fortunately the water ran off into Nine Mile Run. Another flash flood. I’m glad I was not on the trail.
As crazy as this is, it should not be a surprise. Pittsburgh is prone to flash floods, especially in Allegheny County as shown in the 35-year map of Flash Flood Reports from the National Weather Service.
We don’t need a particularly wet year for this to happen. Pittsburgh’s 2021 rainfall is actually 0.93 inches below normal as of today. The problem is that the rain falls all at once, especially in June and July.
Climate change is making the problem worse. A 2019 study found that extreme precipitation has increased 55% in the Northeastern US in my lifetime.
This trend will continue in southwestern PA through the 21st century. (Click here to see where frequent heavy downpours will increase in the U.S.)
Brace yourself, Pittsburgh, for a lot of flash floods in the future. Sometimes every day.
About Nine Mile Run per Upstream Pgh (formerly Nine Mile Run Watershed Association): “Nine Mile Run is a small stream that flows through Pittsburgh’s East End, mostly underground. The 7 square mile Nine Mile Run watershed is home to the largest urban stream restoration in the United States, completed by the U.S. Army Corps of Engineers in 2006.” Upstream Pgh got its start with this project and now works throughout the region on community-oriented stormwater management projects, large and small, plus much more. Click here for their website.
p.s. If you’re not from the area you might not realize that “Pgh” is an abbreviation for Pittsburgh. We’re the only Pittsburgh with an “h.”
(photo by Kate St. John, videos from Upstream Pittsburgh and CBS Pittsburgh, maps from NWS Pittsburgh an climate.gov; click on the captions to see the originals)