Extreme Rainfall in Pennsylvania to Increase, Finds New Report

A new report suggests that the devastation from extreme rain events that claim lives, cause billions of dollars in damage, and pollute local waters, will become more severe in Pennsylvania if left unchecked, thanks to climate change.

The report “Extreme Precipitation in a Warming Climate,” by the science communication group Climate Central, predicts that on average, U.S. counties are likely to experience a 17 percent increase in extreme rainfall on the heaviest 1 percent of days. Much of the data in the report is from the U.S. Global Change Research Program’s Fifth National Climate Assessment.

For every 1°F of warming, the air can hold an extra 4 percent moisture, according to the report. 

The Chesapeake Bay Foundation (CBF) notes that one inch of rain falling on an acre of hardened surface, such as streets and parking lots, produces 27,000 gallons of runoff. Stormwater passing over impervious surfaces often picks up oil, grease, dirt, and nutrients (nitrogen and phosphorus) and flows into nearby waterways.

According to the state’s latest report on impaired waters, 3,828 miles of Pennsylvania’s rivers and streams do not meet water quality standards because of polluted urban stormwater runoff and storm sewers. It is the third leading source of impairment.

The report details how much more rainfall is likely to fall in each Pennsylvania county during severe storms if the climate warms by 3.6 °F (2 °C). That includes Bucks County, 23.9 percent; Philadelphia, 22.5 percent; Centre County, 30.0 percent; Allegheny County, 17.4 percent; Lancaster County, 16.9; and Luzerne County, 15.8.

The Climate Central report notes that as the climate warmed between 1958 and 2021, the heaviest storms now drop 60 percent more rain in the Northeast (including Pennsylvania, Maryland, the District of Columbia, New York, and West Virginia) and 37 percent in the Southeast (including Virginia).

This increases the chances of heavier downpours that contribute to flash floods like those that killed seven people in Southeastern Pennsylvania last summer.

A grandmother, mother, and her two children were swept away by rushing waters after nearly 7 inches of rain fell on Bucks County in 45 minutes last July. The mother and 2-year-old daughter perished and the search for the 9-month-old son was called off. The father, a 4-year-old son, and the grandmother survived. Four other people died in Bucks County floodwaters that day.

Climate Central reported that flood damage costs in Pennsylvania totaled roughly $2.8 billion in 2020 and that figure could go up by 8 percent by 2050 as global temperatures continue to rise.

Harry Campbell, CBF Science Policy and Advocacy Director for Pennsylvania, issued the following statement:

“The impacts of climate change go well beyond warmer summertime highs.

“The Susquehanna River is already one of the most flood prone in the nation, and coupled with Pennsylvania’s aging infrastructure in many towns, costly flooding events are likely to increase in the coming years.

“The intense precipitation is also likely to increase the amounts of polluted runoff flowing into our lakes, rivers, and streams.

“When rainfall lands on a hot surface, like a parking lot, the solar energy is transferred to the cooler rainfall as it makes its way to the nearest waterbody. This superheated stormwater runoff can be so hot that it can critically harm critters that rely on clean, cool water to survive, like brook trout and the eastern hellbender.

“Because hotter water holds less dissolved oxygen than cold water, many critters will find it harder to breathe. If it doesn’t kill them, it can render them susceptible pathogens and diseases. To make matters worse, some pollutants can become more toxic in warmer waters.

“There’s one thing nearly every homeowner, community, and business can do to mitigate these negative impacts. Planting and preserving native trees, shrubs, and other vegetation alongside streams, streets, and other sensitive areas helps slow down, spread out, and soak in precipitation, filter out pollutants, and cools runoff.” 

B.J. Small 90x110

B.J. Small

Pennsylvania Communications & Media Relations Manager, CBF

[email protected]

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