Philly is one of top cities most at risk of extreme rains

Storms galloped at times like wild stallions through the region starting last Friday, stomping on some towns while barely muddying their hooves on others.

But those whose cars plunged into flooded roads or whose possessions bobbed in basement water during the rash of no-named storms might not be surprised that experts say extreme rains are increasing in the Philadelphia region.

Upper Darby got drenched with nearly five inches of rain through Wednesday. Northeast Philly, Bala Cynwyd, and Chadds Ford all got well over four inches — the amount normally received for the entire month of June.

More heavy rains are possible Sunday and Sunday night, forecasters say.

The federal government has been underestimating the potential for extreme precipitation in many areas of the country, and Philadelphia is one of the top cities where that disconnect is greatest, according to a report released this week by First Street Foundation, a nonprofit that studies threats related to climate change. Though there’s no precise definition, extreme precipitation refers to intense, localized heavy rainfall within a short time.

The report looked at the widely used Atlas 14 flood precipitation frequency estimates by the federal National Oceanic and Atmospheric Administration (NOAA). It concluded that they fall short because they are based on an outdated method that doesn’t account for climate change.

That’s no small thing: While little known to outsiders, Atlas 14 is a gold standard used by government officials, civil engineers, and others to build infrastructure that can withstand flooding. Currently, state and local officials around the U.S. are planning how to use billions of dollars from the federal Infrastructure Investment and Jobs Act, passed in 2021 during the pandemic.

Consider that the New Jersey Department of Environmental Protection stated in a scientific report on climate change that extreme precipitation events have increased in the Northeast by 71% over the last 50 years, which is at a faster rate over the last three decades than any other region in the U.S.

Consider that the New Jersey Department of Environmental Protection (DEP) stated in a scientific report on climate change that id which is at a faster rate over the last three decades than anywhere else in the United States.

And a Pennsylvania Department of Environmental Protection assessment of climate threats states: “Increasing average temperatures and heavy precipitation and inland flooding emerged as the two highest-risk hazards by midcentury.”

To assist planners, First Street Foundation created its own model that it says captures climate-driven changes contributing to heavy rainfall and lays out flood risk across the U.S. The precipitation model used in the report has been published in the Journal of Hydrology.

The authors found significant differences across the country in NOAA Atlas precipitation estimates compared with actual rainfall risk in the area. That means current predictions of flood risk on people, communities, and property are too low, the authors say.

Philadelphia, according to First Street data, is one of the major cities at risk of more frequent severe flooding compared with NOAA Atlas 14 estimates for a “1-in-100-year flood.”

Think of flood estimates this way: A 10-year, 24-hour storm has a 10% chance of occurring in any single year. A 50-year storm has a 2% chance, and a 100-year storm, a 1% chance. Storm water management systems are designed to withstand storms based on these chances.

The current Atlas 14 model estimates that Philly could get about three inches of rain an hour during a 100-year flood. First Street’s adjusted estimates say nearly five inches are possible.

That means Philly has a chance of getting what NOAA calls a 1-in-100-year flood every 16 years. But those are just probabilities. In reality, 100-year floods can occur more than once in a year, or every few years.

“Our current understanding of precipitation risk is underestimated,” said Jeremy Porter, a Columbia University professor and head of climate implications at First Street Foundation. “Infrastructure and personal property are at a higher risk than design standards have been built to protect them from.”

The study cites a project in East Brunswick, N.J., using $86 million from the federal infrastructure money that’s designed to alleviate roadway flooding. But it’s based on Atlas 14 modeling to withstand a 15-year event. First Street estimates the same amount of rainfall, however, is expected to occur every four years.

John Moore, a NOAA meteorologist and spokesperson, said the agency doesn’t comment on outside research but called Atlas 14 estimates “the current authoritative source for precipitation frequency information and the national standard referenced in engineering design manuals and regulations.” It’s used for city and regional planning and the design of civil engineering and transportation infrastructure nationwide, he said.

The estimates are pulled from NOAA’s “scientific rigor and extensive observational network,” he said. But he noted NOAA is creating a new model, Atlas 15, that will take climate change into account. It won’t be ready until 2027, but NOAA will release a pilot version in 2024 for public comment.

Locally, Chester County is estimated to experience what’s now considered a 100-year storm nearly every decade.

Robert Kopp, a climate scientist at Rutgers University’s Department of Earth and Planetary Sciences, said shortcomings with Atlas 14 are well-known. He said First Street Foundation’s new data set is a “reasonable approximation as to what we’ll see when NOAA releases their new Atlas.”

Rising precipitation is a key signal of climate change, along with an increase in temperature, Kopp said.

“Rainfall has become a lot more intense,” he said.

“We’ve seen a lot of rain-driven flooding in our region,” Kopp said. “And if it rains intensely, rivers flood; groundwater floods. If you don’t have a storm water system to carry that away, you get the things that happened in Hurricane Ida, an iconic example.”

Indeed, the remnants of Ida battered the Philadelphia region, flooding the Schuylkill Expressway and spawning tornadoes across suburban Pennsylvania and New Jersey. The rainfall totals from Ida on Sept. 2, 2021, were eye-opening: more than nine inches in Coatesville, eight in Perkasie, and eight in Phoenixville.

In Central Jersey, Mercer and Hunterdon Counties saw nine or more inches.

Scientists at the time said the storm was supercharged by climate change. Ida was fueled by warmer-than-normal temperatures in the Gulf of Mexico. When it arrived in Philly, the weather in August had been much hotter and wetter than normal: 2.3 degrees above normal and 6.18 inches of rain — 144% over the average for the last 20 years.

“Climate change makes everything more intense,” Kopp said.