A sweeping new flood analysis has revealed that large portions of the Washington region — from heavily traveled roads to rail lines and bus stops — face far greater risk from flooding than federal data previously suggested.

The findings come from the National Capital Region Transportation Planning Board, operating under the Metropolitan Washington Council of Governments, which set out to create a more complete picture of the region’s vulnerability. To do so, it produced two separate flood maps — one using FEMA’s existing data, and another relying on an independent model from U.K.-based firm Fathom.

The differences between the two are striking. FEMA’s map largely identifies flood zones near rivers and waterways. Fathom’s version, however, goes further — showing where flash floods, extreme rain events, and overburdened storm drains could cause high water, even far from any shoreline.

According to the board’s analysis, those broader factors place nearly 40 percent of the region’s road miles in flood risk zones under the Fathom model, compared with just 14 percent in FEMA’s. For rail lines, the disparity is even greater: 64 percent at risk compared with 39 percent. Bus stops are also more exposed — 11 percent in the Fathom map versus 4 percent in FEMA’s.

STORY HIGHLIGHTS — READ

• Roads at Risk: 40% of roadway miles flagged in Fathom model vs. 14% in FEMA maps.

• Rail Lines: 64% at risk in Fathom model vs. 39% in FEMA data.

• Bus Stops: 11% vulnerable in Fathom model vs. 4% in FEMA analysis.

• Beyond Waterways: Risk zones include inland urban areas, not just riverside locations.

• Example: 600 block of Rhode Island Ave NE — absent in FEMA risk zones but site of fatal 2023 flash flood.

• Weather Shift: Extreme rainfall events are increasing in severity and frequency.

• Future Outlook: By 2050, an additional 3% of roads in the region projected to be at flood risk.

For transportation resiliency planner Katherine Rainone, the expanded mapping offers more than just new statistics.

“This is one of the best ways we can, as a region, prepare for the impacts and prioritize which adaptation measures will get us the most bang for our buck,” Rainone said. “And then putting this information in the hands of public safety and emergency response professionals can help with effective response when there is an event that might occur.”

The difference in perspective is already evident on the ground. The 600 block of Rhode Island Avenue NE — the site of a deadly flash flood in August 2023 that killed 10 dogs in a day care center — does not appear on FEMA’s map. Yet on the Fathom-informed map, it is marked high-risk.

Similar gaps appeared in Silver Spring. On July 19 this year, a slow-moving storm dumped up to six inches of rain in less than two hours. Water surged along East-West Highway at Sundale Drive, submerging a car. The location did not appear on FEMA’s risk map, but it did in Fathom’s.

“I think it’s a good idea to use this mapping … to better understand what the long-term impacts could potentially be,” Rainone said. “What vulnerabilities are there that we could take steps to provide solutions for in the future.”

The summer’s weather has underscored the urgency. This year — the most humid on record so far — has brought repeated downpours. On July 14, torrential rain forced road closures from Vienna to Laurel, where Route 1 saw water three feet deep. Days later, a stream gauge on Sligo Creek in Silver Spring recorded a 10-foot rise in just 30 minutes.

Captain Joe Bell, who leads the Montgomery County Fire and Rescue Service’s swift water rescue team, said crews know some flood-prone areas well, but are increasingly encountering high water in places where they have never seen it before.

“I couldn’t think of any time where we’ve seen that much water in kind of that sort of urban environment there,” Bell said of the July flooding in Potomac Village.

The changes are linked to a broader climate shift. Over the past 150 years, D.C.’s average temperature has climbed about six degrees, increasing evaporation, raising humidity, and giving storms more moisture to draw from. Extreme rainfall events are now more common.

In 2018, the city saw its wettest year ever recorded — 66.28 inches of rain — surpassing the previous record by five inches. That year also brought 24 days with at least one inch of rain, a record at the time. In 2020, the city recorded seven days with two inches or more, another first.

Such downpours have fueled catastrophic flash floods, including those in Ellicott City, Maryland, in 2016 and 2018. Montgomery County has responded by installing flood gates at vulnerable streets and sensors in creeks to send alerts when water rises to dangerous levels.

Projections suggest that heavier rains are likely to become the norm. By 2100, the Mid-Atlantic Regional Integrated Sciences and Assessments team expects a 33 percent increase in days each year with at least one inch of rain, and a 53 percent increase in days with two inches or more — even under low-emission scenarios.

Using the Fathom model, the Transportation Planning Board projects that by 2050, an additional 3 percent of roadway miles in the D.C. region will be at risk of flooding. Virginia is expected to have the largest share of medium- and high-risk roads.

The expanded flood mapping offers a clearer, and in many cases more sobering, view of the Washington region’s vulnerabilities. By revealing risks that extend beyond riverbanks into dense urban areas, the Fathom-informed data gives local officials, emergency responders, and planners a stronger foundation for action. As extreme rainfall grows more frequent and intense, the challenge will be turning this sharper understanding into real-world protections — before the next flash flood turns another “unlikely” location into the center of a disaster.

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