Washington Post and WAMU call on Jeffrey Halverson to explain causes of devastating Ellicott City flood

The devastating flash flood that hit Ellicott City, Maryland, last Saturday night was so severe that meteorologists called it a once-in-a-thousand-years event. What conditions colluded to transform a quaint main street into a torrent of water in a matter of minutes? The Washington Post and WAMU radio both turned to Jeffrey Halverson, professor of geography and environmental systems, to explain.

Air currents from across the region converged over Ellicott City “in a manner that would literally squeeze moisture out of the atmosphere,” wrote Halverson in the Post. The extremely high humidity meant there was plenty to squeeze out. But what made it even worse was the large amount of buoyant energy feeding updraft currents. The more energy, “the more water vapor is lofted into the clouds and processed as rain,” says Halverson.

On top of that, a process called “back-building convection” resulted in several storm cells moving through the area, creating a much longer-lasting event than the traditional summer thunderstorm.

Perhaps the scariest part, though, is that even the most sophisticated computer models cannot predict the exact location of these events. “It becomes a ‘now-casting’ exercise,” says Halverson, “and there is almost never any lead time.”

But it’s not only the meteorological conditions that are to blame for the devastation Saturday night. “The flood was as much about the nature of the underlying land surface as it was about the large amount of rain falling from the sky,” Halverson told WAMU. The narrow streets, large amount of pavement, and valley geography of Old Ellicott City make it especially vulnerable to a flash flood.

“Ellicott City happened to be ground zero for a process that had very laser-like focus,” Halverson explained. “Humble rain, often gentle, is life-sustaining, nourishing, and thus benign 99 percent of the time,” he says. “But what we take for granted sometimes quickly turns deadly.”

UMBC President Freeman Hrabowski and Provost Philip Rous reached out to the UMBC community following the storm, expressing condolences to the families of two individuals who lost their lives, one of whom was an employee at University of Baltimore. Pres. Hrabowski also called on campus experts, like Halverson, to “share their knowledge and resources to help address issues to reduce future flood risk in the area.”

Recovery is a major, long-term process, but one that the area’s residents are already supporting through donations and volunteer service. As Pres. Hrabowski shared, “Ellicott City is a part of us.”

Image: Damage in Ellicott City following the July 31 flash flood; Brian Krista, Baltimore Sun Media Group.