Perhaps it was a breeze off the Chesapeake Bay, or off of another nearby storm, that stirred raindrops to begin falling over Baltimore on Tuesday.
Whatever it was, it created lift that sent the hot, humid air over the region rising into relatively colder levels of the atmosphere. That triggered a release of instability in the clouds over Baltimore — in the form of downpours.
“It pretty much formed right over the city,” said Cody Ledbetter, a National Weather service meteorologist. “There was some instability, so it just kind of popped off.”
That was the first bad turn of fortune. Then there was the fact that the flow of air over the region, typically steering storms at perhaps 30 mph or 50 mph, was nearly stagnant. The average flow overhead was about 12 mph, Ledbetter said.
Storms developed and then would die out only for new cells to form right behind them, making it appear that a single, massive storm cloud was nearly stationary over Baltimore.
“They were moving, but just kept forming on the back end [of the storm system] and moving over the same area,” Ledbetter said.
On some days, storm activity is widespread, such as when a large cold front moves across the area. But on Tuesday, the atmospheric action was more scattered, leaving some areas with sunny skies and making storms more isolated.
It was not an unusual setup for summer in Baltimore, but because the storms were so slow and persistent, they dropped an unusually large amount of rain in areas prone to urban flooding. Radar estimates suggest as much as 5 inches of rain fell within a couple of hours — on par with the deluges that flooded Ellicott City in 2016 and 2018.
That was despite what Ledbetter called “pretty average” levels of moisture in the air.
More storms are in the forecast Wednesday, but they aren’t forecast to cause more flooding. Unlike Tuesday’s storms, they should pass through quickly.