Locations around Ellicott City and Catonsville saw between 5.36 inches and 10.38 inches of rain on Sunday, said Kyle Pallozzi, a meteorologist with the National Weather Service’s Baltimore-Washington forecast office.
Several weather factors combined to produce Sunday’s torrential conditions, according to meteorologists: A slow-moving cold front coming from the north and east, along with slow-moving thunderstorms. In addition, multiple storms converged in the same location, a situation meteorologists call “training convection.”
Pallozzi said the term compares storms to freight cars stacking up behind a train engine — and there’s just one track to travel on. In this case, storms kept forming in the same area because of the stalled cold front.
“They kept regenerating and moving over the same areas,” Pallozzi said. “It was a moisture-rich environment. The storms had a lot of moisture to work with.”
Training thunderstorms is more common in other parts of the country, such as the Great Plains, Pallozzi said.
The heavy rain sent the Hudson and Tiber tributaries over their banks, with the water coursing down Main Street.
A gauge in the Hudson peaked at 3.06 feet above flood stage — which was half a foot higher than the peak in 2016, Pallozzi said.
The Patapsco River, at the bottom of Main Street, peaked about 4 feet lower than it did in 2016. But a little farther downstream, at Patapsco Valley State Park, the river peaked nearly a foot higher on Sunday than it did in 2016.
In the midst of the storm, meteorologists like Chicago-based Zac Flamig used weather radar to predict how much rain was falling.
While 2016’s storm was a 50- to 75-year storm (that is, one recurring every 50 to 75 years), according to Flamig, initial rainfall estimates show that Sunday’s storm was on par with a 200-year storm — one occurring every 200 years or more.
“The best estimates that we have in real time are saying that this is a worse flood,” Flamig said.
The deluge of rain that caused flash flooding in Ellicott City and across the Baltimore region Sunday prompted the evacuation of at least three businesses in the city’s Woodberry neighborhood, according to the Mayor’s Office of Emergency Management.
In 2016, experts called it a “thousand-year storm,” meaning such intense rainfall is likely to occur only once every 1,000 years. However, Flamig said such models can be misleading since they overreact to small changes in flood levels.
“Because of the way we estimate them, small changes in rainfall at the extremes can have big effects,” he said.