A large portion of the Mid-Atlantic and the Northeast was under threat of flash flooding on Monday night as powerful rain moved into major population centers, including New York City, from the east, unleashing showers and thunderstorms from the Ohio Valley to the Atlantic Coast.
Key Things to Know
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Intense rain fell in several places, including New York City, Philadelphia and Washington, D.C. Localized totals over five inches “certainly seems plausible,” forecasters said Monday afternoon.
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” rel=”noopener noreferrer” target=”_blank”>Flash flood watches were in effect from North Carolina to New York for much of the day. A watch means conditions are favorable for severe weather, and a warning is issued once an event is occurring, or about to.
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By 6 p.m., the National Weather Service had issued several flash-flood warnings for areas that included Washington, southeastern Pennsylvania and New York City.
The Forecast
Forecasters said the system’s slow speed and the presence of unusually hot and humid air were contributing to the high flood risk, particularly along the Interstate 95 corridor from Washington to northern New Jersey.
“The concern is one storm will follow another, after another, and multiple rounds over areas that can’t handle that much rain, all falling in a short amount of time,” said Joseph Wegman, a meteorologist with the Weather Prediction Center.
The heaviest rainfall and the most widespread thunderstorms are expected to develop from Northern Virginia through southern New England in the afternoon. The Weather Prediction Center has issued a “moderate” (Level 3 out of 4) risk for flash flooding in that area through Tuesday morning.
Rainfall rates could reach two to three inches per hour in some areas, with widespread totals of two to four inches and isolated areas nearing six inches.
With the potential of heavy rain falling in a short period of time, forecasters were concerned about urban areas where flash flooding could occur rapidly as water collects on concrete, unable to soak into the ground as it does on grass.
In the New York City area, the National Weather Service warned of “scattered instances of flash flooding,” and issued a Level 2 out of 4 risk for flash flooding on Monday. The highest threat for flooding was in New Jersey, just west of the Hudson River. In some parts of the region, up to three inches of rainfall per hour was possible. Mr. Wegman said any storms in the city would most likely come later in the evening, between 7 and 9 p.m.
“But by that point, you’re late enough in the day that you’re losing the heating that acts as an energy source for the storms,” he said. Still, he warned, a stronger storm could move over the city, which would most likely lead to a higher threat of flash flooding.
While the most significant storms were not expected until later in the day, scattered thunderstorms had already begun to form early Monday in northeastern Pennsylvania and in parts of southeastern New York, including the Catskills and the Hudson Valley.
The Level 2 out of 4 risk for excessive rainfall extends from northern North Carolina through parts of New England, and into southeast New York State. Mr. Wegman said the risk of flash flooding in these areas was lower, in part because they are less urbanized and are expected to receive somewhat less rain overall.
On Tuesday, the system is expected to move south, shifting the highest risk of flooding into more of the Mid-Atlantic, from portions of northwestern North Carolina to southern New Jersey. Washington, Baltimore, Philadelphia and parts of Delaware are under a Level 2 out of 4 risk for flash flooding.
While the overall risk may be lower in some areas on Tuesday, Mr. Wegman said, summer thunderstorms are notoriously difficult to predict.
“It’s very possible some areas will have a worse day on Tuesday than Monday,” he said. “The weather models do like to change a lot in the 24 hours leading up to an event.”
Nazaneen Ghaffar is a Times reporter on the Weather team.
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