A severe thunderstorm parked itself over the Baltimore area early Tuesday evening, unloading up to a half-foot of rain in a punishing two-hour torrent. It was an exceptionally intense and rare deluge with a likelihood of less than a half-percent in any given year.

The cloudburst quickly flooded streets, while howling winds gusted up to 70 mph and hail to the size of ping-pong balls slammed to the ground.

Flooding, which concentrated in downtown Baltimore (including around Little Italy and Harbor East) and Fells Point, caused the most problems.

In downtown, the National Weather Service received reports of cars underwater on Central Avenue while high water shut down Charles Street near Penn Station.

Some of the most severe flooding occurred around Fells Point, where waters partially submerged vehicles. One motorist, stranded on the roof of his car in the floodwaters, required rescue.

Fells Point reported a wind gust to 70 mph, and the Weather Service cited numerous reports of tree damage in the southern and eastern portions of Baltimore City.

How it happened

The storm complex erupted practically out of nowhere. As of 5 p.m., radar showed no sign of a storm, but by 5:20 p.m., it towered over the city and was dumping rain and generating frequent lightning. Not long after, simultaneous severe thunderstorm, flash flood and marine warnings were in place.

The storm was very isolated and remained stationary over Baltimore and its southeastern suburbs. Not until just after 7 p.m. did it wind down.

The estimated rainfall from Doppler rainfall, shown above, indicates about three inches (pink shade) fell in downtown Baltimore but more than five inches (yellow shade) just to the southeast. And these radar-indicated amounts are often underestimates.

Where the rainfall exceeded five inches in two hours, this was an exceptionally rare event with a probability of between one in 500 and one in 1,000 in a given a year.

The radar snapshot (below) shows this system at the height of its intensity.

How did the storm develop? A long-lived (two-hour) system such as this requires converging air currents at low levels, in an unstable atmosphere. The storm developed at the northern end of a zone of low pressure close to the ground, into which winds streamed together from the southeast and the southwest.

Additionally, the storm developed along the western margin of the Chesapeake Bay, where a bay breeze wind circulation may have helped nudge air upward. Finally, large urban regions such as Baltimore are known to generate their own windy circulations, because of the urban heat-island effect, and these local winds may have come into play.

There was enough wind shear (increase in winds with altitude) to organize ordinary, “pop-up” storm cells into a larger, coherent complex. Interestingly, it was this same pocket of strong winds aloft — mainly confined to the Eastern Shore and Delmarva — that probably helped trigger a supercell thunderstorm and a waterspout, earlier in the afternoon, near Ocean City.

Why did it rain so hard, for so long, in one spot? Even though the storm complex itself was stationary, radar and wind data suggest that individual cells moved repeatedly through the larger complex, from west to east, as suggested in the figure above. This process is called cell training. Each short-lived cell dumped a quick inch of rain … the succession of many cells over two hours, in the same location, contributed to a prodigious volume of total storm water.

This is the second exceptional rainfall event to affect the Washington-Baltimore region this summer. On July 8, a cloudburst dumped more than three inches of rain in a single hour in Arlington and Washington, leading to severe flooding. It was one of the most intense hourly rain events in recorded history in Washington, with a probability of occurrence of less than 1 percent in a given year.

This rain also comes on the heels of two exceptional flash floods in Ellicott City in 2016 and 2018.

Published data show the most heavy precipitation events in the United States are increasing in intensity because of climate change. “The frequency and intensity of heavy precipitation events across the United States have increased more than average precipitation and are expected to continue to increase over the coming century,” according to the U.S. government’s 2018 National Climate Assessment.

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