Clash of Air Masses Causes Extreme Flooding Event in Ellicott City, MD

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A large pool of cool ocean water south of Greenland is reflected in the upper atmosphere by a persistent polar vortex in eastern Canada. Meanwhile a large envelope of humid subtropical air surrounds Alberto in the Southeast U.S. Yesterday, the two air masses met in the form of a back door cold front in the Baltimore area and an 8-in. rainstorm evolved in Ellicott City.

Discussion: A back door cold front moving south into the Mid-Atlantic region on Sunday arrived near Baltimore, MD just at the right time – during maximum heating in a very unstable atmosphere – to cause an unusually heavy rainfall episode which caused catastrophic flooding centered on Ellicott City located about 13 miles west of Baltimore.

Prior to the arrival of the back door cold front convective available potential energy (CAPE) index was a rousing 2000 to 2500 j/kg supporting severe thunderstorms. A line of thunderstorms developed along the backdoor cold front mid-afternoon just north of Baltimore City expanding westward and intensifying. Over a 3 hour period radar estimates of 4-8 in. of rain occurred due to this narrow band of thunderstorms. In Ellicott City, 8 in. of rain occurred and a flash-flood was underway.

Ahead of the back door cold front temperatures soared into the upper 80’s and dew pointes were in the 74-77F range. This type of humidity normally waits until the second half of summer to evolve.

The high humidity leading to the very unstable air mass was present to the envelope of subtropical air surrounding Subtropical Storm Alberto. Meanwhile the south-moving cold front was quite strong propelled southward by an intense polar vortex in eastern Canada present somewhat due to the immense cool poll of water in the North Atlantic south of Greenland. The macro-scale climate definitely was a driver of this event.

On Saturday a flash flood event of nearly the same intensity occurred just north of Philadelphia.