On May 27, 6 to 10 inches of rain fell around Ellicott City, Md., over the course of just a few hours and caused catastrophic flash flooding in the historical mill town. Days earlier, Frederick, Md., was struck by an intense line of thunderstorms that dumped six inches of rain and baseball-size hail in two hours. The flooding drenched homes and businesses and prompted several high-water rescues.
On the whole, the Washington region just had its wettest 30-day period on record. Several weather stations logged record rainfall since May 6, including Baltimore, Beltsville, Md., and Damascus, Md.
But a mere train ride away, cities farther north are in the opposite situation. It has been the driest 30-day period for several Northeast cities, including Portland, Maine. In New Hampshire, Concord has had its second-driest period, and Lebanon its third.
While rainfall totals generally exceed 12 inches in the D.C. area, they are below two inches north of Massachusetts.
Two forces came together in May to focus the rain on Washington and the Southeast.
The first was a large ridge of high pressure over the Atlantic Ocean, called the Bermuda High. With its clockwise winds, it was forcing warm, moist air north — but not far enough north to reach New England.
May also brought the season’s first tropical storm. Alberto was slow-moving and followed a path around the Bermuda High, focusing its rainfall on the gulf states and the Mid-Atlantic. But again, it didn’t track far enough north to quench New England.