In the afternoon of May 27, 2018, after over 8 inches (20 cm) of rain in a span of two hours, the historic Main Street in Ellicott City, Maryland was flooded, just before the new flood emergency alert system was supposed to become operational. Flooding occurred throughout the Patapsco Valley, in the adjacent communities of Catonsville, Arbutus, and Elkridge, as well as the Jones Falls Valley in Baltimore.

The Patapsco Valley is a small valley surrounding the Patapsco River in central Maryland. The region is known for its historical significance as a major economic and industrial center in the eighteenth and nineteenth centuries.