Tuesday is already Baltimore's rainiest day since at least April 19, and with heavier showers possibly to come, forecasters cautioned of possible localized flooding in the evening hours.
More than half an inch of rain had fallen at Baltimore-Washington International Thurgood Marshall Airport from 9 a.m. through 2 p.m. Another half an inch is possible in the afternoon and evening hours, according to the National Weather Service.
Showers are expected to continue into the early hours of Wednesday morning for Central Maryland, according to the weather service. "Moderately to locally heavy rain" is expected, according to a hazardous weather outlook for the region.
Parts of northern and central Virginia were meanwhile under a flood watch.
A small craft advisory is in effect on the Chesapeake Bay through midnight.
Three-hundredths of an inch of rain had been measured so far this month at BWI before Tuesday, all of them falling late Monday.
Tuesday morning's rainfall was the most since 0.79 inches were measured at BWI on April 19. April ended an inch short of its normal rainfall total, with 2.2 inches.
May is typically among the wettest months of the year in Baltimore, with about 4 inches of rain in an average year.