As much as 4 to 6 inches of rain could fall on Central Maryland from Wednesday night through Thursday, prompting warnings of possible flooding.
A slow-moving low-pressure system is forecast to drop from the Great Lakes toward the mid-Atlantic. Meanwhile, winds laden with sub-tropical moisture are forecast to come in from the east.
"It could rain a lot," said Andy Woodcock, a meteorologist with the National Weather Service's Baltimore/Washington forecast office.
Though the ground is relatively dry -- just three-quarters of an inch of precipitation has fallen at Baltimore/Washington International Thurgood Marshall Airport this month -- flooding is still possible, he said.
"That should not allow us to get our guard down," Woodcock said. "People should be prepared."
Forecasters expect the heaviest rain to fall from Wednesday night through Thursday afternoon. A flash flood watch is in effect across Central Maryland from late Wednesday through early Friday morning.
According to the National Oceanic and Atmospheric Administration's Weather Prediction Center, the region has a 50 percent chance of at least 4 inches of rain and a 10 percent chance of 6 inches of rain or more.