Wayne Bockstanz, a Hagerstown police communications officer, said motorists should avoid roads that are covered by water.
"If you don't know how deep it is, that's the big thing," Bockstanz said. Because of high water, one vehicle was towed from the Memorial Boulevard underpass, which frequently floods, Bockstanz said. The Mitchell Avenue underpass typically fills with water in heavy rains, too, he said.
According to i4.weather.net, a Web site maintained by Hagerstown weather observer Greg Keefer, Sunday's rains had dumped 3 inches on Hagerstown by about 10 p.m. A trace of rain fell Saturday.
AccuWeather and National Weather Service spokespeople said rain showers could continue all week.
"We are seeing a very, very long line of storms that are headed northeast, and it looks like Hagerstown is in the path," National Weather Service spokesperson Jackie Hale said. "It doesn't look like it's going to let up anytime soon."
A National Weather Service flash flood watch for areas throughout Maryland, including western portions of the state, was set to expire this morning, Hale said. According to information included with the watch, some areas could get as much as 5 inches of rain or as much as 2 inches of rain in an hour.
Area rivers have been running low, Hale said. If the rains continue into the end of the week, Hale said river levels could become a concern.
"They will probably not start responding to all this rain for a day or so," Hale said.
Rain accumulations could double by tonight, Erickson said. According to radar estimates, about 2.8 inches had fallen by about 9:30 p.m. at Hagerstown Regional Airport.
The National Weather Service kept a flash flood watch in effect for Franklin and Fulton counties in Pennsylvania into late Sunday. The same watch stretched into this morning for the Eastern Panhandle of West Virginia.
Pennsylvania State Police in McConnellsburg, Pa., said they had several minor vehicle accidents throughout the day, but none while it was raining.
The 911 center in Berkeley County, W.Va., also had several reports of accidents, but none with injuries. The center also received calls about flash flooding, according to a dispatcher who did not have information available about the areas with reported flooding.
A dispatcher in Jefferson County, W.Va., said the 911 center had very few calls throughout the day.
"We've just had a few basement floodings, maybe three, since 3 o'clock," said Ben Rice, a 911 dispatcher in Franklin County.
Other than reports of drainage problems outside the Arby's restaurant on John Wayne Drive, the 911 center received few calls related to the weather, he said.
Franklin County weather watchers reported the heaviest rains between noon and 6 p.m., bringing the daily totals from 1.46 inches in the Chambersburg area to 2.28 inches in Waynesboro, Pa., 2.01 in Rouzerville, Pa., 2.7 inches in Quincy, Pa., 1.16 inches in Mercersburg, Pa., and 2.88 inches in Greencastle, Pa.
Staff writer Jennifer Fitch contributed to this story.