The service says a failure of the College Lake Dam could flood parts of Lynchburg with 17 feet (5 metres) of water in just seven minutes.
Officials say if the dam does fail, the water could rise several feet in just a matter of minutes with swift currents.
Authorities said concerns about the dam's failure will continue to be a threat until the water levels can safely be lowered.
The nearest downstream town is Lynchburg, located about 1 mile from the dam.
A press conference is scheduled at 3 p.m. Friday.
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Local outlet WDBJ7 said a number of people whose homes were deemed to be at risk had been evacuated to nearby schools, along with their pets. The News & Advance reports 12 to 18 inches of water were flowing out of the dam, over a road, and into a creek after three inches of rain fell in a single hour.
The City of Lynchburg has created an interactive map to see if you're in a potential flooding area should the College Lake Dam fail.
The west-central Virginia city near the Blue Ridge Mountains has received plenty of runoff from recent rains, with 4 to 6 inches falling in the area Thursday evening and filling College Lake beyond its capacity. The city owns the dam, but the university owns the lake.
More than 150 people had been removed from the possible path of dam waters, Luann Hunt, a spokesperson for the city of Lynchburg, told ABC News.
There is some good news in the forecast: Following two weeks of relentlessly wet weather in the eastern US, a drier weather pattern will move in over the weekend and into early next week, AccuWeather said.