The service says a failure of dam could flood parts of Lynchburg with 17 feet (5 metres) of water in just seven minutes.
"Should a dam failure occur, water along Blackwater and Ivy Creeks may rise several feet within a few minutes, with very swift currents", the bulletin said.
At 919 PM EDT, county dispatch reported the imminent failure of College Lake Dam on the Blackwater Creek.
President Donald Trump receives new letter from North Korea
It has taken longer than Washington had hoped, but a U.S. state department official says the process so far has been smooth. North Korea previously returned the purported remains of foreign soldiers that turned out to be misidentified.
Lynchburg Director of Water Resources Tim Mitchell said at a 3 p.m. news conference that the dam would continue to be watched around the clock.
Local fire and police sent boats to rescue people to area shelters, including E.C.Glass High School and Sandusky Middle School.
The west-central Virginia city near the Blue Ridge Mountains has received plenty of runoff from recent rains, and 4 to 6 inches of rain fell in the area Thursday evening, filling College Lake beyond its capacity. The area impacted would be the city of Lynchburg, which is one mile downstream from the dam. The city owns the dam, but the university owns the lake.
The NWS issued a flash flood watch for the area that lasts until 11 a.m. Friday.
Zimbabwe police fire tear gas at stone-throwing opposition supporters
So far, the ZEC has said that of 210 parliamentary seats, 205 had been counted with ZANU-PF winning 144 and the MDC Alliance 61. No amount of results manipulation will alter your will", he tweeted before the army was deployed on Wednesday.
The heavy rain in Virginia is part of a sprawling weather system bringing the threat of flooding all the way from the Carolinas to New England.
Officials say people who were evacuated may not be able to return to their homes for at least 24 hours, WDBJ reports.
The dam was built in the early '30s and, like many dams in the US, doesn't meet current safety standards.
Arduous road ahead for Zimbabwe after election results
The ZANU-PF leader tweeted: "At this crucial time, I call on everyone to desist from provocative declarations and statements". As results have yet to be officially announced by the ZEC sources counting the ballots have said it is "close".
"At this time, the dam has not failed and is being closely monitored by emergency personnel for any signs of structural failure", the weather service said in a statement.