Eagles club bartender Mary Hart said a 6-foot-tall wall of water barreled through one of the closed doors leading to the Blue Room, knocking one woman off a chair.
Hart said attendees were startled as the water entered the building's main room, eventually leaving a few inches of water through the length of the facility.
"Everybody jumped up, screamed and started heading for the door," Hart said.
Hart said she believed people were fortunate that rain kept away many karaoke night regulars because it made evacuation easier. Hart estimated at least 30 people were inside at the time of the incident.
About 2 inches of rain fell between 8 and 9 p.m., according to Hagerstown weather observer Greg Keefer's Web site.
The force of the rain began to ease after 9:30 p.m., and 2.88 inches of rain had fallen by 10 p.m.
"It's coming down so fast it can't run off quick enough," said National Weather Service forecaster Jim DeCarufel. "Just one of those summertime deals."
Washington County Sheriff's Deputy Tracey Peyton said authorities scrambled to close portions of many water-covered roads including Halfway Boulevard, Garis Shop Road, Mapleville Road and York Road.
"Pretty much anyplace where there's a bridge, flat road or no grade to the road, we've had standing water," Peyton said. "We've had more than a half-dozen vehicles trapped in high-standing water."
Maryland State Police Cpl. Patrick Morris said the rain forced closures of Md. 66 approaching Smithsburg, Dual Highway at Edgewood Drive and most roads in Halfway.
Morris said authorities and emergency workers had to deal with trees down along Md. 63 and "floating cars." Morris said firefighters rescued at least 10 people from cars floating on Virginia Avenue and Halfway Boulevard when the storm appeared to be at its peak, shortly after 9 p.m.
"The fire department did a great job of getting those people out," Morris said.
No one was injured in the rescues, which were made by members of the Volunteer Fire Company of Halfway, according to a county emergency dispatcher.
Water also flooded Hagerstown Springworks at 437 E. Baltimore St. in Hagerstown, damaging about 20 vehicles that were parked at the business for repairs, owner Henry Physioc said.
Physioc said he was able to get three cars to higher ground before the water started rising in the flood-prone area.
"The water's rolling in as we speak. It's coming in here like a river," Physioc said in a telephone interview. "It's a devastation to the business."
Physioc said he didn't know what he was going to tell his customers in the morning, including the owner of a $50,000 motor home that was waterlogged by the flooding.
Flooding conditions and continuous rain wreaked havoc on roads throughout the county, although police agencies reported no serious car accidents or major injuries.
At about 9:45 p.m., a small truck from the Funkstown Fire Department on its way to a call hydroplaned through the barricaded stretch and continued north across Dual Highway toward Robinwood Drive.
A firetruck close behind had to take a longer, but drier, path.
Rain turned part of Edgewood Drive into a temporary lake. Orange cones formed a barricade near the intersection of Edgewood and Dual Highway. A segment of the road in front of First United Bank & Trust was under several inches of water, which also buried the nearby entrance to the shopping plaza.
The storm knocked out power to about 300 households, including about 200 north of Smithsburg and 100 in Saint James Village south of Hagerstown, Allegheny Power spokesman Guy Fletcher said.
Rainwater flooded numerous basements in the county, and in some cases, first floors of residences, according to Washington County Fire and Rescue Communications.
High water washed out the backyards of homes on West Church Street in Williamsport and the green areas in the Londontowne apartment complex, on the eastern edge of Hagerstown, a dispatcher said.
Staff writer Andrew Schotz contributed to this story.