Six and a half inches of rain dumped on Ellicott City in about two hours Saturday night, a deluge expected to occur only once every thousand years.
More than 4.5 inches fell within one hour, from 7:30 p.m. to 8:30 p.m., according to a Howard County rain gauge.
The massive burst of precipitation sent a wave of floodwaters cascading down the hillsides in the historic downtown where it turned into a wall of water smashing down Main Street, sweeping cars downhill, sending restaurant-goers scurrying for higher ground and carving away the road and sidewalks, leaving behind massive sinkholes.
Two people in cars that were swept away died in the floods, according to officials in Baltimore County, where their bodies were found nearly two miles down the Patapsco River.
Records are not kept for rainfall in Ellicott City specifically, though residents used to moderate flooding events said this one was the worst in recent memory. The last major flood to hit Ellicott City was in 2011, and this one was about 3 feet higher, said Jason Elliott, a hydrologist with the National Weather Service's Baltimore/Washington forecast office.
“This was different,” said David Dempster, who owns Still Life Gallery on Main Street with his wife, Sara Arditti. “This was crazy.”
After the 2011 flood, Dempster built 20-inch-high walls in the rear of the gallery and the side alley. But the Tiber River, a Patapsco tributary that flows just behind the gallery, rose 12 or 15 feet to fill the basement to its ceiling, he said.
“I don’t think anything would have stopped this,” he said.
Howard County Executive Allan Kittleman said this is the worst flooding the town has seen in modern times — including Hurricane Agnes in 1972, when he was a teenager. Another historic flood in 1868 killed 43 people.
The Patapsco River rose 14 feet from about 7:20 p.m. to 9 p.m., according to the weather service.
Based on records for a gauge five miles away in Woodstock, there is a less than 0.1 percent chance of such intense rainfall happening in any given year, Elliott said — making this a once-in-1,000-years storm.
“We were in an extremely moist air mass leading up to this, and to some extent, still are,” Elliott said. “Every bit of moisture there was to wring out of the atmosphere after this heat wave we’ve had all came out mainly over that one hour.”
The difference for Ellicott City was that it was caught in an area where, as one storm system moved off to the east, another developed right behind it. Other parts of the region saw a single 30- or 45-minute burst of heavy rain, instead, Elliott said.
The storm also stacks up with record-breaking rainfall events around the region in recent years.
On Aug. 13, 2014, 6.3 inches of rain fell at Baltimore/Washington International Thurgood Marshall Airport, the second-rainiest day at Baltimore’s point of record. In the midst of that storm, 3.91 inches of rain fell within about an hour, something that only would have been expected to occur once every 500 to 1,000 years, meteorologists said.
Other parts of Howard County also got a month’s worth of rain in the span of a few hours.
Trained weather spotters (volunteers who take weather service classes on meteorology basics) reported 5.69 inches of rain near Woodstock and 3.6 inches near Columbia, according to the weather service.
Other rainfall reports included 5.1 inches near Damascus in Montgomery County, 3.5 inches near Westminster and 2 inches in Northwest Baltimore.
At the Inner Harbor, 2.28 inches fell between 7 p.m. and 10 p.m., including 1.43 inches in the 9 o'clock hour alone. At BWI, 1.8 inches fell over that span, with 1.08 inches in the 8 o'clock hour.
More storm chances were forecast overnight Sunday and Monday night. After that, dry conditions with seasonable humidity and high temperatures in the 80s are forecast through the week.
Timeline of the rain
The floodwaters that inundated Ellicott City came with a warning, but still took its historic Main Street district by surprise. Witnesses described waters rising several feet in a matter of minutes.
6:45 p.m.: Rain begins to fall in Ellicott City.
7:18 p.m.: National Weather Service issues flash flood warning across Howard County, Baltimore and southern Baltimore County.
7:30 p.m.: Heaviest rain moves in to Ellicott City, with 4.56 inches of rain over the next hour.
8:15 p.m.: Patapsco River begins to swell, rising 6 feet in an hour.
8:45 p.m.: As rain tapers off, floodwaters begin to surge down Main Street, with nearly 6 inches of precipitation in two hours.
9 p.m.: As the flash flooding on Main Street eases, the Patapsco River crests, rising 14 feet in an hour and a half.