BOULDER, CO - SEPTEMBER 12: Motorists attempt to drive through downtown Boulder after three days of heavy rainfall September 12, 2013 in Boulder, Colorado. An estimated 6-10 inches of rain fell in 12-18 hours and more is expected throughout the day. Flash flood sirens warned people to stay away from Boulder Creek and seek higher ground. (Photo by Dana Romanoff/Getty Images)
A 30-foot deep "surge of water, mud, rocks and debris" was heading toward the outskirts of Boulder, Colo., early Friday, emergency officials warned.
Area residents were told to move to higher ground after a witness spotted the surge -- which included cars -- heading down Fourmile Creek toward Boulder Creek around 11:10 p.m. local time (1:10 a.m. ET), according to the Boulder Office of Emergency Management. The National Weather Service described the incident as "potentially life threatening."
Some 4,000 people in the city were given mandatory evacuation orders and another 4,000 were told to shelter in place, police said. At least three people died in flash floods in the area on Thursday.
But around 2 a.m. local time (4 a.m. ET), emergency officials had still seen no sign of the surge reported upstream some three hours earlier.
Fire Chief Gerry Morrell in Lafayette, Colorado released video of crews rescuing a man from a car trapped in flood waters.
"This could be for a number of reasons," Boulder Office of Emergency Management spokesman James Burrus said. "A surge is going to have more impact at Fourmile Canyon, where the canyon is steeper, than down toward Boulder Creek where it flattens out.
"Also that area is a burn scar area - there was a wildfire up there and all the grass and trees are gone -- so when the surge gets down into a more vegetated area it might have more tendency to have its movement impeded."
Officials said that flood-monitoring stations outside the city had stopped reporting and it was believed they may have been knocked out by the wave of debris.
Boulder Police Deputy Chief Greg Testa said early Friday that water levels appeared to be "slowly receding."
He said the flow of water in Boulder Creek was running at 5,000 cubic feet per second at 8:45 p.m. local time (10:45 p.m. ET) Thursday, the time when the evacuation order was given. But at 2 a.m. Friday (4 a.m. ET), it was down to 3,800 cubic feet per second.
The National Weather Service also issued flash flood warnings until 6 a.m. local time (8 a.m. ET) for northern Jefferson County and Boulder County.
"At 11:53 p.m. MDT emergency management reported a new round of severe flash flooding in Fourmile Creek, Boulder Creek and Lefthand Canyon in Boulder County," the NWS said. "If you are near Bouder or Fourmile Creeks, get to higher ground now. Do not try to outrun this flash flood!"
Earlier, the Boulder Office of Emergency Management issued two emergency alerts to 8,000 phone numbers for residents along Boulder Creek, telling some to move to higher ground without crossing the creek and others to shelter in place but to move to higher floors if possible.
Scores of Boulder residents remained sheltered in a local YMCA early Friday as the region washed by days of rain and now flash floods braced against the continuing storms.
About 6.8 inches of rain fell over the city in a 24-hour period, according to the National Weather Service.
Officials were uncertain as to when the period of intense weather would end.
"We have been told it is supposed to be raining half and inch to an inch per hour but we don't know when it is going to abate," Burrus added. "Supposedly it could stop Friday evening but it's really hard to tell. It's a slow-moving system which is locked in. On our radar you can just see the stuff rolling up the mountains of New Mexico. If this was December we would be looking at 10 feet of snow."
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Alexander Smith and Jason Cumming, NBC News