BOULDER, CO - An entire community cut off, firefighters huddled on the side of a mountain after water swept their truck away, and -- with rescue helicopters grounded -- no way to reach them.
This is the scene facing authorities Thursday in Boulder County, Colorado, in the wake of what Sheriff Joe Pelle called a "devastating storm" that dumped more than half a foot of rain on the region during a 19-hour period.
The widespread flash flooding washed out roads, pushed dams to their limits and beyond and killed at least three people along Colorado's Rocky Mountain range, from Boulder south to Colorado Springs.
The worst of the reported damage has come in Boulder County, where the National Weather Service reported that a 20-foot wall of water roared down a mountain canyon north of the city, temporarily trapping a firefighter in a tree. Although injured, the firefighter made it to a nearby home, sheriff's Cmdr. Heidi Prentup said.
Lyons, a small town of 2,000 near the foothills of the Rocky Mountains, was cut off by what city officials described as a 500-year flood. They took to Facebook to urge residents to prepare for up to three days on their own.
"We ask you to conserve resources during this time," the city said. "We are currently not able to get water or food into the town."
People were using the city's Facebook page to search for loved ones, hunt for medicine and even ask for pet food while waiting for rescuers to arrive.
Elsewhere, homes collapsed onto residents and a dam in Larimer County broke, flooding some homes and leaving three people trapped, a county spokesman said.
Larimer County Sheriff Justin Smith said some residents there face the dilemma of whether to try to move to safer shelters over bridges that may have been damaged. They will "have to use their own judgment," he said.
An emergency message from the sheriff's office to residents of Big Thompson Canyon said, "If you are cut off because of a compromised bridge, you need to stay at your residence but have a plan to get to higher ground at a moment's notice."
Pelle said emergency crews were worried about a Lyons Fire Department crew stuck on a mountainside near a washed-out road. Hypothermia from the cold, wet conditions was of particular concern, he said.
And with so much uncertainty about what had happened in the middle of the night, Pelle said the death toll could well rise.
"This was a devastating overnight storm in the area, and I anticipate that as the day goes on, we are likely to find other people who are victims of this storm," the sheriff said Thursday. "We are bracing ourselves."
As dawn broke over the region, Pelle said he was "amassing a large-scale effort" to rescue those who are trapped, reach those who are stranded and deliver much needed aid to places like Lyons, where floodwater overtopped several dams protecting the town.
Boulder County has requested Colorado National Guard vehicles capable of fording deep water as well as rescue helicopters, which are currently grounded because of fog, low clouds and rain, he said.
A Federal Emergency Management Agency urban search-and-rescue team was on the way, he said, with some members already working on the scene.
Despite the efforts, rescuers had been constantly frustrated overnight by debris, impassable roads, mudslides and darkness in their efforts to reach those affected by the flooding.
"This is not your ordinary disaster," Pelle said. "All the preparation in the world, all the want-to in the world, can't put people up those canyons while debris and walls of water are coming down."
The rain started falling in earnest about 6 p.m. Wednesday and continued into Thursday, sometimes at the rate of about an inch an hour, according to radar estimates. That added up to about 6 to 7 inches of total rainfall.
"The rain, it almost feels like hail, the drops are so thick," University of Colorado Boulder student Ryan Colla told CNN affiliate KUSA. "It just keeps coming and coming, and when you think it's going to subside, it starts to rain down harder."
The sudden influx of water turned Boulder Creek -- which runs through the campus and other parts of the city bearing its name -- into a high, fast, muddy and dangerous torrent, Colla told the station.
"It freaked us out," he said.
At its peak, Boulder Creek was flowing at 16 times its normal rate for this time of year, city spokeswoman Sara Huntley said.
But that was not the only stream causing trouble, Pelle said. Unlike the last devastating flood in Boulder in 1969, this storm caused virtually every waterway in the area to overflow, he said.
Water rushed through Aurora, east of Denver, swirling and breaking like an ocean hitting a beach. CNN affiliate KCNC captured video of one person stumbling dangerously while trying to cross an Aurora street and finally struggling at the edge of the water. Three onlookers pulled that person to safety.
In Estes Park in Laramie County, KCNC video showed the Big Thompson River