Forecasters in Colorado are warning residents near Denver to brace for torrential overnight rains, as raging flood waters roar through the region and cloud cover hampers helicopter rescue efforts.
Days of heavy rains and flooding have turned the state's Rocky Mountain foothills into high risk zones, with dozens of washed out roads and bridges turning entire communities into disaster areas short on supplies and services. At least four people were known dead by late Sunday, with two others officially missing and presumed dead.
Authorities say about 500 people were unaccounted for Sunday, while cautioning that some may be unable to communicate with loved ones while stranded and awaiting rescue.
Rocky Mountain rescue management officials say as many as 1,000 people were awaiting rescue north of Denver, in an area that includes the hard-hit mountain community of Estes Park. But all airlifts were grounded Sunday because of new rains and poor visibility.
Local news outlets say 50 bridges linking rural communities have been destroyed or damaged, and warn of a growing threat from mudslides and rockslides as roads crumble. Boulder County Sheriff Joe Pelle, speaking to reporters Saturday, said authorities were only beginning to understand the scope of the crisis.
President Barack Obama has declared Colorado a major disaster zone and ordered federal help to boost state and local recovery efforts.
Helicopters have plucked hundreds of people from inundated homes. One helicopter carrying Colorado Governor John Hickenlooper on a tour of the region made two stops to pick up residents waving to be rescued on Saturday.
John Hoffenberg watches the flow of water increase in Boulder, Colorado, Sept. 15, 2013.
An aerial photo of a flood-affected area of northern Colorado along the Big Thompson River, Sept. 14, 2013. (U.S. Air National Guard Handout photo)
An aerial view of vehicles submerged in flood waters along the South Platte River near Greenley, Colorado, Sept. 14, 2013.
Children board a rescue helicopter flown by the U.S. Air National Guard after severe flooding shut down major roads leading out of Jamestown, Colorado, Sept. 14, 2013. (U.S. Air National Guard Handout photo)
Will Pitner is rescued by emergency workers and neighbor Jeff Writer after a night trapped outside on high ground above his home as it filled with water after days of record rain and flooding at the base of Boulder Canyon, Colorado, Sept. 13, 2013.
Boulder Creek flows at high speed next to a road closed off by debris from days of rain and flooding, at the base of Boulder Canyon, Colorado, Sept. 13, 2013.
Nick Carter shovels debris as heavy rains cause severe flooding in Boulder, Colorado, Sept. 12, 2013.
A home and car are stranded after a flash flood in Coal Creek destroyed the bridge near Golden, Colorado, Sept. 12, 2013.
A section of Highway 72 is missing after a flash flood tore through Coal Creek near Golden, Colorado, Sept. 12, 2013.
Matthew Messner looks for a way to cross the sidewalk covered by heavy rains in Boulder, Colorado, Sept. 12, 2013.