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A woman looks at the flooded Boulder Creek in Boulder, Colorado, on Thursday, September 12. An estimated 6-10 inches of rain fell during 12-18 hours along Colorado's Rocky Mountain Range, from Boulder south to Colorado Springs, causing residents to be evacuated, washing out roads and killing at least three people.

Three vehicles crashed into a creek after the road washed out from beneath them in Broomfield, Colorado, on September 12. Three people were rescued.

Nicky Toor, 15, floats on the flooded lawn of North Boulder Park in Boulder, Colorado, on September 12.

Joey Schusler rides through flooded Canyon Boulevard in Boulder, Colorado, on September 12.

Residents view a road washed out by a torrent of water after overnight flash flooding near Left Hand Canyon, Colorado, on September 12.

A city worker talks on his phone while surveying high water levels from Boulder Creek after flash flooding in downtown Boulder, Colorado, on September 12.

Flash flood waters rush over a walking path in downtown Boulder on September 12.

A police officer blocks off a road in Boulder on September 12.

A man walks past the swelled Boulder Creek in Boulder on September 12.

People stand at the edge of floodwaters in Boulder on September 12.

A dive rescue team moves toward floodwaters in Boulder on September 12.

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Boulder, Colorado (CNN) -- Help has arrived for Lyons, the small town cut off by flooding in Colorado.

The Colorado National Guard began evacuating the entire town of about 2,000 at daybreak, Gov. John Hickenlooper said.

But the outlook was less upbeat for a broad swath of northern Colorado. State transportation officials issued an emergency alert to residents in four Colorado counties hit hardest by flooding, warning them to stay off roads because many are unstable and could give way without notice. Interstate 25 was closed.

Authorities warned that more rain was on the way, posing the threat of additional flooding.

In Fort Collins, some residents had been urged to leave their homes. And in Denver, police responded when a man was swept into a drainage pipe with his dog. Both were saved after traveling two blocks in the water, police said on Twitter.

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Deadly flooding in Colorado

The situation is the result of 6 to 8 inches of rain dumped on parts of northeastern Colorado in the last 24 hours, the National Weather Service said. The rains sent virtually every waterway in Boulder County, one of the hardest-hit areas, coursing out of its banks, and massive water flows washed away roads and bridges, flooded homes and stressed numerous other bridges.

Three deaths had been reported: two in Boulder County and one in El Paso County. About 20 people have been reported missing by relatives in Boulder County, Sheriff's Cmdr. Heidi Prentup said.

Lyons rescue

The National Guard effort to get residents out of Lyons began shortly after daybreak. About 100 National Guard troops in 21 heavy vehicles able to ford high waters streamed into the city to begin moving residents out, Gov. John Hickenlooper said.

Residents had been entirely cut off, without water or sewer service, in many cases without electricity, facing what Fire Chief J.J. Hoffman said in a Facebook posting was a "very large disaster."

Not even National Guard helicopters -- grounded by poor weather -- could reach the residents Thursday.

It was unclear when the evacuation would be complete.

"I encourage all of you -- stay strong!" Hoffman wrote on the fire department's Facebook page. "We will make it through this, we are here for you and doing the absolute best we can with the resources we have to get to each and every one of you!"

Lyons follows fellow Boulder County towns of Jamestown and Eldorado Springs to be evacuated as a result of the storm, which began around 6 p.m. Wednesday, Boulder County Sheriff Joe Pelle said.

Up to another half-inch of rain an hour is possible this afternoon, authorities said.

While the forecast called for less rain Friday than the region had received the last few days, meteorologists warned that any rainfall would add to the flooding potential in the region, thanks to waterlogged ground unable to absorb any more water. A flash flood warning remained in effect through noon.

Danger elsewhere

Overnight, flood sirens sounded in Boulder County as Colorado emergency officials feared that debris-caked canyons might give way and send another wall of water crashing through the city of Boulder and neighboring communities.

"All residents are warned to go to higher ground immediately due to the potential for flash flooding along the creek," Boulder's Office of Emergency Management said.

Emergency management warned that "there are mudslides at the mouth of Boulder Canyon 400 feet long and four feet deep as the sides of the canyon give way due to the saturation from the days-long rain."

There were dramatic rescues Thursday, including a man pulled from an overturned car in rushing water on live television. But officials have had a difficult time reaching affected areas because of the flooding, debris, mudslides and washed-out roads.

Emergency workers spent most of Thursday and night early Friday playing defense against rapidly rising water, Larimer County Sheriff Justin Smith said -- moving roadblocks farther and farther back as flooding spread.

Rescue crews have yet to launch helicopters to aid in the rescue effort, Smith said. The helicopters have been grounded because of poor weather.

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Boulder County takes a beating

The worst of the damage reported Thursday was in Boulder County, where the National Weather Service said a 20-foot wall of water roared down a mountain canyon north of the city.

One death was confirmed and another feared after a car stopped in the rushing water. Witnesses said a woman emerged from the car and was swept away. A man left the car and tried to reach her and also was overcome, said Prentup, the Boulder County Sheriff's Office commander. She said the man's body had been recovered and the woman was missing.

Bodies also were found in a collapsed home in Jamestown and on a roadway in Colorado Springs.

Elsewhere, homes collapsed onto residents and a dam in Larimer County broke, flooding some houses and trapping three people, a county spokesman said.

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."

Another concern was a team of Lyons firefighters that had been stuck on a mountainside Thursday morning after their vehicle was washed away. As of Thursday evening, authorities in Boulder had not been able to make contact with the Lyons department to confirm the firefighters had been rescued, said Ashlee Herring, a spokeswoman for the city of Boulder.

Dams threatened, roads washed away

Dozens of roads were closed or impassable Friday in Boulder County alone.

Between 25 and 30 roads were closed Thursday afternoon in Boulder County, Prentup said. Some of them had been washed out entirely.

Officials have yet to determine the extent of the damage, but it will be severe, Hickenlooper said.

"This is not going to get fixed in a week," he said. "We have lost a great deal of infrastructure."

Mike Pearson reported and wrote from Atlanta; Ana Cabrera reported from Boulder, Colorado; CNN's Ed Payne, David Simpson, Matt Smith, Sara Weisfeldt, Tina Burnside, Shawn Nottingham and Sherri Pugh also contributed to this report.