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Boulder, Colorado (CNN) -- Three more days.
That's how long it may be before all the rain goes away in Colorado, where flash flooding has cut off towns, ripped out roads and killed at least three people.
More rain is forecast through Sunday for the region, on top of the 15 inches some parts of the state have already received. While meteorologists aren't sure which way the water will go, state officials warned that "very heavy rainfall" is likely again Friday.
"This isn't over," CNN meteorologist Chad Myers said.
Nineteen counties remained under high threat of flooding Friday, the state Office of Emergency Management said.
They include Boulder County, where National Guard troops were evacuating the entire town of Lyons, which had been cut off by flash floods.
The others are Arapahoe, Weld, Park, Jefferson, Larimer, Clear Creek, Adams, Douglas, Broomfield, Gilpin, Denver, Boulder, Logan, Morgan, Washington, El Paso, Teller, Pueblo and Elbert.
State transportation officials issued an emergency alert to residents in some of the hardest-hit counties, warning them to stay off roads because many are unstable and could give way without notice. They also closed Interstate 25 from the Wyoming line south to Denver. Part of Interstate 70 also was shut down.
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.
The rains sent virtually every waterway in Boulder County 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 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 canyonsmight 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.
HLNTV.com: 8 stunning Colorado flooding Twitter photos
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."
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."
Boulder, Colorado (CNN) -- Three more days.
