By Ben Brumfield and Nick Valencia
BOULDER, Colorado (CNN) -- Heavy storms returned to northeastern Colorado Saturday as rescuers scrambled to take advantage of breaks in the weather to continue reaching stranded residents.
The National Weather Service issued severe thunderstorm warnings for parts of Arapahoe and Adams counties and eastern Denver. The service said 1.73 inches of rain fell in less than 30 minutes at one spot in southeastern Denver. The warning areas are southeast of the counties most heavily flooded earlier, but the weather service expected thunderstorms across the region Saturday evening and more rain at least through Sunday.
Authorities are worried that any additional water on flood-soaked ground will cause more flooding and dislodge mud and debris.
At least four people have been killed.
In addition, a 60-year-old woman was presumed dead after witnesses saw her being swept away by waters that demolished her home, said Nick Christensen, executive officer of the Larimer County Sheriff's Office. Neighbors tried unsuccessfully to rescue the woman, Christensen said. Her body had not been recovered.
The sheriff's office said on Twitter that about 100 people were unaccounted for at midday Saturday. In neighboring Boulder County, 172 were on the "unaccounted for" list, which includes anyone relatives were trying to contact. That number was shrinking as authorities located more people.
"We're assuming some of them have been stranded. We're assuming that some made their way out and simply haven't contacted us or friends and family to get off the list. We're assuming that there may be further loss of life or injuries," Boulder County Sheriff Joe Pelle said.
Gov. John Hickenlooper said on his Twitter account that his surveillance flight Saturday afternoon was diverted for two rescues. "Picked up four stranded people, a dog and a cat during helicopter tour of flooded area," Hickenlooper's Twitter account said. A later tweet said, "Picked up two more stranded people. Dropping all off in Loveland now."
Evacuations continued Saturday, but were hindered by drainage issues and flooded roadways, he said. Many residents are isolated.
"It's a sinking feeling," Sheriff Pelle said of knowing that emergency responders may not be able to reach everyone who needs help.
The nightmare is far from over as the state awaits more rainfall, threatening to send swollen rivers gushing through streets choked with debris.
It will not be as much as the 15 inches dumped in some spots this week, but it could cause more flooding in areas where water has already receded, forecasters warned.
Residents got some relief Friday and Saturday when rains subsided, giving a clear view of towns turned into abrupt lakes, homes and businesses inundated with muddy water and bridges devoured by raging creeks. Homes dangled off cliffs.
In Larimer County, there were 46 medical rescues on Friday, and teams continued looking for those unaccounted for on Saturday.
In expansive Weld County, 140 roads were closed because of the flooding.
Hundreds of residents were evacuated Friday, including 162 people transported by air from Jamestown because roads to the city were impassible, Boulder County EMS spokesman Ben Pennymon said.
Hickenlooper warned an extensive recovery is ahead for the affected area from the state's center into the northeast.
"This is not going to get fixed in a week," he said. "We have lost a great deal of infrastructure."
Currents swept away a woman who got out of her car Thursday in Boulder County. A man jumped out of the car to save her. Both drowned.
Authorities have recovered both bodies, Pelle said. Another body was found in a collapsed home in Jamestown in the same county. Rescuers recovered another body on a roadway in Colorado Springs in El Paso County.
In Denver, rushing waters swept a man into a drainage pipe with his dog. Both were saved after traveling two blocks in the water, police said.
All the people unaccounted for were in Boulder County.
"These are people whose family or friends haven't been able to reach or account for them, not necessarily in harm or dire need," said county spokeswoman Gabrielle Boerkircher.
President Barack Obama declared an emergency for Boulder, Larimer and El Paso counties, FEMA announced Friday. The declaration allowed FEMA to bring in four rescue teams, the largest ever deployment in Colorado, officials said.
The clear skies allowed for an uptick in evacuations Friday and Saturday.
National Guard troops using "high-profile" trucks to wade through water evacuated 550 people from the Boulder County town of Lyons, CNN affiliate KUSA reported.
It had been cut off since the flooding began Wednesday night -- without water or sewer service, in many cases without electricity.
Melinda Villa was stranded in her apartment with her 1-month-old baby in the inundated town. She had no phone service, no water and was running out of formula and food.
Then the National Guard arrived.
"It just really felt like God came down and saved us," she said.
Some had to rescue themselves.
Catherine Smith and Mandy Stepanovsky lived in a part of Lyons that is accessible only by bridges.
"When those became compromised -- one bridge completely blew out and the other one was very much impassible -- we started looking at other options," Smith said.
So the couple decided to hike for 2 miles to safety -- with their 8-month-old toddler in their arms. Walking was the only way out.
They hiked to Smith's brother's house, where they showered and ate a meal before the weather caught up with them again.
A mudslide suddenly brought mud, debris and water through the house, Smith said. They were forced to run to higher ground.
"It was terrifying," Smith said.
Jonathan Linenberger described a Noah's Ark-style evacuation as he, his fiancee, four dogs and three cats greeted the National Guard truck.
"We had to go (through) knee-deep water, at least. We had to wade our animals across into the truck to get them there," said Linenberger. "That was the first thing you can grab, your loved ones -- and that's what we have."
The National Guard also was evacuating the entire population -- 285 people -- from the town of Jamestown by helicopter, CNN affiliate KCNC-TV reported.
In Larimer County, Sheriff Justin Smith surveyed the heavily damaged Big Thompson Canyon by air Friday. Some people remain stranded in homes there, he said, "How we're going to get them out -- it's going to take a damn long time."
However, he said the break in the rain allowed school buses to begin evacuating students who had been stranded at a school.
CNN's Ben Brumfield reported and wrote from Atlanta; Nick Valencia reported from Longmont, Colorado. George Howell reported from Boulder; and Ana Cabrera reported from Lyons. CNN's AnneClaire Stapleton, David Simpson, Jack Hanna, Janet DiGiacomo, and John Branch contributed to this report.
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