Colorado floods: More than 500 still unaccounted for as 'devastating' rain looms

• Heavy rain on "burn scar" could cause more flooding in El Paso County
• Boulder spared Saturday evening but faces possibly "devastating" rain Sunday
• Missing woman is presumed dead, in addition to four confirmed dead

Boulder, Colorado (CNN) -- Authorities who still haven't reached all the stranded victims of floods in northeastern Colorado are bracing for a new round of storms Sunday.

Boulder County officials said they received a forecast of up to 4 inches of rain for Sunday afternoon.

Sheriff Joe Pelle said rain that heavy "could be devastating."

Authorities are worried that any additional water on ground already soaked by up to 15 inches of rain will cause more flooding and dislodge mud and debris.

At least four deaths have been blamed on the flooding, and a fifth person is presumed dead. More than 500 were "unaccounted for," although authorities cautioned that designation included people who simply have not yet contacted concerned relatives elsewhere.

Elected officials were looking past the crisis to plan the recovery.

Gov. John Hickenlooper said he spoke by phone with U.S. Transportation Secretary Anthony Foxx, who "was adamant that the $5 million that was released Friday was just the beginning" of federal assistance.

"We're going to come back and rebuild better than it was before," the governor said.

Hickenlooper said experts from Vermont will arrive next week to share lessons learned about improved road-building in the wake of Hurricane Irene.

Boulder County alone will need an estimated $150 million to repair 100 to 150 miles of roadway and 20 to 30 bridges, county transportation director George Gerstle said. The repair bill will be "10 to 15 times our annual budget," he said. The governor aid he saw many damaged roads with "not just the asphalt taken away, but the entire roadbed, and bridge after bridge missing."

A helicopter surveillance mission Saturday carrying Hickenlooper and members of Colorado's congressional delegation was diverted twice to pick up people waving to be rescued.

After the officials' delayed arrival at a Boulder airport, U.S. Sen. Mark Udall said, "That dog and the cat and those seven people on those two helicopters didn't ask us whether we were Democrats or Republicans." He promised a bipartisan push in Congress for federal aid for flood recovery.

Some storms appeared Saturday slightly east of the most flood-damaged areas, pounding southeast Denver with 1.73 inches of rain in less than 30 minutes.

But skies were clear for much of the day elsewhere, allowing rescues and a more complete count of those not yet located.

The Laramie County sheriff's office said that about 350 people were unaccounted for in Larimer County. That number jumped sharply Saturday afternoon as rescuers reached more empty homes, even though authorities believe those residents got to safety.

In neighboring Boulder County, 231 people were on the "unaccounted for" list as of 7 p.m. MT (9 p.m. ET), said Gabrielle Boerkircher, spokeswoman for the county office of emergency management. She said that number was fluctuating as some people were found safe even as the county received new requests to locate people.

Members of the Colorado National Guard help Boulder County authorities evacuate residents of Lyons, Colorado, to Longmont, Colorado, on Friday, September 13. Flooding has hit the area hard, washing out roads, damaging bridges and destroying homes.

The National Guard had rescued more than 1,200 people in Boulder County by Saturday afternoon, Lt. Col Mitch Utterback said.

Boulder County Sheriff Joe Pelle said authorities have to be "realistic" about the chances that the death toll will rise as rescuers penetrate further into isolated areas.

No new deaths were confirmed Saturday, but Larimer County officials said a 60-year-old woman was presumed dead after witnesses saw her being swept away by waters that demolished her home. Neighbors tried unsuccessfully to rescue the woman, said Nick Christensen, executive officer of the Larimer County Sheriff's Office. Her body had not been recovered.

The four confirmed deaths included a man and a woman, both 19, who were swept away after leaving their car Thursday in Boulder County. Authorities said the woman left the car first, and the man jumped out to try to save her. Authorities recovered both bodies.

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.

CNN's David Simpson reported and wrote from Atlanta; Nick Valencia reported from Longmont, Colorado. George Howell reported from Boulder.Ana Cabrera reported from Lyons. CNN's Jack Hannah, Janet DiGiacomo, and John Branch contributed to this report.

Dave Jackson closes a mailbox with his foot after delivering the mail to a home surrounded by water from the flooded Cheyenne Creek in Colorado Springs, Colorado, on Friday, September 13. Flooding in Colorado has washed away roads and bridges and flooded homes. Authorities warned more rain was on the way, threatening more flooding. At least four people have been killed and 218 are unaccounted for, officials say.