(Adds condition of professor, voluntary evacuations, second avalanche)
By Laura Zuckerman
Feb 28 (Reuters) - An avalanche swept down a mountainside into a Missoula, Montana, neighborhood on Friday and crushed the two-story home of an elderly couple, burying them in snow along with a young boy nearby, but all three were later found alive, police said.
Large, destructive avalanches are rare in populated areas of the United States, as is the survival of anyone trapped under the snow for as long as it took rescuers to find the three victims of Friday's slide.
The elderly man was conscious and talking when he was plucked from his snow-engulfed home some 2-1/2 hours after the slide occurred, police and hospital officials said.
He was identified as Fred Allendorf, a professor emeritus in conservation and genetics at the University of Montana campus several blocks away, and was listed in critical condition at St. Patrick Hospital in Missoula.
His wife also survived and was taken to the same hospital. She was not immediately identified, and neither of their ages were given.
The 8-year-old boy, who was elsewhere in the vicinity of the avalanche when it struck, was pulled separately from the snow by rescue workers and volunteers digging with shovels more than an hour before Allendorf was found, city police Sergeant Travis Welsh said.
Welsh said he was not aware of any relationship between the boy and the couple whose house was struck. The child also was hospitalized. Neither his condition nor that of Allendorf's wife was immediately known.
The slide roared into the community at about 4 p.m. local time (2300 GMT) during a blizzard that forced the closure of the University of Montana campus and public schools in Missoula.
Local authorities had warned earlier in the day of a heightened avalanche risk in the area, which lies at the foot of a peak known as Mount Jumbo, northeast of downtown.
Police later went door to door in the neighborhood, warning residents that unstable snow in the high country could unleash further slides and urging them to leave their homes as a precaution, Welsh said.
A second avalanche raced down another mountainside in the southern end of the city, but there were no immediate reports of property damage or injuries.
Finding Friday's three avalanche victims alive more than an hour after they were buried was fortuitous given the typical odds of survival.
According to the U.S. Forest Service's National Avalanche Center in Utah, half of all avalanche victims completely buried in snow die within the first 25 minutes, and 95 percent are dead within two hours.
Three-quarters of victims die from asphyxiation, the result of being cut off from a fresh supply of air under the snow, and a quarter from trauma caused by debris or hitting fixed objects like trees. Just 2 percent live long enough to die of hypothermia, experts say.
Avalanches in residential areas are exceedingly rare, with 99.9 percent of all snow slide accidents occurring in the back country, and most victims being snowmobilers, skiers and snow-boarders. A typical dry-snow avalanche travels downhill at about 60 to 80 miles per hour (97 to 129 km per hour). (Reporting by Laura Zuckerman in Salmon, Idaho; Writing by Steve Gorman; Editing by Cynthia Johnston and Lisa Shumaker)