Driving down from skiing in Alta, Jessica Hernandez noticed a little snow powder blowing off the side of the mountain near Tanner Flats.

It was the only warning she and her boyfriend, Hayes Hill, got.Suddenly, a thunderous snowslide swept the 20-year-old's Bronco across the road, pushing it within 3 feet of the edge. The force caused both side windows to explode as snow poured in waist deep.

"I saw it, heard it, and felt it all in one moment," said Hill, 24, who was riding in the passenger seat. "There were thousands of pounds of snow just blowing on my face all of a sudden."

Authorities say Hernandez and Hill are lucky that Thursday's avalanche caught them at the end of its descent. The Bronco was only partially buried and the couple was able to exit the car before help arrived.

Experts at the U.S. Forest Service's Utah Avalanche Forecast Center estimate the slide dropped about 3,500 vertical feet and ran between 3 to 5 feet deep and 450 feet wide.

"That's about a Class 4 on a scale of 1 to 5," center public information officer Keith Clapier said. "That's a fair amount of snow and a long ways for an avalanche to slide."

The slide broke off as a slab at the top of the ridge, Clapier said. Avalanche control crews had done work in the area earlier in the day Thursday but were unable to reach a portion of the west side of the mountain, where it appears the slide began. It was most likely triggered by heavy new snowfall combined with high ridgeline winds, Clapier said.

Another avalanche warning was issued for Little Cottonwood Canyon Friday morning, and all canyon roads were closed at press time.

Hill sustained minor facial cuts and other scratches from the shattered glass and refused to be treated by paramedics, said Salt Lake County Fire Capt. Bill Brass. Hernandez was badly shaken but not injured.

A similar utility vehicle traveling behind the Bronco was swept off the road onto the embankment, but its two occupants escaped unharmed, Brass said. The two men, whose names were not available, were helped out of their vehicle by search and rescue crews.

"I'm all right. It looks worse than it is," said Hill, while waiting for a deputy sheriff to give him and Hernandez a ride home to Salt Lake City.

Although Hill was on the side of the car closest to the avalanche, he did not notice anything until it struck.

"I first heard the window shatter," he said. Then everything happened simultaneously until the car came to rest about half-covered with snow. "It almost fell off the edge."

Prior to the 2:45 p.m. slide, avalanche control crews had "shot" the area at least twice, said Salt Lake County Sheriff's Sgt. Jim Potter. The procedure consists of shooting explosives into avalanche prone areas, causing excessive snow pack to fall off the higher mountain altitudes in a controlled way.

"This is always a problem we have, especially during heavy snow like this," Potter said. "They check these slide areas constantly, but there is no way to tell when this (type of slide) is happening."

Travelers on both ends of the road witnessed the avalanche and stopped to help the victims, Potter said. Snow covered at least 100 feet of the road, leaving it impassable until front-end loaders arrived.

It took work crews a few hours to clean the road, but, while search and rescue crews scanned the area for other possible victims, a second smaller avalanche above the first prompted authorities to clear everyone out.

Additional shots were then fired to force the remaining unstable snowpack down.

Ski resort patrons and other eastbound travelers stuck on the east side of the avalanche were allowed to descend the canyon at about 6:30 p.m., Brass said. Otherwise, the road was expected to remain closed overnight.

More than 4 feet of snow were reported in the Alta area since Tuesday's storm blew over the Wasatch Front, Brass said. Up to 10 additional inches were expected to fall overnight.