A tropical storm that caused deadly floods and mudslides in California’s San Gabriel Mountains on Sunday was the kind of event seen only once about every 500 years, the National Weather Service said.

Starting at about 2:45 p.m., the storm dropped nearly 4 inches of rain onto Mount Baldy in a single hour, triggering mudslides and floods that killed one motorist and severely damaged more than 30 homes.

The deluge also cut off the community of Forest Falls after slides of up to 10 feet high buried the lone road connecting it to California 38. San Bernardino County firefighters were still assessing the damage Tuesday but said about 100 buildings had been damage.

Roughly 60 structures took some sort of hit to their exteriors, while six had moderate or major structural damage, officials said. Three outbuildings — another term for a shack — were destroyed. Two dogs were also killed in Sunday’s deluge.

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At Mount Baldy, 33 homes sustained moderate to major structural damage, said firefighter Chris Prater. At least six of the homes were red-tagged, meaning they were uninhabitable, officials said.

The community of Highland was also damaged by flash flooding.

The rain and subsequent floods overwhelmed washes, creeks and roads. A 48-year-old El Segundo man, Joo Hwan Lee, was killed when his vehicle went off a mountain road and into a creek Sunday.

Forecasters said that at times the rain was so thick, drivers would have been unable to see far in front of them.

Residents said they couldn’t believe the storm’s fury.

At Forest Falls, Doug Roath, 48, recalled hearing a roar above his house after a half-hour of rain. He evacuated to higher ground and brought a family of nearby hikers with him. From above, they watched the rocky slurry crash into his home.

“It had us surrounded. These people had two little kids with them and asked if they were going to die,” he said. “My doghouse with my dogs in it was just spinning in a whirlpool.”

Roath rescued the dogs when the flood eased. He spent Monday sifting through the sediment, picking up belongings. Community members helped him dig out, and town residents have offered him financial help and housing.

The storm was the product of an “orographic flow” — when moisture-saturated air is pushed up by a mountain’s natural topography and is squeezed like a sponge. A wave of tropical air blown north from Central America gave the storm extra ammunition, climatologists said.