The tropical storm that left thousands temporarily stranded and at least one person dead in the mountains of Southern California brought a near 500-year rain event, dumping about 4 inches of rain in just one hour, according to the National Weather Service.
The rainfall was recorded at Mt. Baldy Fire Station between 3:45 and 4:45 p.m. on Sunday. The NWS uses the "rain event" terminology to describe the odds a certain amount of rainfall will occur at any point during a year. In a 500-year rain event, the chance that area will receive four inches of rain in an hour again is only two tenths of one percent — an extremely rare occurrence.
The sudden torrent caused mud and debris to slide off mountainsides and swell streams, flipping cars and burying homes.
Crews continued to work on Tuesday to dig out and clean up vehicles and buildings, while displaced residents were directed to local shelters, according to San Bernardino County Fire spokesman Ryan Beckers.
"In the Forest Falls area, many homes have been inundated with mud and debris that came sliding down the mountain from all this rainfall, and there's just going to be a lot of digging out and clearing out and sort of getting to the bottom of this huge, huge pile of mud, basically," Beckers said.
Five homes have sustained structural damage and three outbuildings have been completely destroyed, Beckers said, adding that further damage assessments were still under way. Beckers said between 50 and 60 homes had minor damage, which he said was being described as "yard damage from the debris flow."
About 465 Southern California Edison customers have been without power since Monday morning. The power was cut as part of the recovery effort.
"During the flooding, the power never actually went out, but some lines were damaged and in order for our crews to get through that last sort of wall of mud to open up the road, Edison needed to come in and shut down the power so they could repair the lines," Beckers said.
Power was expected to be restored by 8 p.m. Tuesday.
The Red Cross planned to open a shelter at Magnolia Recreation Center (651 W. 15th Street in Upland) for about 20 residents displaced from the Mt. Baldy area.
Beckers said about 19 homes were damaged or destroyed in that area.