100 mph winds, 3 feet of snow, debris flows all possible in Tahoe area

A moisture-rich October storm is forecast to wallop the northern Sierra Nevada and Lake Tahoe area with heavy rain, snow and wind into Tuesday, potentially triggering debris flows on 2021 wildfire burning scars, including the hillsides torched by the Dixie and Caldor fires, the National Weather Service said.

The atmospheric river aimed at Northern California was expected to funnel moisture toward the region, lowering mountain snow levels to elevations of about 6,000 feet on Monday and making travel difficult over mountain passes.

The weather service issued a winter storm warning for elevations above 6,000 feet in the Sierra for Sunday night through Monday night. "Periods of heavy snow and gusty winds will create hazardous travel conditions," the weather service said.

Chain control was in effect over Donner Sunday morning and officials said hazardous driving conditions could develop on Interstate 80 between Reno and Truckee, Calif.

An atmospheric river is a long, wide plume of upper-atmosphere moisture pushed ashore from the Pacific Ocean. The weather service said the storm is likely to be moderate in strength, and it's unusual for Northern California to see a system of this strength in October.

A flash flood and debris flow warning was issued Sunday morning for the Dixie fire region. "Heavy rain is starting to move in and this will cause debris flows within the Dixie fire," the weather service's Sacramento office said at about 6 a.m. The warning is in effect through 3 a.m. Monday with rainfall rates of up to an inch an hour expected.

A flood watch was issued Saturday night through Monday for the central Sierra Nevada, including Reno and regions affected by the Dixie, Caldor, Numbers and Tamarack fires.

A debris flow is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope with the help of gravity. A debris flow can overwhelm homes and vehicles in its path. Areas recently burned by wildfires are particularly susceptible to flash floods and debris flows during rainstorms.

The weather service's Sacramento office said Saturday debris flows are likely on 2021 burn scars, especially on steep slopes, on Sunday.

"If you are in the vicinity of a recent burn scar and haven't already, prepare now for likely debris flows," the weather service said. "If you are told to evacuate by local officials, or you feel threatened, do not hesitate to do so. If it is too late to evacuate, get to higher ground."

Truckee and South Lake Tahoe could receive up to 2 to 4 inches of rain, while Reno could see up to 2 inches. Creeks and streams could flood in foothill and urban areas.

Winds could gust to 100 mph on mountain ridges, with 3 feet of snow at higher elevations above 7,000 feet, the weather service said.

The Associated Press contributed to this report.