A powerful storm system along the West Coast is dropping heavy snow in the mountains of the Sierra Nevada and the Cascade Range, while bringing substantial wind and rain at lower elevations from the Pacific Northwest to Southern California.
Mountain snow will be measured in feet with this storm, with more than five feet possible in the higher elevations of the Sierras. The lower elevations of central California have the greatest risk of flooding.
“Total snow accumulations will be tremendous,” the National Weather Service in Sacramento wrote in a forecast discussion, calling it “easily the biggest snowstorm so far this season.”
Sure, you've heard of "Elf on the Shelf", but how about "Fountain on the Mountain"?— National Weather Service (@NWS) December 13, 2021
5+ FEET of snow are forecast for parts of the Sierra Nevada through Wednesday, making travel in the mountains extremely difficult to impossible.https://t.co/VyWINDk3xP pic.twitter.com/1aFtmE6M9e
They also warned of “extremely difficult to impossible travel” over the mountains. Strong winds are expected to cause whiteout conditions and downed trees.
According to the Center for Western Weather and Water Extremes, a frontal system is merging with a weak atmospheric river over Northern California to prolong and intensify precipitation in the state. The long duration of the event combined with low freezing levels is ratcheting up snow potential, resulting in “treacherous travel conditions but an extremely beneficial contribution to the depleted California snowpack.”
Both a drought-denter and a flood risk
The storm marks a pattern change from a warm and dry spell that persisted for much of November, prompting worries about another dry winter on top of existing drought. The month ended up the second warmest on record in California, and in several other Western states, according to a monthly climate report from the National Oceanic and Atmospheric Administration.
As in much of the West, California snowpack has been running far behind for the time of year and sits at just 19 percent of normal statewide as of Dec. 10.
Along the coast at lower elevations, the storm system will bring heavy rain and the potential for flooding and debris flows as the atmospheric river stalls in the vicinity of the Central Coast before continuing into Southern California late Monday and into Tuesday.
“That’s something that we’re continuously watching as to where exactly the heaviest rains will be,” Rick Canepa, a meteorologist with the National Weather Service’s San Francisco Bay area office, said in an interview Sunday. “A few tens of miles can make a big difference.”
The threat for significant heavy rainfall continues in California. A Moderate Risk of Excessive Rainfall is in effect for Monday with a Slight Risk Tuesday. The most concerning impacts are flash flooding along coastal mountain ranges and debris flows over burn scars. pic.twitter.com/VVGmaGfDkK— NWS Weather Prediction Center (@NWSWPC) December 13, 2021
Current forecasts suggest the Central Coast and the Santa Cruz mountains will see the brunt of any impacts as the system intensifies Monday. A widespread three to six inches of rain are expected, with some locations receiving up to 10 or 12 inches, Canepa said.
The Weather Prediction Center has highlighted the risk for excessive rainfall and flooding along the Central Coast, and flash flood watches were issued through Tuesday for the 2020 Dolan Fire burn scar near Big Sur and the 2021 Alisal Fire near Santa Barbara. Officials have issued preemptive evacuation warnings in some areas at high risk for debris flows.
Portions of Highway 1, which hugs the rugged Big Sur coastline, were closed ahead of the storm.
Highway 1 will close tonight at 8 pm until Tuesday, Dec. 14 from Ragged Point to Deetjen’s Big Sur Inn. The closure was requested by law enforcement so residents in the evacuation warning areas near the Dolan Fire burn scar can evacuate safely before rainfall. @CaltransD5 pic.twitter.com/8tkrV2ryNG— Caltrans District 5 (@CaltransD5) December 13, 2021
Last January, a debris flow washed out a significant portion of the famed highway just below the Dolan Fire scar when a stalled atmospheric river brought intense rain to the area.
“We’re expecting to see rapid rises on rivers, streams and creeks, especially if the system stalls,” Canepa said.
As the storm moves south Tuesday, much of Southern California could see one to three inches of rain at the coast and in valleys, and three to six inches in the mountains and foothills, along with one to three feet of snow at the highest elevations. That’s significant precipitation for the region and a welcome shift from a very warm and dry November, which saw increasing fire danger and little or no rainfall at many locations.
Shifting to a wet pattern in the West
This week’s atmospheric river may be the beginning of a generally wet pattern that could continue for much of the month. Another weaker system will move through the state later this week, and the Climate Prediction Center outlook indicates a good chance for more rain and snow next week.
Canepa said October’s atmospheric river combined with the current shift to a cold, wet pattern amounts to a strong start to the water year at last for Northern California.
“Even by mid- to late December, we may have a blast of Arctic air move down the West Coast if current model trends verify,” he said, though he cautioned that the forecast is a long way off. “For rainfall and snow … it’s certainly excellent news.”