As if someone just flipped a switch, California’s meager rainy season will transition from deficit to abundance this week as a parade of storms strike the state. Record rain and snow totals are possible, which will help reverse drought conditions but also bring a myriad of hazards to the region, including flash flooding, mudslides and avalanche risks.
“This is a blockbuster winter storm event for us” said Mark Deutschendorf, a meteorologist at the National Weather Service in Reno, Nevada. “Travel is not recommended from late Tuesday night to Friday across the Sierra and Lake Tahoe area, including western Nevada. If you do venture outdoors, be prepared to spend time an extended period of time in your car.”
This is just another example of California stuck in a “weather whiplash” – moving from one weather extreme to the opposite extreme.
Feet of snow forecast for Sierra
A dramatic shift in scenery will occur as winter kicks into high gear.
“Parts of the Sierra will need a yardstick to measure the forecast snow in the forecast,” tweeted the NWS in Sacramento Monday.
The foothills could see 6 inches to a foot of snow while the mountains could see anywhere from 3 to 5 feet. In some areas, up to 7 feet is forecast across the Sierra Nevada’s favored terrain through Friday.
“This storm remains projected to be a major headache for the region and will bring heavy snowfall to the Sierra as well as portions of western Nevada,” said NWS Reno.
A backcountry avalanche watch is in effect for much of the region, including the Tahoe Basin, through Friday. The combination of heavy snowfall and strong winds will lead to extreme avalanche danger, says the NWS.
Ridgetops in the Sierra could see gusts exceeding 125 mph.
This heavy snow is all thanks to a category 3 – out of 5 – atmospheric river.
Atmospheric rivers are long, narrow regions in the atmosphere – like rivers in the sky – that transport water vapor, according to the National Oceanic and Atmospheric Administration.
The moisture laden air from this atmospheric river (AR) event will be forced to rise over the mountains’ crest and loft upwards, cooling as it rises. As the air cools, it condenses, forming heavy precipitation, which will flirt with snowfall duration records.
“The Owens Valley (Great Basin Desert) could receive near-record amounts of snow over a two-day period. Their previous 48-hour snowfall record was 23 inches set back in 1969,” NWS Meteorologist Jenn Varian told CNN.
Atmospheric river creates flash flood potential
As the atmospheric river points its firehose along the central coast, excessive rainfall totals of 4 to 8 inches will threaten flash flooding in the region.
San Francisco averages 4.19 inches of rain during January. Suppose the atmospheric river concentrates the heaviest rain across the Bay Area: In that case, it’s possible the city could receive a month’s worth of precipitation in just a few days.
Depending on the precise placement of the atmospheric river, coastal locations south of San Francisco to Santa Barbara could get between 5 to 10 inches of rain with locally higher amounts. Lesser but still significant rain totals of 1 to 2 inches are likely from Los Angeles to San Diego.
Evacuations underway as risk for debris flows increases
This heavy rainfall combined with the burn scars of the record wildfire season of 2020 brings a dangerous risk of mudslides and debris flows.
Forecasters are especially concerned about Monterey County and Santa Cruz County where evacuations are underway, according to a statement released by California’s Department of Forestry and Fire Protection.
Intense wildfires can compromise soil structure, making sloped terrain more vulnerable to debris flows and mudslides during heavy rain events.
If enough rain falls upon a recent burn scar, a torrent of mud, rock and debris can cascade downhill and put communities in danger. Properties directly affected by recent fires or those located directly downstream from burn areas are at the most risk.
The storms could relieve current drought conditions
Although atmospheric rivers can be hazardous, residents rely upon them to bring beneficial precipitation to the region.
“Thirty to 50 percent of annual precipitation on the west coast occurs in just a few AR events,” according to NOAA.
California’s winter snowpack, fueled in part by atmospheric rivers, is crucial to the state’s water supply. Warmer spring temperatures melt the snow and fill reservoirs, so freshwater is available during the dryer summer months.
This AR event can be viewed as much needed relief because the current statewide snowpack is just 40% of average to date.
This storm, along with an active weather pattern forecast into early February, will help alleviate drought conditions across California and the Great Basin.
Severe drought conditions cover nearly 80% of California.
However, just like anything in moderation, it’s possible to have too much of a good thing.
This ‘weather whiplash’ will happen more often
Research shows that the US West Coast, specifically California, can expect more climate extremes in the future.
Precipitation extremes – such as the current atmospheric river – will intensify while dry spells are likely to become longer and more frequent, according to climate scientists.
“In a place like California, we really need to be thinking about both risks [drought and flood] simultaneously,” said Daniel Swain, a University of California, Los Angeles climate scientist and lead author of a study on the subject.
Dramatic swings are becoming more common and will continue to do so in the coming decades thanks to man-made climate change, according to a study published in the journal Nature Climate Change.
Drastic swings from extremely wet to extremely dry and vice versa will be nearly twice as likely, occurring on average once every 25 years, by 2100.
Climate variability is due, in part, to human activity such as the burning of fossil fuels.