The rain and snow is good news in a state experiencing a severe to extreme drought, but prompts flash flood concerns because of how quickly it’s set to fall.
Winter storm warnings blanket the Sierra Nevada, where wind gusts of 100 mph or more could yield extreme blizzard conditions amid prolific snowfall rates during the back-to-back storms.
In the San Francisco Bay area, high wind watches have been hoisted for the potential for 60- to 70-mph wind gusts Tuesday and Wednesday, with two to four inches of soaking rainfall on the way, with more rain and wind likely later this week. Flash flood watches are also in effect.
The system will affect the Los Angeles metro area late this week with high winds and heavy rainfall, and bring heavy precipitation to the Sacramento River Valley.
A soaking atmospheric river
Driving the wild weather is a strong jet stream, or corridor of winds in the upper atmosphere, that will be screaming over California through the end of January. That will yank a narrow corridor of moisture-rich air toward the coast ahead of a strong storm system diving southeast from the Aleutians. That juiced-up conveyor belt of moisture is called an atmospheric river. Such events are responsible for the majority of California’s cold season rainfall, but can be extremely destructive when they bring extreme amounts of precipitation in short time spans.
Moisture from the atmospheric river will begin to brush against the coast shortly after lunchtime Tuesday, forced upward and concentrated by the mountains. That will lead to extremely heavy snow above 5,000 feet, with rain in the lowlands.
A strong atmospheric river such as this event can transport an amount of water vapor that’s about equivalent to 7.5 to 15 times the average flow of water at the mouth of the Mississippi River, according to a National Oceanic and Atmospheric Administration fact sheet.
Wind and flood concerns in San Francisco, Los Angeles
For San Francisco, torrential downpours and damaging winds will accompany the arrival of the atmospheric river Tuesday night, where the local National Weather Service office wrote “gusts in the 60 to 70 mph are definitely possible over the higher terrain and immediate coastline.” A few thunderstorms are possible, too, with embedded lightning strikes and small hail.
“If you live near a recent burn area from the 2020 fire season, or know someone who does pay attention the forecast Tuesday through Thursday of this week,” warned the Weather Service. Concern is growing for potential debris flows as well, since recently burned areas can’t absorb as much water as can forested areas.
Farther east, cold temperatures may briefly bring snow levels down to 2,000 feet, with a hard freeze watch even up for the San Joaquin Valley.
The atmospheric river will get hung up on the central and southern Sierra Nevada before pivoting southward and scraping along the coast toward Los Angeles by late week. A general 1½ to four inches of rain is likely, with up to eight inches in San Luis Obispo and San Bernardino counties.
Lesser rainfall is likely to fall in the Central Valley, generally on the order of one to two inches.
Thereafter, a renewed fire hose of moisture will buffet the coast starting late Friday night into Saturday.
Extreme moisture leads to blockbuster mountain snows
When all is said and done, the entire Sierra Nevada, which has about half of its typical snow pack for this time of year, can expect blockbuster snow totals this week.
Since atmospheric rivers transport most of their moisture several thousand feet above the ground, the greatest rain and snow totals will occur in the mountains, where summits jutting into the moisture stream will force the air to rise, cool, condense and dump rain and snow.
The National Weather Service was calling for a broad four to six feet of snow at the highest peaks, including at popular ski destinations such as Mammoth Lakes, and two to four feet in places such as Lake Tahoe.
Some computer models indicate a few mountain peaks could see up to 10 feet of snow.
Winds gusting over 50 mph in the lower elevations and topping 80 mph in the mountains could bring visibilities to near zero and grind travel to a dangerous halt, and snowfall rates of three inches per hour are possible.
The dynamic storm system is even expected to bring a rare snowfall to the northern Sacramento Valley, with 3 to 6 inches likely late Tuesday into Wednesday. Snow levels could descend to 500 feet elevation, visiting cities like Redding. A winter storm watch is in effect.
The sudden influx of heavy rain and snow is in stark contrast to the dry pattern that’s dominated most of the winter. But it won’t be enough to end the drought that’s settled into the state and much of the West.
“It’s completely different,” said Bill South, a senior meteorologist at the National Weather Service in Hanford, Calif., in an interview. “This winter, most of November and December was just dry, dry dry. Here in the San Joaquin Valley, we usually get more precipitation to this point in the [season]. But [this season] we had one event. It’s been dry as a bone all winter long.”
The theme has been feast or famine — and this week, the needle is pointed toward feast.
The Weather Service in Reno, Nev., warned that “even a walk can be deadly in these [predicted] conditions.”
Impacts on water resources
The Sierra’s snowpack is a vital component of California’s water resources and has been significantly depleted in recent years. While this week’s extreme snowfall will make a dent in the deficit, there remains a long way to go to make a meaningful difference.
“This will help the snowpack,” said South. “Comparatively speaking, it will be significantly helpful, and every little bit matters … but I don’t think this single event will [fix it] for us.”
Climate change is exacerbating the pattern of all-or-nothing precipitation extremes in California. A 2018 study in the journal Nature Climate Change found that as the climate warms, a dramatic uptick in precipitation volatility is expected in California, despite little change in yearly averaged precipitation amounts.
That tendency will have a bearing on drought and wildfire concerns, as well as water resources and management, in the years ahead.