Thanksgiving is still two weeks away, but the Bay Area could be about to shift into winter weather mode.
A storm from the Gulf of Alaska is expected to bring the first significant rainfall of the winter season to Northern California next week.
Forecasters say it is still early, and the numbers could change. But on Thursday the National Weather Service predicted there is roughly a 70% chance that parts of the Greater Bay Area — including the North Bay, San Francisco, the Santa Cruz Mountains and Big Sur — will receive 2 inches or more of rain next Wednesday through Friday.
San Jose and the East Bay are expected to receive less, with specific forecasted totals coming in the next few days. In the Sierra, the storm is forecast to dump at least 1 foot of snow above 6,000 feet, the National Weather Service says. That would more than double the total that has fallen so far since Oct. 1, bringing it up to historical averages for this time of year after a relatively dry autumn so far.
“This looks like the first widespread rainfall event of the season,” said Alexis Clouser, a meteorologist with the National Weather Service in Monterey. “We’ve had a few storms that have brought light rain, but we haven’t had a good soaking rain across the Bay Area. This appears to be that.”
There is a 50% chance that the storm will be an atmospheric river of at least level 1, the mildest of 5 levels, according to the Center for Western Weather and Water Extremes at UC-San Diego.
📢Big pattern change next week bringing the 1st widespread precipitation event this wet season. Moderate to heavy rain over lower elevations & moderate snow for the Sierra. The forecast will change between now & then, but increasing conf for several inches of rain #cawx (1/2) pic.twitter.com/k5aWJZnRxz
— NWS Bay Area 🌉 (@NWSBayArea) November 9, 2023
On Friday, the first ski resort will open for the winter season in the Lake Tahoe area when Mt. Rose Ski Tahoe, six miles northeast of Incline Village, welcomes skiers to a terrain of machine-made snow and what resort officials hope will a fair amount of new natural snow on top a week from now. Mount Rose has the highest base elevation, at 8,260 feet, of any Tahoe-area ski resort.
“We benefit from the colder temperatures we experience, which often translates to better snow quality,” said Mike Pierce, a spokesman for Mt. Rose Ski Tahoe. “Our snowmakers have been busy making snow from top-to-bottom, and we’re excited to get the winter season underway.”
In the Bay Area, any significant amount of rain close to the 2 inches forecasted will all but end fire season.
Because last winter was very wet, with the biggest Sierra snow pack in 40 years and soaking conditions across much of the state that filled reservoirs and broke three years of previous drought, 2023 has seen a very mild wildfire year. Moisture levels in the soil and in plants has remained high all year.
So far this year, 319,070 acres has burned statewide in wildfires, according to Cal Fire. That’s just 20% of the average of the past five years. By comparison, in 2020, more than 4.3 million acres burned — 13 times more land than has burned this so far year.
“If we get a couple of days of good rain, fire risk is going to be very low,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay. “You can always have an outlier, but if we get 1 inch that will probably put an end to fire season in most places.”
Scott Stephens, a fire scientist at UC Berkeley, has said that after an area receives a storm of about 1 inch or more of rain in the fall, that unusually triggers the end of most fire risk through the winter.
Null analyzed weather records back to 1849, when they first began in San Francisco, and found that the first storm delivering 1 inch or more of rain has often varied between October and December in the Bay Area. On average over those 174 years, it has occurred on Nov. 29 in San Francisco, and in Los Angeles, where weather records date back to 1877, it’s Dec. 16.
A new wrinkle this year is El Niño.
El Niño conditions — the warming of ocean waters off South America that can alter weather across the globe, including California’s summer temperatures and the amount of rain it might receive next winter — are emerging in the Pacific Ocean for the first time in 4 years, and continuing to strengthen.
On Thursday, NOAA, the parent agency of the National Weather Service, issued a monthly update saying that there is a greater than 55% chance of at least a “strong” El Niño persisting from January through March 2024. There is a 35% chance of the event becoming “historically strong” for the November-January season.
El Niño conditions don’t guarantee wet winters for California. Historically, they have increased the likelihood of wet winters in Southern California, but in the north, have not shown a clear pattern.
However, there have been three very strong El Niño winters, with unusually warm water in the Pacific — 1982-83, 1997-98 and 2015-16.
In two of those three years, 1982-82 and 1997-98, Northern California has received large amounts of rainfall, which triggered flooding and other problems.
“As far as autumns go, 2023 has been mercifully calm,” UCLA climate scientist Daniel Swain wrote in a blog post on Wednesday. But, he added, with next week’s storm system developing: “that relatively quiet weather pattern won’t last much longer in California”
