Bay Area rainfall: When’s it coming and when should we start to worry?

Normally between Oct. 1 and mid-November, if historical averages are any guide, the Bay Area has received nearly 2 inches of rain, and Los Angeles and Fresno each have received about an inch.

But so far this year? None.

To be sure, there was one-hundredth of an inch recorded in San Jose and San Francisco — about the thickness of a few sheets of paper — over the past six weeks. But nearly every city from Sacramento to Silicon Valley to San Diego is showing lots of zeros in the rainfall column for the first two months of California’s winter rainy season.

Fire risk remains high. The air is gritty. Lake Tahoe ski resorts are scheduled to start opening for the season later this week and are hustling to make snow. And no storms are forecast for at least the next 10 days.

“We’ve been high and dry,” said Matt Mehle, a meteorologist with the National Weather Service in Monterey. “The outlook over the next week or so isn’t good. There’s a chance of some drizzle along the coast, but nothing of note.”

Remember the “Ridiculously Resilient Ridge?” That was a wall of high pressure air that parked off the West Coast for an unusually long time between 2012 and 2016, blocking storms and causing California’s historic drought. When that ridge went away in 2017 and soaking atmospheric river storms, also known as “Pineapple Express” storms, barreled through, the drought was broken.

This fall, a similar ridge of high pressure has been sitting off the West Coast.

Mehle said that long-range computer models show some hope that the ridge may break down in a few weeks. But usually, any forecasts beyond a week or so aren’t particularly reliable.

“Maybe the last week of November,” he said. “We see a pattern change which could usher in some storm systems that could bring some rain to the Bay Area. But confidence on that extended forecast is pretty low.”

How does this dry autumn compare historically?

The amount of rain San Francisco received from this July 1 to Oct. 31 — .12 inches — ranks as the 21st driest such period back to 1850, according to Jan Null, a meteorologist with Golden Gate Weather Services in Saratoga.

Null noted that San Francisco’s all-time driest and all-time wettest seasons started out dry, like this year. The driest was the winter of 1850-1851 with a July-to-October total of 0.33 inches and a final seasonal total of just 7.42 inches.

By comparison, the winter of 1861-1862, after seeing only .02 inches through October, finished with a deluge of 49.27 inches. That winter was so drenching that Leland Stanford, who had just been elected California governor, had to take a row boat through the streets of Sacramento to give his inaugural address.

Because of the legendary floods, the state Legislature and state Supreme Court moved to San Francisco. The Legislature moved back to Sacramento, but the court remains in San Francisco to this day.

Null noted that fall last year was nearly as dry, yet rainfall totals around the state, along with the Sierra snow pack, finished in good shape. Traditionally, California’s wettest months are January, February and March.

“It’s not time to panic yet at all,” he said. “We still have lots of time left in the winter rainy season.”

The good news is that because of significant rain and snow last winter and the winter before, California’s major reservoirs are in pretty good shape.

Shasta Lake, the state’s largest reservoir, is 71% full, or 120% of its historical average for this time of year. The second-largest, Oroville, is 56% full, or 93% of normal. And Folsom is 58% full, or 115% of average.

The bad news is that every day with dry weather means another day of fire season.

“Fire season isn’t done,” said Scott McLean, deputy chief of Cal Fire, the state’s primary firefighting agency. “Our staffing remains the same. I know we sound like a broken record, but people need to pay attention. We all need to be prepared.”

As of Monday, no major fires were burning around the state, although the Kincade Fire, which burned 77,758 acres in Sonoma County, destroying 374 buildings and causing the evacuation of roughly 188,000 people before being fully contained last Wednesday, was still fresh in the minds of many.

Temperatures should remain about 10 degrees warmer than normal Tuesday, with some cooling Wednesday and Thursday, said Mehle. But humidity levels across Northern California are back up to about 25% to 30% in many places, a big improvement from the 5% to 10% humidity during the Kincade Fire and other fires earlier this month in Southern California.

Most important: The winds have died down.

“Fortunately right now we haven’t been experiencing any big wind events,” Mehle said, “but fires can still start quickly, and people should be mindful of that.”

One group of people watching the weather closely are skiers.

Traditionally, Thanksgiving week is the start of ski season in California. Some resorts, such as Squaw Valley and Alpine Meadows, plan to open this week. But with no snow storms yet, resorts have been cranking up snow-making machines and hoping for the best.

“We’re still unsure of what terrain we’ll have open by Friday, but we will open,” said said Alex Spychalsky, a spokeswoman for Squaw Valley and Alpine Meadows resorts. “It will be great to get back out there. Whether we have one trail open or three trails open, it’s always a great time to get the skis back out.”

The two resorts near Lake Tahoe have 320 snow-making guns, which draw water from storage ponds and operate mostly at night and during the early morning hours when temperatures are the coldest. Crews have been running the machines for nearly a month and should have at least one lift open at each resort, she said.

Last year after a dry start, Squaw Valley ended up with 719 inches of snow. People skied until July 7. That total — nearly 60 feet — was the third-most snow the resort received in any season back to 1970.

“If we get 400 to 450 inches in a year, we’re happy,” she said. “Last year we were ecstatic.”

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