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.