The fact that there's no rain in the current forecast for the Bay Area may not be anything to fret about just yet, according to the National Weather Service (NWS).
Our extended dry spell since the middle of November looks to be extending into the middle of December, with forecast models not showing any sign of precipitation in the next 10 days. But the National Weather Service's Bay Area bureau says that this is fairly typical for a moderate La Niña year like the one we're in.
"Late starts to the wet season are not too uncommon during moderate La Niña events," writes NWS Bay Area on Twitter. "The [1995-96] La Niña year had the driest start, but a strong Dec storm kickstarted a [greater-than-]normal [rainfall pattern] that season."
Late starts to the wet season are not too uncommon during moderate La Niña events.— NWS Bay Area (@NWSBayArea) December 2, 2020
This chart shows normal precip (brown), w/ similar La Niña ONI WYs highlighted
The 95-96 La Niña year had the driest start, but a strong Dec storm kickstarted a >100% normal precip that season. pic.twitter.com/d83YfyfeFt
So, that could mean a drenching around Christmas week, or just a steady influx of rainstorms coming later in the month and continuing into January.
It's not immediately great news for Tahoe ski resorts, which are already dealing with a ski season impacted by a pandemic. But it means the skiing could be getting good just as everyone finishes with their holiday business, or perhaps before.
The last La Niña event in the winter of 2017-18 saw only moderate rainfall into the middle of December, according to the NWS data, with about three inches of rainfall in mid-November and no significant rain after that until early January.
As SFist noted back in September, La Niña years are neither predictably dry nor abnormally wet for the Bay Area, with the effects of the phenomenon often more dramatically felt to our north and south along the West Coast.
The NWS chart above shows that four of the last five La Niña years, based on rainfall totals in downtown San Francisco, looked like just-below-average years for rainfall with late starts to the wetness overall. Each year clocked in between approximately 3 and 7 inches less rain than normal by the time May arrived.
As the Chronicle reports this week, CalFire is keeping itself staffed up pending some significant rainfall around the state — with Southern California still in acute danger of wildfires as we enter December. The agency typically begins its wind-down of staff and winter equipment maintenance around now, but that has not yet been authorized.