An Early Look at the North Bay’s Winter Precipitation Outlook

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The National Weather Service says a moderate La Nina pattern now in place will be an important driving force behind our winter weather here in the West. While other shorter term climatic patterns may also help determine what happens this winter, La Nina will most likely be the largest scale weather driver across much of North America.

The first graphic below shows sea surface temperatures along the equator. In general, cooling there (blue) portends a La Nina winter.

So what does that mean for us in the North Bay?

Although La Nina can often bring a relatively dry winter to much of California, for Northern California the story is often different. La Nina conditions can bring near to above average moisture to the northern half of the state, while often drenching the Pacific Northwest with lots of rain and snow.

In the second graphic below, in four La Nina winters that have occurred since 1955, rainfall for Northern California (but not for central and southern California) has been somewhat above normal.

The third graphic is the “typical” setup during a La Nina year, while the fourth graphic shows the European model forecast for precipitation for the months of December, January and February. It shows above average rain and snow for the northern third of the state, with below average rain and snowfall further south across California.

As always, long range outlooks are notoriously unreliable. La Nina years have been both wet and dry for us. The most recent was a weak La Nina year, 2016-2017, that turned out to be the third wettest on record. So, although La Nina or El Nino conditions can offer a hint of what’s to come, they are not the final word. And with climate change a complicating factor, the outlook is far from certain for the coming months.

One final thought. In 4 of 7 years since 1970 in which precipitation at Donner Summit was 20% or less of average in October, the rain and snowfall for the year actually ended up above average.

So while the past is a guide, California winter weather is famously difficult to predict in the long term. Hang in there!