When Will It Rain in the North Bay? The Long Range Forecast Offers Hints.

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As the North Bay and the rest of Northern California begin to emerge from yet another offshore wind event there is some good news. At this point at least, forecasters do not see another significant wind event showing up in the long range weather outlook. That outlook goes about two weeks into the future.

So all of this brings us to the obvious question, when will rain arrive across the North Bay and Bay Area?

As always, that is a difficult question to answer with any precision. However, there are indications of a change upcoming, subtle shifts that could tip the scales toward the first significant precipitation of the year across the region.

First, the immediate outlook shows very little chance of rain over the next two weeks. Through November 6th at least, it looks like high pressure will keep us dry.

However, by the second week of November, several atmospheric changes are signaling that rain may begin pushing south from the Pacific Northwest.

First, the Madden Julian Oscillation, or MJO, which measures the location and strength of active tropical weather will be entering a phase conducive to Northern California rainfall. Besides the large scale El Nino-Southern Oscillation (ENSO), this pattern of tropical thunderstorm activity can influence the movement of storms far away in the North Pacific.

After being stalled in October, the MJO is due to move into a more favorable zone by the second week of November.

So that’s good news. You can see current tropical Pacific thunderstorm activity in this image.

And long range forecasts support this. Here is the European model ensemble (average of lots of runs) forecast for rain through November 10th. It shows moisture finally reaching Northern California.

The CFS model, which tracks weekly changes is onboard with this idea as well. It shows above average precipitation right now for the third week of November.

Is any of this certain? This far out, the answer is, no. But it is encouraging that important atmospheric factors are pointing to a possible return of rainfall across Northern California and the North Bay by mid-November.

The end of November forecast continues this trend on the current CFS forecast.

As for December, some early indications of at least average rain and snowfall across Northern California.

One final word. Long range forecasts are just a snapshot. They take model simulations, based on current data, and extend them into the future. The longer range the forecast, the more uncertainty and less accuracy.

We are entering into what looks like a moderate La Nina winter, which is forecast to strengthen to a strong La Nina by January. That usually means we have a pretty equal chance of above or below average precipitation in Northern California.

Interestingly, of the 7 strong La Nina years since 1950, four have brought above average snowfall to the Northern Sierra, and 3 have brought below average. This corresponds to similar rainfall patterns for us in the lowlands as well. And the average for those years, is 103% of normal. So, there is reason to hope that we could have a fairly normal rainfall year.

With all of that said, we will update you with additional posts offering insights into the possible return of rain as we move forward into November.