Weather Models Provide Hopeful Early Look at Fall Across California

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We’ll begin this post with a caveat. As we all know, long range weather forecasting is a tricky business, with VERY imperfect results. However, with this in mind, several global forecast models are beginning to offer glimpses of our fall weather, here in the North Bay, and across California. Since this is an early look, plenty of cautions apply as to the validity of the forecasts, but since there is some agreement among global models, we wanted to present them to you, our readers, for consideration.

These models use present conditions, sea surface temperatures, global pattern analysis, and complex computer simulations to produce best guesses. We’ll leave it at that. This is the best information that we have in the first days of August. This will be updated as we approach fall. But since the timing and extent of fall rain is of critical importance to our fire season in California, this information is offered as a first glimpse of what we may have coming our way.

First off, the overall outlook is promising. Both the Euro Long Range and the CFS outlook (American Climate Forecast System) show a good chance for average, or above average precipitation for the start of fall. This is in marked contrast to the past few years when rains were slow to arrive or non-existent.

Now the charts. This is the CFS V2 outlook for precipitation in September. (Image 1)

The Euro Long Range agrees, showing an above average chance of enhanced precipitation for the months of August, September and October. (Image 2)

This is significant, as it shows both models see the same pattern potentially emerging. In fact, the CFS sees October still trending wet, or at least not abnormally dry, for Northern California. (Image 3)

This is in agreement with the Euro forecast. When models are in agreement, using different algorithms and data sets, forecasters have more confidence in their conclusions.

Now, looking further ahead the CFS V2 sees the month of November potentially bringing wet weather. (Image 4) But does the Euro agree?

The Euro Long Range is in general agreement, at least for now. Here is the North American precipitation anomaly map for September, October and November. A rough match with the CFS. (Image 5)

And the October, November and December plot shows the same general trend. (Image 6)

And, what about El Nino and La Nina? As of now, most global models, based on current and expected sea surface temperatures, predict an ENSO (El Nino Southern Oscillation) neutral fall (neither El Nino or La Nina) with near normal sea surface temperatures. (Image 7) Then, later in the fall the best chance is for a weak to moderate La Nina (cooler sea surface temperatures). That generally means increased rain north, drier south in California. But various seasonal factors can greatly impact that, driving rain to our north, or bringing it south. The models seem to think, at this point, that our fall may bring average or above precipitation.

Finally, here is a fire hazard outlook for the North Bay and North Coast, showing above average fire potential for our region through November. (Image This forecast takes into account the persistent drought, soil and fuel dryness, and other factors. The conclusions are valid and not inconsistent with the forecasts above. Since we have been so dry, for two consecutive years, fire potential will remain elevated even if normal rains return.

We will be updating this forecast as new data comes in, and as new forecasts either confirm these general trends, or point in a different direction. Stay tuned!