FALL OUTLOOK FOR NORCAL. COOL AND WET FOR SEPT AND OCT?
THE BIG PICTURE
As we approach fall across Northern California, lots of us anxiously await the first rains of the season. Not just because the arrival of moisture is a welcome reprieve from the hot, dry months of summer, but also because a delayed wet season can contribute to a dangerous fall fire season. As we know so well, a couple of well-timed rains can do wonders to temper California’s wildland fire outlook.
Below we’ll go into the factors that are in play this year, and how that may impact our fall weather. If you’d like to skip the detail, you can go to the BIG TAKEAWAYS section below. But for those who like to know the details, read on….
ANALOG YEARS
Each year meterologists look to past “analog” seasons, years when the conditions were similar to what we are experiencing this year. That is, when the major seasonal factors that impact our wet season were lining up in a similar way.
We mainly look at these variables:
- The El Nino Southern Oscillation, or ENSO conditions
- The temperature of the Pacific Ocean, near California and how it’s trending
- The temperature of the North Atlantic Ocean
- The so called Quasi-biennial oscillation
- The North American Monsoon season
- Hurricane Season
It is important to note that not all of these factors have a major influence on our fall weather. But each of them can nudge upper air wind and pressure patterns in such a way that our fall precipation and temperature can be impacted.
Let’s take a look at how these factors currently line up.
ENSO
We are currently in a ENSO neutral range, meaning ocean temperatures off the coast of South America are somewhere between warm (El Nino) and cool (La Nina). But the forecast is trending toward A WEAK La Nina (cool) set up this winter.
You can see that here in the ENSO forecast. Especially in the first part of the rainy seaon through December, weak La Nina conditions (the blue bar) are most likely.
You can see all of this in more detail below. The various models agree on a WEAK La Nina, especially for the fall and early winter (dipping below the center line), but also forecast the La Nina conditions to return to NEUTRAL sometime around mid-winter. Focus on the green and purple lines.
You can see that even at it’s lowest (coolest) point, the La Nina ocean conditions are not expected to be that pronounced, at around .5 degree celsius cooler than average.
NORTH ATLANTIC
It’s recently been discovered that ocean temperatures in the North Atlantic can impact global weather. This variation in temperature there, called the Atlantic Quadpole Mode, has been shown to be a contributing factor in our West Coast weather, mainly because of how it impacts atmospheric pressure patterns. So, what’s it doing?
As we see below, there is forecast to be a cooling trend underway in the North Atlantic through the fall and early winter. I’ve circled the area we’re talking about.
This cool phase of the AQM can have an impact on fall rains in the western U.S.
PACIFIC OCEAN TEMPERATURES NEAR CALIFORNIA
It has been known for quite some time that ocean temperatures near the West Coast can influence pressure patterns. For this fall, the major models are forecasting a slightly cool ocean signal along the CA coast. This could be a strong enough factor to impact our fall weather, as it can encourage troughing (low pressure) off the CA coast. This cool ocean signal is expected to weaken later in the winter season.
SOUTHWEST MONSOON
The pressure and wind pattern that brings summer rain to the Southwest U.S. and even California is called the seasonal monsoon. It started slow this year but has picked up in a big way, with heavy flooding rains in parts of the SOCAL desert and the Sierra Nevada and beneficial rains in Arizona. This late season surge in monsoonal activity can impact our fall weather pattern.
As mention above, other factors like hurricane season and the Quasi Biennial Oscillation (wind patterns near the equator) can also have some influence, but less so, on our weather in the West.
ANALOG YEARS
So, when looking at our chances for fall rains, are there some recent analog years we can compare with, when the conditions mentioned above lined up in a similar way? Yes, there certainly are!
The major patterns that we’ve talked about correspond with similar late summer/fall conditions in six recent seasons…. 1989, 1996, 2000, 2013, 2016, and 2017.
THE BIG TAKEAWAYS
When comparing 2025 with past years in which similar conditions prevailed (ENSO, Ocean Temperatures, Monsoon, etc) there are some definite hints as to what our fall may bring.
For the West Coast, years with a similar set-up generally brought cooler and wetter than average conditions to California (especially Northern California) in September and October, with warming and drying conditions often returning in November.
In fact, one winter season forecast site called Open Snow did an analysis of the six analog years, and found that similar conditions in past seasons generally brought a cool trough to the coast of California in the early fall...suggesting that cool and wet conditions could prevail during September and October.
ANALOG PRESSURE PATTERNS
And, here are the graphics for early fall temperatures and precipitation during the same 6 analog years.
ANALOG TEMPERATURES AND PRECIPITATION
CONCLUSION
Of course, none of this is a lock. But we can be somewhat optimistic, based upon recent years with similar conditions, that we could see a rather cool and wet September and October.
As we all know, long range forecasts are tricky. But in this case at least we have past experience to draw from.
We’ll keep you updated on the outlook as our fall season gets underway. Stay tuned!
Author: newsofthenorthbay
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