FIRST LOOK: WINTER 2024-2025 ACROSS THE NORTH BAY. WILL IT BE WET?

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THE BIG PICTURE: WINTER 2024-2025

It’s always tricky to try to predict weather conditions months in advance, but conditions quite similar to our upcoming winter of 2024-2025 have been seen in recent decades, and so we’ll use those so called “analog” years as a basis for a first look at the upcoming winter.

ENSO OUTLOOK: LA NINA

After three straight La Nina years, the winter of 2023-2024 brought a moderate to strong El Nino to the West Coast of North America. Now, after a short one year pause, La Nina looks to return.

In broad terms, La Nina occurs when the prevailing tropical winds blow from east to west. This is the opposite of El Nino. During a La Nina year warm ocean water tends to get pushed to the west, piling up near Indonesia and Australia. Cooler water comes to the surface off of the coast of South America.

All of this impacts the position and strength of various high and low pressure centers and atmospheric circulation across the Northern Hemisphere. This can bring Northern California cooler, wetter winters. But as you can see, we’re right on the dividing line between “typical” conditions. Some La Nina years can be drier than normal too.

WHAT ABOUT “ANALOG” YEARS?

A La Nina year can be strong or weak, and is also impact by other atmospheric disturbances and oscillations…it can get quite complicated. That’s why scientists often look at “analog” years, in other words, years when very similar conditions prevailed during the leadup to the winter, and also featured the same “flavor” of La Nina.

So, what kind of La Nina year is on tap? This chart tracks what the major forecast models are predicting. While all models forecast a cooling of the ENSO region off the coast of South America (lines dropping below the horizonal line), most are predicting a weak La Nina which doesn’t really kick in until the fall.

SO WHAT DOES THAT MEAN FOR US?

To determine this, we have to look at those analog years when conditions were similar, on balance, to what we’re seeing now.

Since 1990 there have been 6 winters when weak La Nina conditions have occurred. They are 2022-2033, 2017-2018, 2016-2017, 2008-2009, 2005-2006, and 2000-2001.

Overall, those winters have produced more rain and snow than average over our region of Northern California.

And, they have been cooler than average as well.

BUT WAIT!

Yes, there’s a catch. 3 of the 6 analog winters were much wetter and snowier than average. A couple were even huge, like the winter of 2022-2023. Those wet winters tended to bring the overall average up for the 6 comparison years. Meanwhile 2 of the analog years were much drier than average and one was about normal. So while there appears to be a big “upside” potential in a weak La Nina year (here in Northern California), there is also a 50% percent chance (based upon this small sample) that the winter will wind up on the drier side.

Finally, many other regional atmospheric circulations can play a role in how our winter plays out. One that forecasters are looking at right now is the QBO, or Quasi-biennial oscillation which impacts trade winds high in the atmosphere. It is forecast to be in a western phase (west to east), which tends to correlate with increased rain and snow across Northern California. So that’s a positive sign.

THE BOTTOM LINE

As you already know, long range forecasts are by their nature unreliable. We can only look at trends and past results and go from there. Right now we seem to have a pretty equal chance of a wet, cool winter and a drier one as well. Let’s cross our fingers and wait for more data to come in.

We’ll know more as fall gets underway in about a month from now and forecasts are sharpened and revised. Stay tuned!