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As we post this, El Nino is rapidly weakening across the Northern Hemisphere and being replaced by La Nina conditions. What might this big change mean for our summer weather here in the North Bay and across California? Read on.


In the region of the tropical Pacific where sea surface temperatures are measured to guage El Nino/La Nina cycles, colder than normal water (blue in this graphic) is rapidly replacing warmer than normal water. The transition began a few months ago, and is intensifying. You can also see a blob of colder than average water off the West Coast of the U.S.

As you can see, La Nina conditions are expected to be in full force by late summer through the coming winter.


This type of transition from El Nino to La Nina conditions has happened 5 times in the past 30 years, in 1995,1998, 2007, 2010, and 2016, so it may be helpful to look at what occurred in those years to get a handle on what we might expect this summer.

In those years, upper level low pressure (blue) has been more prevalent than average over parts of the Western U.S. including California.

As we know, low pressure can be associated with cooler than average temperatures, and that’s what was seen across the West in these 5 comparison years. Interestingly, most of the rest of the country had above average summertime temperatures.

For us in California it is often from late summer into fall when the hottest temperatures kick in and set the stage for high fire danger. However, in our comparison years the cooler than average temperatures (in the West) continue into early fall. This graphic looks at August and September.

While the signal is less strong, there are also hints in our comparison years that California and especially the Pacific Northwest can be a bit wetter than average through the summer. This would be expected with the overall pattern of increased low pressure “troughing” over the western region of the U.S.

So, what to make of all this?

Long range outlooks can be pretty fickle, and it is very hard to rely on these forecasts. But one place to look for signs of change is in our Spring weather so far, because if the transition is already underway, we should have seen some of the effects (cooler than average) already. And, as we know in the North Bay, we’ve had a pretty nice, cool spring so far.

And that’s exactly what we do find, overall, for the month of May so far, with most of the Eastern U.S. hotter than average, and most of the West near average or a bit cooler.

And the forecast for the next month is relatively cool, at least for California.

Still, in the era of climate change, there is only so much past experience can tell us. In fact, in their “official” (if unreliable) seasonal outlook for the summer, the NWS foresees slightly warmer than average temperatures in the West, and in fact almost the entire country.

Very little of the “cool summer” effect shows up in the seasonal forecast across the West.

Perhaps the impacts of climate change will overcome the cooler conditions that, based on past years, the strengthening La Nina suggests may be coming our way. We’ll just have to wait and see.

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