What NOAA’s new snow maps say about California’s upcoming winter

New maps from the National Oceanic and Atmospheric Administration show that a large part of California’s Sierra Nevada has, on average, seen above-normal snowfall during El Niño events. With a strong El Niño pattern predicted to occur this winter, you may jump to the conclusion that the Golden State could see a massive snow pile-up this year.

But Michelle L’Heureux, a physical scientist at NOAA’s Climate Prediction Center, cautions against viewing the maps as a forecast for what is likely to happen this winter. Rather, they are historical data that show how snowfall deviated from the average in winters marked by El Niño weather patterns. On the map, which uses data from 1959 to 2023, brown depicts less-than-normal snowfall and blue is above normal.

The key is to recognize that within the average, the Sierra has seen wild swings in snowfall, with some years recording above-normal snowfall and others below-normal. In fact, L’Heureux and her team found that in the Sierra, depending on the region of the mountain range, there were six to eight El Niño years with above-average snowfall and five to seven with below-average.

In those above-average years, the deviation was greater than in the below-average ones. Because of this, the big snow years dominate the map, giving the impression above-average snowfall is likely in an El Niño year. In reality, what happens in the Sierra is “more of a coin flip,” L’Heureux wrote in an email. 

El Niño develops in the equatorial waters of the Pacific Ocean when the trade winds ease and warm ocean water collects at the equator in the central and eastern Pacific. The warmer sea surface temperature in an El Niño year can shift the jet stream, high-altitude winds that generally move west to east over the continents. Movement in the jet stream can impact weather across the globe, especially in winter months, leading some areas to be drier and some areas to be wetter than is normal. 

“The jet stream tends to extend eastward and shift southward during El Niño winters,” L’Heureux and her team wrote in a blog post featuring the maps. “You can think of the jet stream as a river of air, which carries more moisture and precipitation along the southern tier of the United States during El Niño. As a result, it is not surprising to see a stripe of increased snowfall (blue shading) over the southern half of the country.”

Sometimes in an El Niño year, the jet stream sets up in a way that brings increased snow to California, but that’s not always the case. El Niño conditions developed over summer and are expected to persist into the spring, according to the most recent forecast from the Climate Prediction Center. There’s a 75-85% chance a strong El Niño will occur November through January. 

California state climatologist Michael Anderson called the new maps “a great data product for climate analyses,” but he said it’s important to understand the caveats.     

“A key caveat is that the maps depict average conditions and each storm event, and each winter is likely to be different than the average,” Anderson wrote in an email. “This is particularly true for California as we are seeing more extremes on both ends of the wet/dry spectrum.  In 2015 (a weak El Niño year), California had its lowest snowpack on record, while in 2019 (a weak to moderate El Niño year), California experienced one of the 5 largest snowpacks on record. The super El Niño of 2016 produced the only near-average snowpack in the past decade. This variability can’t be ignored in California. 

“Bottom line, the new data set is a great way to map snowfall across North America and evaluate changes as the world warms, but averages associated with El Nino and Southern Oscillation phases are not always good predictors for seasonal outcomes for California.”

Anderson said the Department of Water Resources is working with a team of researchers to explore how to produce “more skillful” long-range season outlooks for the state that incorporate other climate influences, including weather patterns beyond El Niño and the inverse La Niña. The Madden-Julian Oscillation, marked by thunderstorms that circle the equator and help foster storms, may have helped drive the historically wet 2022-23 winter.

Until more accurate outlooks are available for California, it’s good to be prepared for any of one three scenarios in the Sierra: above-normal, below normal or even ordinary, normal snowfall.