What will La Niña mean for the Pacific Northwest mountain snowpack?
The Climate Prediction Center asserts: “La Niña is favored to emerge in September-November 2024 (60% chance) and is expected to persist through January-March 2025.” Like most things in life, the devil is in the details. So how do we answer the questions you’ve likely been daydreaming about? Are we really going to get a La Niña winter? What will that mean for our snowpack? We explore how the snowpack in our region responds to the 3-month moving average of Sea Surface Temperature (SST) anomalies in the middle of our winter season (also known as December-January-February or “DJF” anomalies).
The El Niño-Southern Oscillation (ENSO) is a period of fluctuation in SST and the air pressure of the overlying atmosphere across the equatorial Pacific Ocean. ENSO has three phases: El Niño, La Niña, and Neutral. The October 10th CPC forecast predicts the DJF anomaly this season will either be a 5% chance of a strong La Niña, 25% chance of a moderate La Niña, 41% chance of a weak La Niña, 29% chance of neutral conditions, or a 1% chance we enter weak El Niño conditions in early 2025. Quite a range of potential outcomes!
Before your mind wanders too far with all these possibilities, let’s compare the percentage of normal peak seasonal snow depth (inches) against the DJF anomaly strength (ENSO phase) using our snowpack climatology dataset.
Snowpack Climatology Dataset
A moderate La Niña tends to be quite favorable for snowfall along the west slopes and crest of the Cascades (>110% of normal). Peak snow depth is markedly lower for a weak La Niña, except at Mission Ridge, but remains near normal. The outcome tilts slightly more negative in an ENSO neutral phase where slightly below normal conditions are favored at all stations except Timberline. Well-below-average peak snow depths (76-84% of normal) would be anticipated should we tip into a weak El Niño by January (1% chance). Note that Mission Ridge (on the east slopes of the Cascades) has a weaker signal due to ENSO than the stations near or west of the Cascade Crest and therefore doesn’t follow the same trend.
If we look specifically at an average of the four sites with the longest and most robust data records (Mt Baker, Stevens Pass, Snoqualmie Pass, and Paradise), we can see the scatter of the data. The graph below illustrates the relationship between DJF SST anomalies and peak snow depths averaged across these sites. For every 1°C increase in SST anomaly, peak snow depth decreases by 1 foot. This relationship is statistically significant, with a p-value of 0.0052, confirming that warmer winters (associated with El Niño) lead to lower snowpacks across the Pacific Northwest. If you recall the recent 2023-24 season (star on the plot below), it was a shallower peak snowpack than we would have anticipated from the ENSO regression trendline.
If we zoom in a bit closer, we can see data in the range of possible outcomes for the 2024-25 winter:
All moderate and strong La Niña seasons in the dataset have peak snow depths greater than or equal to the mean for ENSO neutral. And all of these moderate and strong La Niña years exceed typical snow depth values of an El Niño year by 5% or more. All that to say, if the negative Sea Surface Temperature anomaly intensifies sufficiently, we’re in store for a great season!
Some less-than-stellar snowpack years start coming into the mix as we head into the most probable ENSO phases for the 2024-25 season–a weak or neutral La Niña–with little to no weighting on the dice. Some of the worst seasons in PNW history have occurred in a weak El Niño, so fingers crossed the Niño 3.4 region of the Pacific Ocean (where we measure ENSO SST anomalies) doesn’t enter El Niño territory by January. Fortunately, the odds of that are very low!
To summarize, the strength of an anticipated La Niña ENSO phase has a major influence on our chances for an above-average snow season in our forecast zones. A moderate La Niña or stronger gives us very high chances of normal to above normal peak snowpacks. Whereas a weak La Niña or ENSO neutral conditions give us a range of potential outcomes close to the overall average for our region.
So if you want to load the dice in favor of a cold, snowy winter in our mountains, root for strong upwelling of cold water off the coast of South America!
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Disclaimers:
- The datasets are not fully quality-controlled.
- The impact of long-term climatic changes has not been incorporated into this study, but preliminary analysis suggests that ENSO impacts remain larger than the overall climate trends.