The winter of 2024-2025 served as a stark reminder that a weak La Niña does not automatically translate into the robust, deep mid-winter snowpacks that outdoor enthusiasts and water managers eagerly anticipate. This year’s forecast, particularly its comparison to the previous season and the influence of other climatic predictors, warrants a closer examination of the El Niño-Southern Oscillation (ENSO) and its complex relationship with Pacific Northwest snow accumulation.
Understanding ENSO: The Global Weather Driver
The El Niño-Southern Oscillation (ENSO) is a naturally occurring phenomenon characterized by cyclical fluctuations in tropical Pacific sea surface temperatures, profoundly influencing weather patterns across the globe. ENSO operates in three distinct phases: El Niño (warm phase), La Niña (cool phase), and Neutral conditions. Understanding the probabilities of these phases is crucial for forecasting seasonal weather.
ENSO Probability Forecasts: A Tale of Similarities and Subtle Shifts
October forecasts for mid-winter ENSO probabilities reveal striking similarities between the upcoming 2025-2026 cool season and the most recent 2024-2025 season. However, a subtle weakening of the La Niña signal is evident for the 2025-2026 period. Both forecasts indicate the highest likelihood of cool-phase conditions prevailing during the autumn and early winter months. This consistency in initial probabilities suggests that the atmospheric and oceanic drivers are pointing towards a similar trajectory, albeit with a slightly diminished intensity for the favored La Niña phase in the upcoming season.
The Oceanic Niño Index (ONI), a primary metric for assessing and predicting ENSO, is derived from sea surface temperature (SST) departures from the average in the Niño 3.4 region. During the 2024-2025 season, the December-January-February (DJF) ONI registered -0.6°C, just barely crossing into weak La Niña territory. This marginal deviation from neutral conditions underscored the challenges in achieving substantial snowpack accumulation, even with a nominal La Niña presence.
Current data for the approaching winter presents a parallel scenario. The latest weekly SST departure stands at -0.6°C, again placing us on the cusp of a weak La Niña. Mid-October Oceanic SST models project a November minimum ONI of approximately -0.8°C. However, these models anticipate a decrease in ONI values to around -0.3°C by midwinter (DJF). This projected trend strongly suggests that neutral ENSO conditions are the most probable outcome for the mid-winter period of 2025-2026. This forecast is a critical data point for those relying on predictable winter weather patterns.
Historical Snowpack Response to ENSO Phases
To contextualize these ENSO forecasts, it is essential to analyze how Pacific Northwest mountain snowpack has historically responded to different ENSO phases. Utilizing an updated Snowpack Climatology Dataset, researchers have identified distinct patterns:
- Weak La Niña Winters: In these scenarios, most weather stations typically register snowpack levels hovering near the historical average. Only a few select locations, such as Mission Ridge and Timberline, have historically shown modest gains. This suggests that while a weak La Niña may offer some benefit, it is often insufficient to drive significant snow accumulation across the broader region.
- Moderate and Strong La Niña Winters: These phases are demonstrably more favorable for the western slopes and crest of the Cascade Mountains. During these periods, peak snow depths have historically been recorded at 6-24% above normal. This highlights the potential for significantly improved snowpack when La Niña conditions are more pronounced.
- Neutral Winters: Neutral ENSO conditions generally lead to slightly below-average snowpack across the region. This outcome reflects a balance in atmospheric influences, without a strong leaning towards either extreme.
- El Niño Winters: Conversely, most El Niño conditions strongly favor reduced snowfall. The warmer sea surface temperatures associated with El Niño often result in drier and warmer weather patterns, significantly diminishing snow accumulation potential.
Analyzing the 2025-2026 Forecast: Probability vs. Likelihood
The DJF ENSO probability distribution for the upcoming winter shows a 18% chance of moderate to strong La Niña conditions and a 2% chance of El Niño. These "long tails" in the probability distribution, while representing lower likelihoods, can still exert a significant influence on the mean outcome.
To assess the most probable snowpack scenarios, analysis focuses on the average data from four sites with the longest and most robust historical records: Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise.
The 2024-2025 season, which experienced a weak La Niña, saw an average peak snow depth of 114% of normal. This figure falls around the 19th percentile for data within the -1 to 0°C ENSO Index interval and was 24% below the regression trendline. The snowpack reduction was particularly noticeable at lower elevation stations, primarily attributed to warmer temperatures.
To mitigate the impact of longer-term climate trends on this analysis, researchers have also examined a dataset restricted to 21st-century data. This shortened dataset, while lowering the mean snow depth across all ENSO phases, still indicates that the ENSO forecast predicts an "almost perfectly ‘average’" winter. This suggests that, even when accounting for recent climate shifts, ENSO alone is not a definitive predictor of extreme snow events.
Beyond ENSO: The Role of Sea Surface Temperatures
It is critical to acknowledge that ENSO is not the sole determinant of Pacific Northwest snowpack. Linear best-fit trendlines indicate that ENSO explains only 11% to 19% of the data variability in snowpack, meaning over 80% of season-to-season variability is not explained by ENSO alone. This points to the importance of other climatic factors, such as regional sea surface temperatures.
The presence of a marine heatwave in September of the previous year prompted an analysis comparing SSTs from September and October of 2025 against those of 2024 and 2014. All three of these seasons exhibited mostly positive SST anomalies. While slightly positive anomalies have become increasingly common due to long-term climate warming, the intensity and location of these anomalies deserve closer scrutiny.
The 2014-2015 season, though characterized by a very weak El Niño, was marked by anomalously warm SSTs just off the coast. Scientists have attributed this warmth, in part, to a notably poor snow year, with the four-station average snowpack reading falling to less than 50% of the 2024-2025 season’s values.
In the current context, September 2025 began with significantly warm SSTs across the northeastern Pacific. However, these waters have since cooled considerably, and the waters proximal to the Pacific Northwest are now approaching normal levels. This cooling trend is a positive development for potential snow accumulation.
The Pacific Decadal Oscillation (PDO) and Nearshore SSTs: A Combined Influence
When the influence of proximal West Coast Marine waters, often referred to as "The Blob," is combined with ENSO phase, researchers can explain approximately 25% of the snowpack’s inter-seasonal variability. The eastward spread of exceptionally warm waters in the western North Pacific remains a factor to monitor.
Crucially, as long as these extraordinarily warm waters remain west of Hawaii, the region is likely to remain in the negative phase of the Pacific Decadal Oscillation (PDO). The State Climate Office notes that a negative PDO phase correlates with cooler and snowier periods. A similar analysis of PDO versus ENSO phase explains 20% of the snowpack variance, underscoring the interconnectedness of these large-scale climate drivers.
Conclusion: A Call for Balanced Expectations
The outlook for the 2025-2026 winter in the Pacific Northwest suggests a nuanced picture. "Magic goggles" for predicting snowpack with absolute certainty are unavailable. At best, approximately 25% of the variability in a 75-year dataset can be explained by ENSO and regional Sea Surface Temperatures.
The impending weak La Niña is anticipated to transition into neutral conditions by mid-winter, portending a fairly typical snowpack for the region. The probability of a moderate or strong La Niña, while low, remains a factor. Should such conditions materialize, they would significantly improve the odds of deeper snowpacks.
Regional sea surface temperatures have cooled to near-normal levels. However, surrounding warmer waters may temper the potential for further cooling. Therefore, the success of achieving above-average snowpacks will likely depend on a confluence of favorable factors: sustained upwelling off the coasts of South and North America, continued cool-phase PDO conditions, and the flourishing of marine ecosystems, symbolized by healthy sardine and salmon populations.
Disclaimers:
This analysis is based on current climate model projections and historical data. Actual weather conditions can vary significantly due to the inherent complexities of atmospheric and oceanic systems. The information provided is for informational purposes and should not be considered a substitute for professional meteorological advice.
