The winter of 2024-2025 presents a complex forecast for Pacific Northwest snow enthusiasts and water managers, with initial indications pointing towards a weak La Niña potentially transitioning into neutral conditions. This nuanced outlook, informed by a deeper understanding of the El Niño-Southern Oscillation (ENSO) cycle and regional oceanographic factors, suggests a season that may not deliver the consistently deep snowpacks of stronger La Niña events. The prevailing scientific consensus indicates a higher probability of near-average snowfall, deviating from the hopes for exceptionally wet and cool conditions often associated with this phase of ENSO.
Understanding ENSO: The Driving Force Behind Seasonal Weather Patterns
The El Niño-Southern Oscillation (ENSO) is a naturally occurring climate pattern characterized by fluctuations in sea surface temperatures across the central and eastern tropical Pacific Ocean. This cyclical phenomenon significantly influences global weather patterns, including precipitation and temperature across North America. ENSO operates in three distinct phases: El Niño, characterized by warmer-than-average sea surface temperatures; La Niña, marked by cooler-than-average temperatures; and Neutral conditions, where temperatures are closer to the long-term average.
The October forecasts for mid-winter ENSO probabilities reveal a striking similarity between the upcoming 2025-2026 cool season and the recently concluded 2024-2025 period. Both forecasts indicate a strong likelihood of cool-phase conditions during autumn and early winter. However, a subtle weakening of the La Niña signal is projected for the 2025-2026 season, suggesting a potential drift towards neutrality.
The Oceanic Niño Index (ONI) serves as a primary metric for monitoring and predicting ENSO. It is calculated based on 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, a value that just barely placed it within the weak La Niña territory. Current data for the approaching season suggests a similar trajectory. Mid-October Oceanic SST models forecast a November minimum ONI of approximately -0.8°C, with projections indicating a decrease to around -0.3°C by mid-winter (DJF). This trend points towards neutral ENSO conditions being the most probable outcome for the upcoming mid-winter snowpack.
Historical Snowpack Response to ENSO Phases
To contextualize these ENSO projections, researchers have analyzed an updated Snowpack Climatology Dataset for the Pacific Northwest. This dataset allows for an examination of how mountain snowpack typically responds to varying strengths of ENSO phases.
- Weak La Niña Winters: In these scenarios, most weather stations tend to record snowpack levels near the average. Only a few locations, such as Mission Ridge and Timberline, have historically shown modest increases in snow depth.
- Moderate and Strong La Niña Winters: These phases are consistently associated with more favorable conditions for the west slopes and crest of the Cascade Mountains. During these periods, peak snow depths have been observed to be 6-24% above normal.
- Neutral ENSO Years: Typically, neutral years result in snowpack slightly below the regional average.
- El Niño Conditions: Most El Niño phases tend to strongly favor reduced snowfall across the region.
Analyzing the 2025-2026 Forecasted ENSO Probabilities
The DJF distribution of forecast ENSO probabilities for the upcoming winter reveals a number of key indicators. There is an 18% chance of a moderate to strong La Niña event, and a mere 2% chance of an El Niño event. These probabilities, while not dominant, exert a significant influence on the overall mean forecast.
To derive the most likely outcomes, a specific analysis was conducted focusing on the average snowpack data from four sites with the longest and most robust historical records: Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise.
The 2024-2025 season, for instance, saw an average peak snow depth of 114 inches. This figure represented approximately the 19th percentile for data within the -1 to 0°C ENSO Index interval and was 24% below the regression trendline. Snowpack reduction was particularly evident at lower elevation stations, largely attributed to warmer temperatures.
To mitigate the influence of longer-term climate trends, an analysis was also performed using only 21st-century data. While this shortened dataset lowered the mean snow depth across all ENSO phases, the fundamental prediction remained consistent: the ENSO forecast for the upcoming winter predicts an almost perfectly "average" winter.
Beyond ENSO: The Role of Regional Sea Surface Temperatures
It is crucial to recognize that ENSO is not the sole determinant of Pacific Northwest snowpack. Linear best-fit trendlines indicate that ENSO explains only 11% and 19% of the data variability when including and excluding 20th-century data, respectively. This implies that over 80% of the year-to-year variability in snowpack is not explained by ENSO alone.
Recent discussions have highlighted a marine heatwave in September of the current year. An analysis comparing September and October SSTs from 2025 against those of 2024 and 2014 provides valuable context. While all three of these seasons exhibited mostly positive anomalies, the deeper shades of warmth in the data warrant attention. The 2014-2015 season, despite being a very weak El Niño, was characterized by anomalously warm SSTs just off the coast. Scientists have partially attributed the unusually poor snow year of 2014-2015 to these warm waters, with the four-station average snowpack reading falling to less than 50% of the values observed in the 2024-2025 season.
While September 2025 began with significantly warm SSTs across the northeastern Pacific, these waters have since cooled considerably, and proximal waters are now approaching normal levels. This cooling trend is a positive sign for potential snowpack development.
Integrating ENSO and Marine Conditions for a Comprehensive Forecast
A more refined understanding of snowpack variability can be achieved by combining ENSO phase with nearshore West Coast Marine waters, often referred to as "The Blob." This combined regression model can explain approximately 25% of the snowpack’s inter-seasonal variability.
The potential for an eastward spread of extraordinarily warm waters in the western North Pacific remains a factor to monitor. As long as these exceptionally 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 has noted that a negative PDO phase correlates with cooler, snowier periods in the Pacific Northwest. A similar analysis of PDO versus ENSO phase explains about 20% of the snowpack variance.
Looking Ahead: Navigating Uncertainty for the Winter of 2025-2026
Forecasting seasonal snowpack is an inherently complex endeavor, with multiple atmospheric and oceanic factors at play. For the impending winter, characterized by a potentially ephemeral La Niña that may transition into neutral conditions, the most optimistic outlook suggests a fairly typical Pacific Northwest snowpack. The current analysis indicates that ENSO and regional Sea Surface Temperatures can explain, at best, about 25% of the variability within a 75-year dataset.
The probability of a moderate or strong La Niña remains low. However, should such an event materialize, it would significantly tilt the odds in favor of deeper snowpacks. Regional sea surface temperatures have shown a welcome cooling trend, nearing normal levels. Nevertheless, the presence of surrounding warmer waters could limit the potential for further substantial cooling.
Ultimately, the success of the upcoming winter’s snow accumulation will likely depend on a confluence of favorable conditions. Meteorologists and climate scientists will be closely watching for sustained upwelling off the coasts of South America and North America, continued cool-phase PDO conditions, and a healthy marine ecosystem that supports the food chain, from sardines to salmon. These interconnected elements play a vital role in shaping the atmospheric patterns that ultimately deliver snow to the mountains of the Pacific Northwest.
Disclaimers: This analysis is based on current climate model projections and historical data. Actual weather patterns can deviate from forecasts due to the inherent variability of the climate system.
