The winter of 2024-2025 served as a stark reminder that a weak La Niña, long anticipated to deliver the deep, mid-winter snowpacks cherished by winter sports enthusiasts and water resource managers alike, does not always fulfill those expectations. This season’s performance underscores the complex interplay of global climate phenomena and their localized impacts, prompting a closer examination of the upcoming 2025-2026 winter outlook. Forecasters are now assessing how this year’s predictions compare to historical patterns and what other climate indicators might hold sway in shaping regional snowfall.
Understanding the El Niño-Southern Oscillation (ENSO)
At the heart of these seasonal weather patterns lies the El Niño-Southern Oscillation (ENSO), a cyclical fluctuation in tropical Pacific Ocean sea surface temperatures that profoundly influences global weather. ENSO operates through three distinct phases: El Niño, characterized by warmer-than-average sea surface temperatures; La Niña, marked by cooler-than-average temperatures; and Neutral, representing a period of no significant temperature anomaly.
Comparing the October forecasts for mid-winter ENSO probabilities between the upcoming 2025-2026 season and the recently concluded 2024-2025 season reveals striking similarities. Both forecasts indicate the highest probability of cool-phase conditions during autumn and early winter. However, there is a subtle, yet significant, weakening of the La Niña signal for the 2025-2026 period, suggesting a diminished likelihood of robust La Niña influence.
The Oceanic Niño Index (ONI) is the 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 of the Pacific. 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 weekly SST departures are also hovering around -0.6°C, indicating that the region is once again on the cusp of a weak La Niña.
However, mid-October Oceanic SST models project a November minimum ONI of approximately -0.8°C, with a projected decrease to around -0.3°C by mid-winter (DJF). This forecast strongly suggests that neutral ENSO conditions are the most probable outcome for the upcoming mid-winter, a scenario that carries distinct implications for Pacific Northwest snowpack accumulation.
Historical Snowpack Response to ENSO Phases
To understand the potential impacts of the anticipated ENSO conditions, researchers have analyzed an updated Snowpack Climatology Dataset for the Pacific Northwest. This dataset offers valuable insights into how mountain snowpack typically responds to different ENSO phase strengths.
In weak La Niña winters, the historical data reveals that most weather stations tend to experience snowpack levels close to the average. Only a few select locations, such as Mission Ridge and Timberline, have historically shown modest gains during these periods. The data becomes more compelling when examining moderate and strong La Niña years. These phases consistently favor the western slopes and crest of the Cascade Mountains, leading to peak snow depths that are 6-24% above normal.
Conversely, neutral ENSO years typically yield snowpack slightly below the regional average. El Niño conditions, on the other hand, strongly correlate with reduced snowfall across the region, often tilting the odds significantly toward drier winter months.
Analyzing the 2025-2026 Outlook: Probabilities and Predictions
The DJF forecast for the upcoming winter displays long tails in the distribution of ENSO probabilities. There is an 18% chance of a moderate to strong La Niña and a mere 2% chance of El Niño. These lower probability, but potentially impactful, scenarios can influence the overall mean prediction.
To refine the outlook for the most likely outcomes, analysis has focused on the average of four sites with the longest and most robust historical data records: Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise. These locations provide a stable baseline for assessing long-term trends.
During the 2024-2025 season, the average peak snow depth at these four sites was 114% of the average for data within the -1 to 0°C ENSO Index interval. This reading fell at the 19th percentile and was 24% below the regression trendline. The reduction in snowpack was particularly noticeable at lower elevation stations, largely attributed to warmer temperatures.
To mitigate the influence of longer-term climate trends, researchers examined the data with only 21st-century records included. While this shortened dataset resulted in a slight decrease in the mean snow depth across all ENSO phases, the fundamental ENSO forecast for the upcoming winter remained consistent: an almost perfectly "average" winter is predicted. This prediction suggests that neither extreme La Niña nor El Niño conditions are expected to dominate.
The Limited Role of ENSO in Snowpack Variability
The statistical analysis of ENSO’s influence on snowpack reveals that linear best-fit trendlines explain only 11% (including 20th-century data) and 19% (excluding 20th-century data) of the data variability. This indicates that over 80% of the season-to-season variability in snowpack is not explained by ENSO alone. This finding highlights the importance of considering other atmospheric and oceanic drivers.
The Impact of Marine Heatwaves and the Pacific Decadal Oscillation (PDO)
Recent discussions have highlighted the impact of marine heatwaves, with a notable event occurring in September of this year. To assess this influence, a comparative analysis of SSTs from September and October 2025 against those from 2024 and 2014 was conducted. All three of these seasons exhibited predominantly positive SST anomalies.
While slightly positive SST anomalies have become increasingly common due to long-term climate shifts outpacing the 30-year averaging period, deeper analysis of the data’s nuances is warranted. The 2014-2015 season, despite being a very weak El Niño, was marked 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 seen in the 2024-2025 season.
Currently, while September 2025 began with very warm SSTs across the northeastern Pacific, these waters have cooled significantly. Proximal waters are now approaching normal levels, offering a more favorable outlook compared to the conditions observed in 2014.
Combining ENSO and Nearshore SSTs for a More Comprehensive Model
When ENSO phases are combined with nearshore West Coast Marine SSTs, often referred to as "The Blob," the predictive power for snowpack variability increases to approximately 25%. This suggests that regional ocean temperatures play a crucial, albeit not exclusive, role in determining winter snowfall.
Of particular interest is the potential for an eastward spread of extraordinarily warm waters in the western North Pacific. 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 and snowier periods.
A similar analysis combining PDO and ENSO phase data explains 20% of the snowpack variance, further underscoring the intricate relationships between these climate drivers.
Conclusion: A Near-Average Outlook with Lingering Uncertainties
The prevailing scientific consensus suggests that for the upcoming winter of 2025-2026, there are no "magic goggles" that can definitively predict snowpack accumulation with absolute certainty. Even with sophisticated models incorporating ENSO and regional Sea Surface Temperatures, only about 25% of the variability in a 75-year dataset can be explained.
The current forecast indicates a weak La Niña that is likely to transition into neutral conditions by mid-winter. This scenario portends a fairly typical Pacific Northwest snowpack, with accumulations generally around the historical average for the region.
The probability of a moderate or strong La Niña, which historically favors deeper snowpacks, remains low. However, should such conditions materialize, they would undoubtedly tilt the odds in favor of more substantial snowfall.
Regional sea surface temperatures have cooled to near-normal levels. Nevertheless, the presence of surrounding warmer waters may limit the potential for further cooling, which could otherwise enhance snowfall prospects. Therefore, the outlook for the 2025-2026 winter season will depend on a confluence of favorable factors: sustained upwelling off the coasts of South America and North America, the continuation of cool-phase PDO conditions, and a healthy marine ecosystem supporting robust fish populations like sardines and salmon, which are indicators of a balanced oceanic environment.
The scientific community will continue to monitor these indicators closely, providing updated forecasts as more data becomes available. Residents and stakeholders in the Pacific Northwest are advised to prepare for a winter that, based on current projections, is likely to be near average in terms of snowpack, while remaining aware of the low-probability but potentially significant influence of stronger La Niña events or persistent marine heatwaves.
