The winter of 2024-25 presents a complex forecast for the Pacific Northwest, with early indications pointing towards a weak La Niña that may transition into neutral conditions by mid-winter. This nuanced scenario carries significant implications for the region’s crucial snowpack, a vital resource for water supply, recreation, and ecosystems. Lessons learned from the 2024-25 season, which saw a weak La Niña fail to deliver the anticipated deep snowpack, are informing current predictions and highlighting the intricate interplay of oceanic and atmospheric patterns.
Understanding the El Niño-Southern Oscillation (ENSO) is paramount to deciphering these forecasts. ENSO, a cyclical fluctuation in tropical Pacific sea surface temperatures, profoundly influences global weather. It cycles through three phases: El Niño (warm), La Niña (cool), and Neutral. October forecasts for mid-winter ENSO probabilities for the upcoming cool season reveal striking similarities to the previous year’s outlook, with only a subtle weakening of the La Niña signal anticipated for 2025-26. Both forecasts suggest the highest probability for cool-phase conditions during autumn and early winter.
ENSO Forecasts: A Familiar Pattern with Subtle Shifts
The El Niño-Southern Oscillation (ENSO) index, specifically the Oceanic Niño Index (ONI), measures sea surface temperature (SST) departures from average in the Niño 3.4 region. This index is a primary tool for monitoring, assessing, and predicting ENSO. During the 2024-25 season, the December-January-February (DJF) ONI registered -0.6°C, just barely entering weak La Niña territory. Current data suggests a similar trajectory for the upcoming winter. The latest weekly SST departure stands at -0.6°C, indicating the region is once again on the cusp of a weak La Niña. Oceanic SST models from mid-October project a November minimum ONI of approximately -0.8°C, with values expected to decrease to around -0.3°C by mid-winter (DJF). This trajectory makes neutral ENSO conditions the most likely outcome for mid-winter snowpack accumulation in the Pacific Northwest.
Figure 1, comparing DJF ENSO phase probability forecasts from October 2025 and October 2024, visually underscores this similarity. While both years show a strong likelihood of cool-phase conditions in the early part of the winter, the probabilities for a more robust La Niña appear marginally lower for the 2025-26 season. This suggests that while the influence of La Niña may be present, its intensity and longevity are projected to be less pronounced than in some previous La Niña events.
Snowpack Climatology: Linking ENSO to Regional Snow Depth
To understand how these ENSO predictions translate to snowpack, the region’s Snowpack Climatology Dataset provides valuable insights. This dataset analyzes historical snowpack responses to different ENSO phases across representative weather stations in the Cascade Mountains.
In weak La Niña winters, historical data shows that most stations tend to hover around average snowpack levels. Modest gains have been observed at specific locations like Mission Ridge and Timberline. However, it is during moderate and strong La Niña years that the west slopes and crest of the Cascades consistently experience more favorable conditions, leading to peak snow depths that are 6-24% above normal. Neutral years, on the other hand, typically result in slightly below-average snowpack across the region. El Niño conditions, conversely, strongly favor reduced snowfall.
Figure 2, illustrating the average percentage of normal peak snow depth by ENSO phase across six representative Cascade weather stations, clearly demonstrates these historical trends. The bars for weak La Niña generally remain close to the 100% mark, indicating near-average snowpack. Moderate and strong La Niña phases, however, show a distinct upward trend, signifying above-average snow accumulation. Neutral conditions are depicted as slightly below average, and El Niño phases show a significant dip, indicating reduced snow depth.
Analyzing the 2025-26 Forecasted Snowpack
The long tails of the DJF distribution of forecast ENSO probabilities for the upcoming winter, which include an 18% chance of moderate/strong La Niña and a 2% chance of El Niño, exert the most significant influence on the mean outcome. To evaluate the most probable scenarios, researchers focused on the average data from four sites with the longest and most robust historical records: Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise.
The 2024-25 season, for instance, saw an average peak snow depth of 114% of normal. This figure fell around 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 noticeable at lower elevation stations, primarily attributed to warmer temperatures. To mitigate the impact of long-term climate trends on this analysis, a subset of data including only 21st-century records was examined. While this shortened dataset lowered the mean snow depth across all ENSO phases, the fundamental ENSO forecast for the upcoming winter continues to predict an almost perfectly "average" winter in terms of snowpack accumulation.
Figure 3, which displays the distribution of ENSO phase strength and corresponding snowpack response, with yellow shading highlighting the likely ENSO phase for 2025-26, provides a visual representation of this. The diagram suggests that with the projected weak La Niña transitioning to neutral, the expected snowpack will fall within the average range, neither exceptionally deep nor unusually sparse.
Beyond ENSO: The Influence of Sea Surface Temperatures and PDO
Linear best-fit trendlines explain 11% and 19% of the data variability for the ENSO vs. Snowpack plots, depending on whether 20th-century data is included or excluded, respectively. This indicates that over 80% of the season-to-season variability in snowpack is not directly explained by ENSO alone. This underscores the importance of considering other climatic drivers.
The phenomenon of marine heatwaves, such as the one observed in September of the current year, warrants close examination. A comparative analysis of SSTs from September and October 2025 against those from 2024 and 2014 reveals interesting patterns. While all three seasons exhibited mostly positive SST anomalies, which have become increasingly common due to long-term climate shifts, the intensity and location of these anomalies are critical. The 2014-15 season, characterized by a very weak El Niño, was marked by anomalously warm SSTs just off the coast of the Pacific Northwest. Scientists have linked this to a particularly poor snow year, with the four-station average snowpack reading languishing at less than 50% of the values seen in the 2024-25 season.
Although September 2025 began with notably warm SSTs across the northeastern Pacific, these waters have since cooled significantly, with proximal waters approaching normal levels. This cooling trend is a positive development for snowpack prospects.
Integrating Multiple Climate Predictors
The combination of ENSO phase and regional sea surface temperatures offers a more comprehensive model for predicting snowpack variability. When proximal West Coast Marine waters, often referred to as "The Blob," are considered alongside ENSO phase, approximately 25% of the snowpack inter-seasonal variability can be explained. The potential for an eastward spread of extraordinarily warm waters in the western North Pacific remains a factor to monitor.
The Pacific Decadal Oscillation (PDO) also plays a significant role. As long as exceptionally warm waters remain west of Hawaii, the region is likely locked into the negative phase of the PDO. As noted by the State Climate Office, a negative PDO phase generally correlates with cooler and snowier periods in the Pacific Northwest. An analysis of PDO versus ENSO phase explains about 20% of the snowpack variance, further emphasizing the interconnectedness of these climate phenomena.
Figure 4, presenting a combined ENSO and nearshore SST regression model explaining snowpack variability, visually represents how these factors interact. The model suggests that by integrating both ENSO and SST data, a more significant portion of snowpack variability can be accounted for compared to ENSO alone.
Conclusion: A Year of Cautious Optimism and Vigilance
In summary, the outlook for the winter of 2025-26 in the Pacific Northwest suggests a nuanced scenario. There are no definitive "magic goggles" to provide perfect snowpack predictions. Using a 75-year dataset, ENSO and regional Sea Surface Temperatures can explain approximately 25% of the variability in snowpack. The anticipated weak La Niña is expected to transition into neutral conditions by mid-winter, pointing towards a fairly typical snowpack across the region.
While the probability of a moderate or strong La Niña remains low, its occurrence would significantly increase the likelihood of deeper snowpacks. Regional sea surface temperatures have cooled to near-normal levels, offering some encouragement. However, the presence of surrounding warm waters could potentially limit further cooling and impact snow accumulation. Therefore, continued vigilance and a focus on key indicators will be crucial.
For a more robust winter, stakeholders will be closely monitoring for sustained upwelling off the coasts of South America and North America, the continuation of cool-phase PDO conditions, and the health of marine ecosystems, including sardine and salmon populations, which are often indicative of broader oceanic health. The interplay of these factors will ultimately determine the success of the upcoming winter season for the Pacific Northwest.
