The winter of 2024-2025 in the Pacific Northwest is shaping up to be a complex interplay of atmospheric and oceanic influences, with forecasters pointing towards a potentially neutral El Niño-Southern Oscillation (ENSO) phase that could result in a near-average snowpack. This forecast follows a winter where a weak La Niña failed to deliver the anticipated deep snow accumulations, underscoring the nuanced relationship between ENSO and regional weather patterns.
Understanding ENSO’s Influence on Pacific Northwest Snowfall
The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern characterized by variations in sea surface temperatures (SSTs) across the equatorial Pacific Ocean. It cycles through three phases: El Niño (warm), La Niña (cool), and Neutral. These shifts have a significant impact on global weather, including precipitation patterns and temperatures in the Pacific Northwest.
October forecasts for mid-winter ENSO probabilities in 2025-2026 show a striking similarity to the previous year’s projections. Both indicate a high likelihood of cool-phase conditions during autumn and early winter. However, a subtle weakening of the La Niña signal is anticipated for the upcoming season.
The Oceanic Niño Index (ONI) is a primary metric for monitoring ENSO, calculated based on SST departures from average in the Niño 3.4 region. During the 2024-2025 season, the December-January-February (DJF) ONI registered -0.6°C, just barely qualifying as a weak La Niña. Current weekly SST departures of -0.6°C suggest a similar trajectory for the upcoming winter. 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 midwinter (DJF). This indicates that neutral ENSO conditions are the most probable outcome for the region’s mid-winter snowpack.
Historical Snowpack Trends and ENSO Phases
Analysis of the Pacific Northwest’s Snowpack Climatology Dataset reveals distinct correlations between ENSO phases and mountain snowpack levels.
- Weak La Niña Winters: Historically, weak La Niña events have resulted in snowpack levels near the regional average. Some specific locations, such as Mission Ridge and Timberline, have shown modest gains.
- Moderate and Strong La Niña Winters: These more pronounced La Niña events have consistently favored the west slopes and crest of the Cascade Mountains, leading to peak snow depths ranging from 6% to 24% above normal.
- Neutral ENSO Years: Typically, neutral ENSO conditions are associated with snowpack levels slightly below the regional average.
- El Niño Years: El Niño events, conversely, tend to strongly favor reduced snowfall across the region.
Probabilistic Forecast for 2025-2026
The probabilistic forecast for the upcoming DJF period shows an 18% chance of a moderate to strong La Niña and a 2% chance of El Niño. These lower-probability, higher-impact scenarios, if they materialize, would significantly influence the mean snowpack outcome.
To assess the most likely scenarios, researchers often examine data from long-term, robust weather stations. An average of four such sites – Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise – provides a representative picture. In the 2024-2025 season, the average peak snow depth at these stations was 114% of normal, which corresponds to the 19th percentile for an ENSO Index range of -1 to 0°C. This was 24% below the regression trendline, with lower elevations experiencing reduced snowpack primarily due to warmer temperatures.
To mitigate the influence of long-term climate trends, an analysis using only 21st-century data was conducted. While this shortened dataset lowered the mean snow depth across all ENSO phases, the ENSO forecast for the upcoming winter still predicts an "almost perfectly average" winter when considering this refined dataset.
Beyond ENSO: The Role of Marine Heatwaves and PDO
While ENSO is a primary driver of winter weather, it is not the sole predictor of snowpack variability. Linear regression analyses indicate that ENSO explains only 11% to 19% of the data variability in snowpack, depending on whether 20th-century data is included. This suggests that over 80% of season-to-season snowpack variation is influenced by other factors.
Recent attention has been drawn to marine heatwaves, particularly following a significant event in September of this year. A comparative analysis of SSTs from September and October in 2025 against similar periods in 2024 and 2014 highlights the importance of these oceanic conditions. While positive SST anomalies have become more common due to long-term climate shifts, the intensity and location of these anomalies warrant close examination.
The 2014-2015 season, for instance, experienced a weak El Niño coupled with anomalously warm SSTs just off the coast. Scientists attribute a portion of that year’s poor snow performance 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. Although September 2025 began with very warm SSTs across the northeastern Pacific, these waters have since cooled significantly, with proximal waters approaching normal levels.
The Pacific Decadal Oscillation (PDO) and Nearshore SSTs
Further enhancing predictive capabilities, a combined model incorporating ENSO and nearshore West Coast Marine waters (often referred to as "The Blob") 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 exceptionally warm waters remain west of Hawaii, the Pacific Northwest 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 about 20% of the snowpack variance.
Outlook for the 2025-2026 Winter
Forecasters emphasize that no single model offers a perfect prediction for the upcoming winter. The current forecast suggests a weak La Niña is likely to transition into neutral conditions by mid-winter, pointing towards a fairly typical snowpack for the region.
The probability of a moderate or strong La Niña remains low. However, if such an event were to occur, it would significantly increase the likelihood of deeper snowpacks. Regional sea surface temperatures have cooled to near-normal levels. Nevertheless, the presence of surrounding warm waters may limit the potential for further significant cooling.
The success of a snow-rich winter will therefore depend on a confluence of factors: sustained upwelling of cold ocean currents off the coasts of South America and North America, continued cool-phase PDO conditions, and a healthy marine ecosystem that supports populations of sardines and salmon, which can indirectly influence atmospheric patterns.
Disclaimers
This analysis is based on current climate model projections and historical data. Weather patterns are inherently complex and subject to change. The provided figures and analyses represent statistical probabilities and trends, not definitive outcomes. Users are advised to consult ongoing meteorological forecasts for the most up-to-date information.
