The winter of 2024-2025 served as a stark reminder that a weak La Niña does not automatically guarantee the deep, mid-winter snowpacks that winter enthusiasts and water resource managers eagerly anticipate. This upcoming 2025-2026 season presents a similar, yet subtly nuanced, forecast, prompting a closer examination of the El Niño-Southern Oscillation (ENSO) and other climatic indicators that shape the region’s winter weather patterns.
Understanding ENSO and Its Impact on Pacific Northwest Snowfall
The El Niño-Southern Oscillation (ENSO) is a naturally occurring climate pattern characterized by fluctuations in sea surface temperatures across the tropical Pacific Ocean. These fluctuations influence atmospheric circulation and, consequently, weather patterns globally. ENSO cycles through three distinct phases: El Niño (characterized by warmer-than-average sea surface temperatures), La Niña (cooler-than-average sea surface temperatures), and Neutral conditions.
October forecasts for the upcoming winter’s ENSO probabilities reveal a striking similarity to the previous year’s outlook. Both forecasts indicate the highest likelihood of cool-phase conditions during the autumn and early winter months. However, there is a subtle weakening of the La Niña signal projected for the 2025-2026 season compared to the 2024-2025 season.
The Oceanic Niño Index (ONI) is the primary metric for monitoring ENSO, measuring sea surface temperature anomalies in the Niño 3.4 region of the Pacific. During the 2024-2025 winter, the December-January-February (DJF) ONI registered -0.6°C, just barely falling within the weak La Niña range. Current data suggests a similar trajectory for the upcoming winter. Mid-October Oceanic SST models predict a November ONI minimum of approximately -0.8°C, with a projected decrease to around -0.3°C by mid-winter (DJF). This trend points towards neutral ENSO conditions being the most probable outcome for the mid-winter snowpack in the Pacific Northwest.
Historical Snowpack Response to ENSO Phases
To better understand the potential implications of these ENSO forecasts, an analysis of the Pacific Northwest mountain snowpack’s typical response to ENSO phase strength has been conducted using an updated Snowpack Climatology Dataset. This dataset compiles historical snow depth data from various Cascade weather stations.
Weak La Niña Winters: Historically, weak La Niña conditions have generally resulted in snowpack levels hovering near the average across most stations. Modest increases have been observed at specific locations like Mission Ridge and Timberline.
Moderate to Strong La Niña Winters: The most significant positive impacts on snowpack have consistently been observed during moderate and strong La Niña years. These conditions have historically favored the western slopes and crest of the Cascade Range, leading to peak snow depths ranging from 6% to 24% above normal.
Neutral Years: Neutral ENSO phases typically correlate with slightly below-average snowpack levels across the region.
El Niño Years: Conversely, most El Niño conditions have been associated with a strong tilt in the odds towards reduced snowfall.
Deeper Dive into the 2025-2026 Forecast and Data Analysis
The forecast distribution for the upcoming winter’s ENSO probabilities shows a notable long tail. There is an estimated 18% chance of a moderate to strong La Niña and a 2% chance of El Niño conditions. These less probable, but potentially impactful, scenarios can significantly influence the overall average outcome.
To refine the analysis, researchers focused on the average snowpack data from four stations with the longest and most robust historical records: Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise.
The 2024-2025 season 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 range 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 long-term climate trends, a comparative analysis was performed using only 21st-century data. This shortened dataset, while reducing the mean snow depth across all ENSO phases, still indicated that the ENSO forecast for the upcoming winter predicts an "almost perfectly average" winter.
Linear regression analysis reveals that ENSO phases explain only 11% to 19% of the variability in snowpack, depending on whether 20th-century data is included or excluded. This suggests that over 80% of the year-to-year variation in snowpack is not directly attributable to ENSO.
The Influence of Marine Heatwaves and Pacific Decadal Oscillation (PDO)
The presence of marine heatwaves, such as the one observed in September of the previous year, also warrants consideration. A comparative analysis of sea surface temperatures (SSTs) from September and October of 2025 against those of 2024 and 2014 offers valuable insights. While 2024 and 2014 also exhibited mostly positive SST anomalies, the year 2014-2015, despite a very weak El Niño, was characterized by anomalously warm SSTs off the coast, which scientists have linked to a particularly poor snow year. During that season, the four-station average snowpack reading was less than 50% of the values recorded in the 2024-2025 season.
Although September 2025 began with significantly warm SSTs across the Northeast Pacific, a notable cooling has occurred. Proximity to average in these regional waters is approaching.
When ENSO phases are combined with data from proximal West Coast Marine waters (often referred to as "The Blob"), the model can explain approximately 25% of the inter-seasonal variability in snowpack. The eastward spread of exceptionally warm waters in the western North Pacific remains a factor to monitor.
The state of the Pacific Decadal Oscillation (PDO) also plays a crucial role. As long as exceptionally warm waters remain west of Hawaii, the region is likely to remain in the negative phase of the PDO. This negative PDO phase has been noted by the State Climate Office to correlate with cooler and snowier periods. A similar analysis considering the interplay between PDO and ENSO phases explains about 20% of the snowpack variance.
Expert Commentary and Outlook
"We don’t have any magic goggles for this impending ephemeral La Niña or neutral winter," stated a lead researcher involved in the analysis. "At best, we can explain about 25% of the variability in our 75-year dataset using ENSO and regional Sea Surface Temperatures. An impending weak La Niña is likely to fade into neutral conditions by mid-winter, portending a fairly typical Pacific Northwest snowpack in our region."
The expert further elaborated on the probabilities: "A moderate or strong La Niña remains a low probability, but if it occurs, it would tilt the dice in favor of deeper snowpacks. Regional sea surface temperatures have cooled to near-normal levels, but surrounding warm waters may limit the potential for further cooling."
Factors Favoring Deeper Snowpack
For those hoping for a bountiful snow season, the outlook hinges on several factors:
- Sustained Upwelling: Continued upwelling of cold, nutrient-rich water off the coasts of South America and North America can help regulate ocean temperatures and influence atmospheric patterns.
- Cool-Phase PDO Conditions: A persistent negative phase of the Pacific Decadal Oscillation is historically associated with cooler and snowier periods in the Pacific Northwest.
- Healthy Marine Ecosystems: The abundance of marine life, such as sardines and salmon, can be an indicator of a healthy and cooler ocean environment, which can indirectly support more favorable winter weather patterns.
Broader Implications for the Region
The variability in winter snowpack has significant implications for the Pacific Northwest. Mountain snowpack is a critical source of freshwater for the region, impacting everything from agricultural irrigation and municipal water supplies to hydroelectric power generation. Ski resorts and other winter tourism industries also rely heavily on consistent snowfall.
A year with below-average snowpack can lead to water shortages during the dry summer months, potentially affecting ecosystems and human populations. Conversely, exceptionally deep snowpacks can pose avalanche risks and present challenges for early-season travel and infrastructure.
The analysis underscores the complex interplay of global and regional climate drivers. While ENSO remains a primary indicator, its influence is not absolute, and other factors, such as SST anomalies and the PDO, can significantly modulate its effects. As the 2025-2026 winter season approaches, residents and stakeholders in the Pacific Northwest will be closely monitoring these climatic signals, hopeful for conditions that will lead to a healthy and resilient snowpack.
Disclaimers: This analysis is based on available forecast data and historical climate records. Actual weather patterns can vary significantly due to the inherent complexities of atmospheric science. The information provided is for informational purposes and should not be considered a definitive prediction of future weather conditions.
