The winter of 2024-25 offered a stark reminder that a weak La Niña does not automatically guarantee the deep, mid-winter snowpacks that winter enthusiasts and water managers eagerly anticipate. This past season, the anticipated cool and wet conditions fell short, prompting a closer examination of the predictive power of El Niño-Southern Oscillation (ENSO) and other climate indicators for the upcoming 2025-26 winter season.
Understanding ENSO and Its Influence on Pacific Northwest Weather
The El Niño-Southern Oscillation (ENSO) is a naturally occurring climate phenomenon characterized by fluctuations in sea surface temperatures across the equatorial Pacific Ocean. These cycles significantly influence global weather patterns, impacting temperature and precipitation across continents. ENSO operates in three distinct phases: El Niño, marked by warmer-than-average sea surface temperatures; La Niña, characterized by cooler-than-average sea surface temperatures; and Neutral conditions, where sea surface temperatures are near the long-term average.
Forecasting the ENSO phase for the upcoming winter is a critical step in predicting regional weather patterns. October forecasts for mid-winter ENSO probabilities in 2025-26 show a striking similarity to those of the previous year. However, a subtle weakening of the La Niña signal is observed for the 2025-26 period. Both forecasts suggest the highest likelihood of cool-phase conditions during the autumn and early winter months.
Analyzing the Oceanic Niño Index (ONI) and its Implications
The Oceanic Niño Index (ONI) serves as a principal metric for monitoring, assessing, and predicting ENSO. It is calculated based on sea surface temperature (SST) departures from the average within the Niño 3.4 region of the Pacific Ocean.
During the 2024-25 season, the December-January-February (DJF) ONI registered -0.6°C, just barely entering the threshold for a weak La Niña. As of the latest weekly data, the SST departure stands at -0.6°C, indicating a similar situation as the region hovers on the cusp of another weak La Niña. Mid-October Oceanic SST models 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 strongly suggests that neutral ENSO conditions are the most probable outcome for the mid-winter period of 2025-26.
Historical Snowpack Response to ENSO Phases in the Pacific Northwest
To better understand the potential impact of these ENSO forecasts on regional snowpacks, researchers have analyzed an updated Snowpack Climatology Dataset. This dataset allows for an examination of how Pacific Northwest mountain snowpack has historically responded to varying ENSO phase strengths.
Weak La Niña Winters: In winters classified as weak La Niña, most monitored weather stations have historically shown snowpack levels hovering near the average. Exceptions include Mission Ridge and Timberline, which have occasionally experienced modest gains.
Moderate and Strong La Niña Winters: These phases have historically been the most beneficial for snowpack accumulation on the west slopes and crest of the Cascade Mountains, consistently producing peak snow depths that are 6-24% above normal. This indicates a significant positive correlation between stronger La Niña events and deeper snow accumulations in these key mountain regions.
Neutral ENSO Years: Historically, neutral ENSO conditions tend to yield snowpack levels slightly below the regional average.
El Niño Conditions: Conversely, most El Niño phases have historically tilted the odds significantly towards reduced snowfall across the Pacific Northwest.
Detailed Analysis of Forecasted ENSO Probabilities and Snowpack Variability
The DJF distribution of forecast ENSO probabilities for the upcoming winter reveals some key figures. There is an 18% chance of a moderate to strong La Niña and a mere 2% chance of an El Niño event. These "long tails" in the probability distribution exert the most significant influence on the mean outcome.
To provide a more focused assessment, researchers specifically examined the average snowpack data from four sites with the longest and most robust historical records: Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise.
In the 2024-25 season, the average peak snow depth at these four stations was 114% of the expected average, falling around the 19th percentile for data within the -1 to 0°C ENSO Index interval. This snowpack was notably 24% below the regression trendline, with reduced snowpack observed at lower elevation stations, largely attributed to warmer temperatures.
To mitigate the influence of long-term climate trends, an analysis was conducted using only 21st-century data. This shortened dataset resulted in a reduction of mean snow depth across all ENSO phases. However, even with this truncated dataset, the ENSO forecast for the upcoming winter predicts an "almost perfectly average" winter.
The Limited Predictive Power of ENSO Alone
Statistical analysis reveals that linear best-fit trendlines explain only 11% and 19% of the data variability for the ENSO vs. Snowpack plots, whether including or excluding 20th-century data, respectively. This critical finding underscores that over 80% of the season-to-season variability in snowpack is not explained by ENSO alone.
The Role of Sea Surface Temperatures (SSTs) and Pacific Decadal Oscillation (PDO)
The influence of marine heatwaves, particularly in September, has also been a topic of recent discussion. A comparative analysis of SSTs from September and October 2025 against those of 2024 and 2014 reveals that all three of these seasons exhibited mostly positive anomalies. While slightly positive anomalies have become increasingly common due to long-term climate shifts, the more intense anomalies warrant closer attention.
The 2014-15 season, despite being a very weak El Niño, was characterized by anomalously warm SSTs just off the coast of the Pacific Northwest. Scientists have partially attributed this to an anomalously poor snow year, with the four-station average snowpack reading languishing at less than 50% of the values observed in the 2024-25 season. Although September 2025 began with notably warm SSTs across the Northeast Pacific, these waters have since cooled significantly, and the proximal waters are approaching normal levels.
A Combined Model: ENSO and Nearshore SSTs
When both ENSO phase and nearshore West Coast Marine SSTs (often referred to as "The Blob") are incorporated into a regression model, the predictive power for snowpack variability increases to approximately 25%. This suggests that regional SSTs play a significant, albeit secondary, role alongside ENSO.
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 in the Pacific Northwest. A similar analysis examining the interplay between PDO and ENSO phases explains approximately 20% of the snowpack variance.
Conclusion: A Call for Diverse Climate Factors and Optimism
The prevailing scientific consensus for the upcoming 2025-26 winter season indicates a complex interplay of climate factors, rather than a single dominant predictor. There are no definitive "magic goggles" to precisely forecast snowpack depths. At best, the combined influence of ENSO and regional Sea Surface Temperatures can explain about 25% of the variability observed in the 75-year dataset.
The forecast points towards a weak La Niña that is likely to transition into neutral conditions by mid-winter. This scenario portends a fairly typical snowpack for the Pacific Northwest region. While the probability of a moderate or strong La Niña remains low, its occurrence would significantly increase the odds of deeper snowpacks.
Regional sea surface temperatures have cooled to near-normal levels. However, the presence of surrounding warmer waters may temper the potential for further cooling. Therefore, as the winter season approaches, the focus will be on a confluence of favorable conditions: sustained upwelling off the coasts of South America and North America, continued cool-phase PDO conditions, and a healthy marine ecosystem, including abundant sardine and salmon populations. These factors, in conjunction with a favorable ENSO phase, offer the best hope for a bountiful and snowy winter in the Pacific Northwest.
Disclaimers:
This analysis is based on current climate model projections and historical data. Weather patterns are inherently complex, and unforeseen events can influence outcomes. This report is intended for informational purposes and should not be solely relied upon for critical decision-making.
