The winter of 2024-2025 has arrived with a familiar mix of anticipation and uncertainty for Pacific Northwest residents, particularly concerning snowpack levels. A retrospective look at the preceding winter, 2023-2024, revealed that a weak La Niña phase, a pattern often associated with cooler and wetter conditions, did not necessarily translate into the substantial mid-winter snow accumulations that skiers, outdoor enthusiasts, and water resource managers eagerly anticipate. This upcoming season’s outlook, therefore, is being scrutinized through the lens of historical data and evolving climate indicators, with a particular focus on the El Niño-Southern Oscillation (ENSO) cycle.
Understanding the ENSO Cycle and Its Predictive Power
The El Niño-Southern Oscillation (ENSO) is a naturally occurring climate phenomenon characterized by fluctuations in sea surface temperatures across the tropical Pacific Ocean. These fluctuations significantly influence global weather patterns and are broadly categorized into three distinct phases: El Niño (warm phase), La Niña (cool phase), and Neutral conditions. Forecasts issued in October provide crucial insights into the probable ENSO phase for the upcoming winter.
Comparing the October forecasts for mid-winter ENSO probabilities between the 2024-2025 season and the most recent one, a striking similarity emerges. Both forecasts point towards a strong likelihood of cool-phase conditions prevailing during the autumn and early winter months. However, there is a subtle indication of a weakening La Niña signal for the 2025-2026 period. This suggests a delicate balance in the atmospheric drivers that dictate regional weather.
The Oceanic Niño Index (ONI) and Current Projections
A principal metric for monitoring and predicting ENSO is the Oceanic Niño Index (ONI). This index is derived from sea surface temperature (SST) departures from the average in the Niño 3.4 region of the Pacific. For the 2023-2024 season, the December-January-February (DJF) ONI registered -0.6°C, a value that barely placed it within the weak La Niña territory.
Current data indicates that the upcoming winter is again poised on the edge of a weak La Niña. The latest weekly SST departure stands at -0.6°C, mirroring the conditions of the previous year. Mid-October Oceanic SST models project a minimum ONI of approximately -0.8°C in November, with a subsequent decrease to around -0.3°C by mid-winter (DJF). This trajectory suggests that neutral ENSO conditions are the most probable outcome for the mid-winter period, potentially influencing the depth and duration of Pacific Northwest snowpacks.
Historical Snowpack Responses to ENSO Phases
To better understand the potential implications of these ENSO projections, researchers have analyzed an updated Snowpack Climatology Dataset for the Pacific Northwest mountains. This dataset provides valuable context by illustrating how regional snowpack typically responds to different ENSO phase strengths.
Weak La Niña Winters: During weak La Niña winters, the data indicates that most weather stations in the region tend to experience snowpack levels close to the historical average. A modest increase in snowpack is observed at specific locations like Mission Ridge and Timberline. This phase generally does not deliver the exceptionally deep snow years that many hope for.
Moderate and Strong La Niña Winters: The analysis clearly shows that moderate and strong La Niña years are the most favorable for substantial snow accumulation on the west slopes and crest of the Cascade Mountains. During these periods, peak snow depths can be expected to range from 6% to 24% above the historical average. These are the conditions that often lead to banner ski seasons and robust water reserves.
Neutral ENSO Years: Neutral ENSO conditions typically correlate with slightly below-average snowpack across the Pacific Northwest. While not as detrimental as El Niño, these years can still result in less abundant snowfall compared to strong La Niña events.
El Niño Winters: Conversely, most El Niño conditions strongly tilt the odds towards reduced snowfall in the region. Warmer ocean temperatures associated with El Niño can alter atmospheric circulation patterns, leading to drier and warmer conditions, which are inimical to deep snow accumulation.
Analyzing the Probabilities for 2024-2025
The DJF forecast distribution for ENSO probabilities for the upcoming winter reveals some key figures that will likely shape the snowpack. There is an 18% chance of experiencing a moderate to strong La Niña, and a mere 2% chance of an El Niño event. These probabilities, particularly the lower likelihood of a strong La Niña, underscore the potential for a more average winter.
To refine the outlook, an examination of the average peak snow depth at four sites with the longest and most robust historical data records—Mt. Baker, Stevens Pass, Snoqualmie Pass, and Paradise—provides a more localized perspective.
In the 2023-2024 season, the average peak snow depth at these four stations was 114% of normal. This figure falls within the 19th percentile for data points corresponding to an ENSO Index interval of -1 to 0°C. Notably, the snowpack was 24% below the regression trendline, with reductions most pronounced at lower elevation stations, likely influenced by warmer temperatures.
To mitigate the impact of long-term climate trends on this analysis, a subset of the data including only 21st-century records was examined. This shortened dataset, while lowering the mean snow depth across all ENSO phases, still points towards an "almost perfectly ‘average’" winter for the upcoming season based on the ENSO forecast.
The Limited Predictive Power of ENSO Alone
It is crucial to acknowledge that ENSO is not the sole determinant of Pacific Northwest snowpack. Linear regression models, both including and excluding 20th-century data, indicate that ENSO explains only 11% and 19% of the data variability, respectively. This implies that over 80% of the year-to-year variability in snowpack is not directly attributable to ENSO.
The Influence of Marine Heatwaves and Sea Surface Temperatures
Recent attention has been drawn to marine heatwaves, particularly a notable event in September of the previous year. An analysis comparing SSTs from September and October of 2024, 2025, and 2014 reveals interesting patterns. While all three seasons exhibited mostly positive SST anomalies, a common occurrence as long-term climate shifts, the intensity and location of these anomalies warrant closer inspection.
The 2014-2015 season, despite a very weak El Niño, was characterized by anomalously warm SSTs just off the coast. Scientists have partially attributed the unusually poor snow year of that season to these warm waters. In contrast, the four-station average snowpack reading during that period was less than 50% of the values recorded in the 2023-2024 season.
For the upcoming season, while September 2024 began with significantly warm SSTs across the northeastern Pacific, these waters have since cooled considerably, with proximal waters approaching normal levels. This cooling trend is a positive development for potential snow accumulation.
Combining ENSO and Regional SSTs for a Broader Picture
A more comprehensive model that integrates ENSO phases with regional West Coast Marine SSTs (often referred to as "The Blob") can explain approximately 25% of the snowpack’s inter-seasonal variability. This suggests that while ENSO plays a role, the influence of regional ocean temperatures is also significant.
The Pacific Decadal Oscillation (PDO) and Its Correlation with Snowpack
The Pacific Decadal Oscillation (PDO) is another important climate driver that influences Pacific Northwest weather. The PDO is a long-term pattern of sea surface temperature and atmospheric circulation in the North Pacific. When exceptionally warm waters remain west of Hawaii, it signals a negative phase of the PDO.
As the State Climate Office notes, 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 complex interplay of these climatic factors.
Looking Ahead: A Season of Averages and Lingering Possibilities
In conclusion, the outlook for the 2024-2025 winter in the Pacific Northwest suggests a season of relative averages, with a weak La Niña likely transitioning into neutral ENSO conditions by mid-winter. This scenario portends a typical snowpack for the region. The probability of a moderate or strong La Niña, which would favor deeper snow accumulations, remains low.
Regional sea surface temperatures have cooled to near-normal levels, a positive indicator. However, the presence of surrounding warmer waters may temper the potential for further cooling, acting as a moderating influence.
For optimal snowpack development, the region will be hoping for a confluence of favorable conditions: sustained upwelling of cold water off the coasts of South America and North America, continued cool-phase PDO conditions, and a healthy marine ecosystem supporting sardine and salmon populations, which often correlate with cooler ocean cycles.
While climatic models and historical data provide valuable insights, it is essential to recognize that predicting precise snowpack levels remains a complex endeavor. The interplay of ENSO, regional SSTs, the PDO, and numerous other atmospheric variables means that forecasting snow depth is an inexact science. Residents and stakeholders will undoubtedly be closely monitoring weather patterns throughout the winter, hoping for the best while preparing for the most probable outcomes. The 75-year dataset underscores that while ENSO and regional sea surface temperatures are influential, they account for only a portion of the year-to-year variability in snowpack, leaving room for other factors to play a significant role in shaping the winter’s narrative.
