Following a 2023-2024 winter season that started slow and finished strong in many regions, it's time to shift gears and take a look at the upcoming 2024-2025 winter forecast for the United States.

Winter outlooks contain an inherent degree of uncertainty since so many factors in the atmosphere are not predictable months or even weeks in advance. However, there are a few variables including ENSO (the El Niño Southern Oscillation) that can provide some clues.

La Niña is Likely for 2024-2025

During the winter of 2023-2024, El Niño conditions were present. El Niño began to fade during the spring of 2024 and we have been in a neutral phase (meaning neither El Niño nor La Niña is present) during the summer of 2024.

During the fall of 2024, La Niña conditions are expected to emerge with NOAA's model-based outlook projecting a 74% chance of La Niña conditions prevailing through the winter of 2024-2025.

Image: NOAA Forecast showing the probability of an El Niño, La Niña, and Neutral phase for each three-month period between the summer of 2024 and the spring of 2025. Red bars show the percent chance of an El Niño and the “DJF” label is for “December-January-February”.

What is La Niña?

The term La Niña refers to the large-scale ocean-atmosphere climate phenomenon linked to periodic cooling in sea-surface temperatures across the central and east-central equatorial Pacific.

La Niña represents the cool phase of the ENSO cycle and means that the ocean water temperatures are cooler than average.

La Niña Criteria

1) The average sea surface temperatures in the Niño-3.4 region of the equatorial Pacific Ocean were at least 0.5°C (0.9°F) cooler than average in the preceding month.

2) The average anomaly of at least -0.5°C (-0.9°F) has persisted or is expected to persist for five consecutive, overlapping 3-month periods.

Expected La Niña Strength and the Winter Ahead

The relative strength of a La Niña is also an important factor when it comes to prevailing weather patterns. A given La Niña (or El Niño) phase is rated as weak, moderate, or strong depending on sea surface temperature anomalies in the Niño-3.4 region of the equatorial Pacific Ocean.

So far, La Niña conditions have been slower to emerge than previously forecast earlier this summer. La Nina conditions were originally forecast to emerge by late summer (August or September), but it is now expected to be well into the autumn season before the transition to La Niña occurs.

As of early August 2024, the sea surface temperature anomaly in the Niño-3.4 region is still slightly above average at +0.2ºC, which means we still have a ways to go before reaching La Nina status at -0.5ºC or lower.

In addition to a slower onset of La Nina, most climate models are projecting this to be a weak La Niña episode with sea surface temperatures ranging from -0.5 to -1.0ºC in the Niño-3.4 region of the equatorial Pacific Ocean.

Image: Plot of multiple climate models projecting the phase and strength of ENSO over time. Positive values of +0.5 or greater indicate El Nino, with values of +1.5 or greater over three consecutive 3-month periods indicate a strong El Nino. The dark purple line indicates the dynamical model average, and the “DJF” label is for “December-January-February”.

Since 1990, there have been 6 winters in which weak La Niña conditions were present. The last weak La Niña occurred just two years ago in 2022-2023 (which, by the way, was a huge winter for much of the West).

The image below shows precipitation anomalies during these weak La Niña “analog” years.

Image: Departure from average precipitation from December through March during the weak La Niña analog winters of '22-23, '17-18, '16-17, '08-09, '05-06, and '00-01. Green areas show wetter (which usually means snowier) weather.

The next image shows temperature anomalies during weak La Niña years.

Image: Departure from average temperatures from December through March during the weak La Niña analog winters of '22-23, '17-18, '16-17, '08-09, '05-06, and '00-01. Green areas show colder than average temperatures and yellow/red areas show warmer than average temperatures. 

Weak La Niña Year Takeaways

• Pacific Northwest – Surprisingly, the signals point to drier conditions. Conventional wisdom is that La Niña favors cold and snowy winters in the PNW, and while the colder signal does show up in weak La Niña years, recent history tells us that weak La Niña phases actually favor below-average precipitation. Only two of the six analog years were wetter than average in this region.
  
  However, drier does not always mean less snowy! When examining the six analog years based on April 1st snowpack, 4 of the 6 years had a deeper-than-average snowpack in the Washington and Oregon Cascades. This is likely a result of lower snow levels and higher snow-liquid ratios in weak La Niñas compared to average.
  
  
• California & Tahoe – The overall signal when averaging the six analog years together is for colder and snowier conditions, but with a major caveat. Three of the six analog years were huge winters with well above average precipitation (that skewed the entire average up). However, two of the six analog years were much drier than average, and one of the six was right around average.
  
  In other words, there is high upside potential but only 50% odds of an above-average winter based on recent history. 
  
  
• Rockies – Above-average precipitation is favored overall, with the most consistent signal over the Northern Rockies from the Tetons to the Canadian border where 5 of the 6 analog winters were generally above average.
  
  Utah (SLC region) also has a decent signal with 4 of 6 analog years featuring above-average precipitation, while Colorado is 50/50 with only 3 of 6 analog years coming in above average.
  
  
• East and Midwest – For most of this region, including New England, the signals are weak for these 6 analog years. Exceptions include the Mid-Atlantic where drier conditions are favored, and most of the Great Lakes region where above-average precipitation is favored. 
  
  When averaging all six analog years together, temperature anomalies stray very little from the 1991-2020 average.

QBO Influence on Winter Weather

Another long-range variable that can influence winter patterns is the Quasi-biennial Oscillation (QBO) which describes trade wind patterns in the tropics. A westerly phase of the QBO is expected to occur this winter, which is a phase that has shown some correlation with above-average winter precipitation across the Western U.S.

Although the sample size is small, winters that feature a weak La Niña combined with a westerly phase of the QBO appear to have a better chance of being wetter (and snowier) than average across the Rockies and the Sierra, but still drier than average across the Cascades.

Weak La Nina years featuring a westerly phase of the QBO also tend to be colder than average across the West (based on 1991-2020 averages) and warmer than average across the East. 

Looking Ahead

Keep in mind that no matter how deep or light a winter is compared to average, when it comes to skiing it’s all about timing. Booking a trip 7-10 days in advance and for a general area that looks stormy will increase your chances of scoring deep powder days.

Also, stay tuned for region-specific winter forecast previews this fall from our local forecasters, including regions outside the U.S. such as Canada and Europe. 

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Alan Smith