Back in August, we published our 2024-2025 United States Winter Forecast Preview, indicating that a weak La Niña was likely based on sea surface temperature trends in the eastern equatorial Pacific Ocean and long-range model projections.

However, as of early December 2024, chances of La Niña developing this season have decreased, and the likelihood of an ENSO Neutral season is increasing. 

During a typical La Niña, colder-than-normal sea surface temperature anomalies are found along the equator from the coast of South America westward into the Central Pacific Ocean. 

Image: Sea surface temperature anomalies during a typical La Nina pattern. Blue colors indicate colder than normal sea surface temperatures, red/orange colors indicate warmer than normal sea surface temperatures, and white colors indicate near-normal sea surface temperatures.

During late summer of 2024, we began to see pockets of colder-than-normal sea surface temperatures develop in the eastern equatorial Pacific. But this trend has stalled in recent weeks, and some portions of the eastern equatorial Pacific have even started warming up again.

Take a look at the average sea surface temperature anomalies from November 10 to December 7, 2024. There is little to no indication of an emerging La Niña in the eastern equatorial Pacific, with pockets of below-normal and above-normal sea surface temperatures.

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.

What is an ENSO Neutral Season?

An ENSO Neutral season occurs when neither La Niña or El Nino criteria are met. In other words, average sea surface temperature anomalies fail to exceed -0.5ºC or +0.5ºC during five consecutive, overlapping 3-month periods.

Current ENSO Status

We are currently in a neutral ENSO phase as of early December 2024, and confidence is growing that neutral conditions may persist through the 2024-2025 winter season, rather than La Niña.

In August 2024, NOAA's ENSO Outlook indicated a 74% chance of La Niña conditions developing during the 2024-2025 winter season. In an updated outlook in mid-November, NOAA had decreased its probability of a La Niña developing to 57%.

As of November 19th, many climate models had backed off on La Nina, and were projecting neutral conditions instead with sea surface temperature anomalies failing to exceed -0.5ºC.

Image: Plot of multiple climate models projecting the phase and strength of ENSO over time. Values between +0.5ºC and -0.5ºC indicate neutral conditions in which neither La Nina nor El Nino are present. The x-axis labels are for 3-month periods, with the “DJF” label indicating “December-January-February”.

Since these most recent NOAA outlooks in mid-November, signals have continued to back away from La Niña, and OpenSnow forecasters believe that a neutral winter is becoming more likely.

The Winter Season So Far

Many areas in the West had a strong start to the season in November with above-average snowfall in the Pacific Northwest, California, and Southern Rockies (Colorado and New Mexico).

December has started out much drier than normal, and snowpack percentages have decreased as a result. Snowpack remains healthy over the Sierra, Pacific Northwest, and Colorado Rockies with below-normal snowpack for Northern Utah and much of Wyoming and Montana. 

The East experienced an unseasonably dry autumn season with wildfires burning in parts of the Mid-Atlantic and New England.

However, late November through early December has featured colder than normal temperatures, good snowmaking conditions, and abundant natural snowfall in New England, resulting in a strong start to ski season.

ENSO Neutral Status and What to Expect for the Rest of the Winter

Seasonal forecasting is difficult enough as is, and is never really that accurate to begin with. Removing a clear La Niña or El Nino signal makes the winter outlook even less certain.

But we can still look at past ENSO neutral winters, as well as QBO to see if any signals emerge...

QBO Explained

Another long-range variable we can look at is the Quasi-biennial Oscillation (QBO) which describes trade wind patterns in the tropics. The QBO oscillates between easterly (negative) and westerly (positive) phases roughly every 10-16 months with varying levels of intensity.

The QBO is not as well understood as ENSO, but it does appear to have some influence on winter weather patterns in North America.

A fairly strong westerly phase of the QBO has emerged heading into this winter. The last time we had an ENSO neutral winter and a westerly phase of the QBO was in 2013-2014. 

Analog Years

There have only been four winters since 1980 in which we had ENSO neutral conditions and a westerly phase of the QBO: 2013-2014, 1990-1991, 1985-1986, and 1980-1981. This is an admittedly small sample size, but let's take a look at these past seasons anyways.

The Winter of 2013-2014

This was an interesting winter across the West, as snowpack ended up well above average in the Rockies from Alberta and Montana to Central Colorado, but well below average across California and Oregon as well as the Southwest. Snowpack was right around average in Utah and across the Pacific Northwest.

Image: Percent of average snowpack on April 1, 2014.

This winter season started very slow in the Pacific Northwest in December and January, while the Rockies saw near-average snowfall through the first half of the season. During February and March, the Pacific Northwest saw a big comeback while the Rockies also scored deep snowfall.

A northwest flow pattern was dominant during this winter (storms tracking from northwest to southwest) while persistent high pressure over the Southwest kept California much drier, and even Utah didn't see the same abundant snowfall compared to other parts of the Rockies. 

The winter of 2013-2014 was also very active in the Eastern U.S. Snowfall and snowpack maps for past seasons are not available in the East like they are in the West, so for the sake of this article, I looked at historical snow records at Mt. Mansfield, Vermont and Snowshoe, West Virginia.

In the 2013-2014 season, snowfall was above average in Vermont and right around average in West Virginia, while temperatures were below average. New England also had a strong finish to the season with heavy snowfall in February and March. 

For nationwide temperatures, the Central and Eastern U.S. were colder than normal, while areas west of the Continental Divide were warmer than normal. One exception was Washington and Northern Idaho where temperatures were right around normal. 

The Winter of 1990-1991

The season featured above-normal snowpack in the Southern Rockies around the Four Corners and in the Pacific Northwest and Northern Rockies, with below-normal snowpack in the West Central U.S. Temperatures were below normal throughout the West. 

Image: Percent of average snowpack on April 1, 1991.

This pattern is somewhat similar to what we saw in November 2024, except that Oregon and California fared better in November 2024 (relative to average) than in the winter of 1990-91.

The East did not fare as well this season with below-normal snowfall in Vermont and West Virginia, and warmer-than-normal temperatures throughout the East. 

The Winter of 1985-1986

This season featured above-normal snowpack across the Sierra in California and across the Central and Southern Rockies, including Utah and California. Snowpack was below normal across the Pacific Northwest and Northern Rockies.

Temperatures were also above normal across most of the West in the same areas that had a deeper than average snowpack, and near normal in the drier Pacific Northwest. This indicates there may have been a lot of warm atmospheric river events that impacted the Sierra and Central Rockies. 

Image: Percent of average snowpack on April 1, 1986.

This was also a good winter for the East with above-normal snowfall in Vermont and West Virginia, and near to below-normal temperatures throughout the East.

The Winter of 1980-1981

This was a poor snow year for the West with widespread below-normal snowpack and above-normal temperatures. Interestingly enough, the one area with above-normal snowpack was in Central/Southern Arizona and New Mexico and it's possible that a late-season storm cycle could have resulted in these high anomalies on April 1st.

Image: Percent of average snowpack on April 1, 1981.

This was another good winter for the East with above-normal snowfall in West Virginia, near-normal snowfall in Vermont, and below-normal temperatures across most of the East. 

Main Takeaways:

• Considerable Uncertainty – These maps are fun to look at and may offer some slight clues, but there is a lot we don't know and every winter is unique. Also, 4 analog years out of the past 45 years is a very small sample size.
  
  
• The East tends to be favored in Neutral ENSO/Westerly QBO Years – In three of the four seasons examined, snowfall was near to above normal at the Eastern U.S. climate stations examined, and temperatures were below normal in three of the four seasons.
  
  
• The West tends to be warmer than normal in Neutral ENSO/Westerly QBO Years – In three of the four seasons examined, temperatures were warmer than normal for most of the West. The one exception is the Pacific Northwest, which tends to have closer to normal temperatures. 
  
  
• In Colorado, snowpack tends to be above normal in Neutral ENSO/Westerly QBO Years with mixed results for the rest of the Rockies. In three of the four years examined, snowpack on April 1st was above normal in Colorado. Perhaps this is a coincidence, as results were much more varied in New Mexico, Utah, Wyoming, and Montana.
  
  
• In the Pacific Northwest, snowpack tends to be better the further north you go in Neutral ENSO/Westerly QBO Years. In three of the four years examined, April 1st snowpack was well below normal in Oregon but near to above normal in Washington, Northern Idaho, and British Columbia. 
  
  
• In California, snowpack tends to be below normal in Neutral ENSO/Westerly QBO Years. In three of the four years examined, snowpack on April 1st was below normal in California and across the Sierra Nevada Range. However, there was one season with well above normal-snowpack, so you never know!

And finally, it's important to remember that...

Long-Range Outlooks Are Interesting, But Short-Range Forecasts Are King

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.

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Alan Smith