For the upcoming 2024-2025 winter season, La Niña conditions are growing and are favored to emerge during the late fall. Before getting into the details on what that could mean for Aspen Snowmass, it's important to remember that any winter outlook will contain an inherent degree of uncertainty.

Long-range forecasts are rarely accurate. These forecasts cover 3-6 months but we know that skiing quality improves and degrades with storm cycles that last a few days to a week.

La Niña, Explained

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.

Ski Season Snowfall vs. La Niña

The map below shows winter snowfall during seven La Niña episodes. The lower the number, the stronger the La Niña. The blue dots are above average, the white dots are average, and the orange dots are below average snowfall.

"ENSO Neutral" conditions (neither La Niña nor El Niño) have persisted through the summer of 2024 and long-range models have been projecting a higher-than-average chance of a La Niña taking over during the winter of 2024-2025.

The weekly Nino-3.4 region index (sea surface temperatures in the east-central Pacific) anomaly has dropped from 1.8°C during the 2023-2024 winter season to -0.1°C as of September 9, 2024.

A similar change from El Niño to a weak La Niña last occurred ahead of the 2016-2017 winter season.

Thanks to my colleague Alan Smith, we have identified six winters since 1990 in which weak La Niña conditions were present:

• 2000-2001
• 2005-2006
• 2008-2009
• 2016-2017
• 2017-2018
• 2022-2023

We will use these winter seasons that featured a weak La Niña as “analog” years for this analysis.

Historical Weak La Niña Seasons @ City of Aspen

After looking back at the six weak La Niña winters and comparing them against the 30-year median snowfall of 166 inches for the City of Aspen, I've found that the median snowfall in Aspen during those years is 165 inches or 100% compared to the 30-year normal.

• 1991-2020: 166" (30-year normal)
• 2000-2001: 151" (90%)
• 2005-2006: 159" (95%)
• 2008-2009: 201" (122%)
• 2016-2017: 158" (95%)
• 2017-2018: 127" (76%)
• 2022-2023: 218" (131%)

Two out of the six La Niña years produced well above-normal snowfall, three produced near-normal snowfall, and one produced well below-normal snowfall for the City of Aspen. 

For timing, when looking at those six weak La Niña winters and comparing them against the 30-year median snowfall for the City of Aspen:

• October: 7" (75%)
• November: 25" (108%)
• December: 31" (122%)
• January: 39" (144%)
• February: 18" (65%)
• March: 21" (79%)

This gives us a signal that the shoulder seasons could be below-normal, while the early winter months could be above-normal.

Historical Weak La Niña Seasons @ Independence Pass

After looking back at the six weak La Niña winters and comparing them against the 30-year median snow water equivalent (SWE) on March 31 of 15.8 inches at the Independence Pass SNOTEL station near Aspen, I've found that the median SWE near Aspen during those years is 16.3 inches on March 31 or 103% compared to the 30-year normal.

SWE on March 31 During La Niña

• 1991-2020: 15.8" (30-year normal)
• 2000-2001: 14.3 (91%)
• 2005-2006: 15.9 (100%)
• 2008-2009: 19.7 (125%)
• 2016-2017: 16.7 (106%)
• 2017-2018: 10.5 (66%)
• 2022-2023: 17.0 (108%)

One out of the six La Niña years produced well above-normal snowfall, four produced near-normal snowfall, and one produced well below-normal snowfall.

For timing, when looking at the change in SWE each month during those years and comparing them against the 30-year median change in SWE for that month:

• October: 0.9 (77%)
• November: 2.5 (74%)
• December: 3.3 (116%)
• January: 3.0 (105%)
• February: 2.5 (83%)
• March: 2.5 (89%)

This also gives us a signal that the shoulder seasons could be below-normal, while the early winter months could be above-normal.

2016-2017 Winter Season

As mentioned above, the Niño-3.4 region index anomaly as of September 9, 2024, is down to -0.1°C, which is similar to where it was ahead of a La Niña event during the summer of 2016.

Since this last occurred back in 2016, let's have a little fun by looking back at the snowfall during the 2016-2017 winter season at Aspen Snowmass.

Data courtesy of BestSnow.net:

• Aspen Highlands: 320" (123%)
• Aspen Mountain: 255" (102%)
• Snowmass: 296" (98%)

Overall, history tells us that Aspen Snowmass tends to be right around normal for snowfall during weak La Niña winters.

Having said all of this, for skiers and snowboarders, keep in mind that when it comes to finding the best conditions, it’s all about timing. To have the best chance of enjoying the deepest powder, our recommendation is to book a trip 7-10 days in advance.

Sometimes, longer-range forecasts can identify possible storms 1-2 weeks (or longer) in advance, but often, forecast confidence in the details of each storm only begins to increase when the system is about one week away or closer.

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Sam Collentine