The ski industry is currently going through one of the most important parts of the ski season. Nope, it’s not the arrival of endlessly sunny days and predictable slarvy slashing, nor is it the deluge of college students flooding into the Rockies for spring break. Early spring is a time when many ski areas launch their season pass sales, hoping to secure most of their revenue for next season. Many ski areas in the Midwest and East Coast are riding a wave of record snowfall and a winter to remember, but for ski areas across the West, convincing people that there probably won’t be two horrible winters in a row may be a formidable challenge. The National Oceanic and Atmospheric Administration’s most recent ENSO Forecast predicts a shift from La Niña to El Niño conditions by the start of next ski season.
Many North American skiers are familiar with the El Niño Southern Oscillation, but how well does this model of long-term weather patterns stack up against the turbulent and chaotic nature of our atmosphere? In the context of one of the worst winters on record occurring during a La Niña year, which should have been a winter to remember for different reasons, it is worth taking a moment to remind ourselves of the physics of El Niño and to take a look at some other recent El Niño winters.
El Niño conditions mean that sea surface temperatures in the tropical Pacific Ocean are warmer than usual. These warmer ocean temperatures cause a weakening of the trade winds blowing across the Pacific near the equator, and a dip in the jet stream at higher latitudes. In North America, this mechanism typically means that a drier winter is expected across the Pacific Northwest and Northern Rockies, and a wetter winter is expected in the Sierra Nevada and across the Southwest. The opposite pattern, called La Niña, involves colder ocean temperatures, stronger trade winds, and snowier winters across the Northern part of the continent.
Using the wealth of climate data NOAA has been tracking, we can look at what El Niño conditions have meant for snowfall in the past. The map below shows the difference in seasonal snowfall between El Niño years and the thirty-year average, with red indicating less snow than average and blue indicating more. The Cascades are evident as a strong red line running through Oregon and Washington, and an even larger red blob extends from Idaho’s Panhandle and Western Montana into Alberta and the Western part of British Columbia. Meanwhile, the Sierra Nevada appears to have above-average snowfall, as does Utah, parts of Colorado, and most of the mountains in the Southwest. This pattern seems generally consistent with a snowier winter in mountain ranges closer to the equator and drier conditions in mountain ranges farther north. At least according to this map, if you’re a Pacific Northwest skier, I’d think about booking a trip to Lake Tahoe or Utah next winter.
The data on the map above show the average snowfall during winters with a moderate to strong El Niño index. Looking at these winters individually, using data from SNOTEL sites, the broader trend appears to be reflected at least somewhat in each El Niño winter, with a few exceptions. In 2003, California, Nevada, and Utah all had below-average snowpack, while much of Idaho and Montana had slightly above-average snowpack. Only a small chunk of Arizona and New Mexico was far above average. Similarly, in 1992, almost everywhere in the West was hurting except for the mountain ranges near the Four Corners Area. In 1998, there was plenty of snow everywhere except in the furthest reaches of Northern Montana, and in 2016, despite strong El Niño conditions, the Southwest was far below average. Because of the role that local weather patterns, orographics, and other long-term weather trends play in setting the weather, regional exceptions to the El Niño pattern are present in pretty much every El Niño winter.
Astute observers may have noticed in the map above that the deep red colors abruptly end in Southeast Alaska and switch to deep blue, indicating a much snowier winter. Alaska is certainly far away from the equator, so the snowier winters seem to contradict the general trend for El Niño in North America. However, Alaska is a special place, and Southern Alaska is home not only to incredible heliskiing but also to the Aleutian Low. This semi-permanent storm sits somewhere near the Aleutian Islands and pelts Southern Alaska with snow all winter long, occasionally sending a few storms further south to share the goods with the rest of us. El Niño’s interactions with other weather patterns, such as the Aleutian Low, are poorly understood, and recent research suggests that the Aleutian Low may play a major role in setting the phase of the El Niño cycle.
Snowfall is brought to the mountains by low-pressure storm systems, and another way to understand El Niño’s impacts on winter weather is to examine atmospheric pressure anomalies during El Niño winters. The map below shows deviations in atmospheric pressure during El Niño or La Niña winters. Instead of thinking directly in terms of pressure, atmospheric scientists like to think in terms of what height in the atmosphere a particular pressure might be found, so the colors represent the height of the 300 millibar pressure line, with redder colors corresponding to higher-than-normal atmospheric pressures, and bluer colors corresponding to lower pressures and stormier weather. El Niño is associated with higher-than-normal pressures across the Northern Rockies and Cascades, and near-neutral pressures over the Sierra Nevada and Southwest, but with a huge low-pressure anomaly in the Pacific capable of slinging storms onto the continent.
The pressure anomaly for La Niña appears to be almost exactly the opposite, with high pressure throughout much of the Pacific and low pressure across Canada. For all of the weather nerds, like myself, who have been poring over maps of atmospheric pressure anomalies all winter, a huge dome of high pressure in the Pacific looks awfully familiar. For much of this winter, a very similar dome of high pressure has been centered over California or just off the coast, much farther east than the average La Niña conditions shown above. This high-pressure system blocked almost all storms that tried to form in the Pacific from reaching the mountains, leading to the long January drought. Thus, it appears that La Niña did, in fact, show up this winter in terms of ocean temperatures and atmospheric pressure anomalies, but the positioning of those anomalies led to far less snow than average.
The positioning of the high-pressure system over California may also explain why the Midwest and East Coast had such a phenomenal ski season this winter. In mid-January, when ski areas across the Midwest and East Coast were getting hammered with snow, a high-pressure system off the West Coast was driving strong winds and bringing cold Arctic air across the eastern part of the continent. Even now, as temperatures soar at ski areas throughout the West, the Midwest and East Coast were again buried in snow over the weekend.
Similar to El Niño winters, on a winter-by-winter basis, La Niña patterns can be identified in the SNOTEL data, but there are many instances of significant deviations from expected behavior. In 2000, 2008, and 2011, essentially every mountain range reported an average or above-average snowpack. Despite strong La Niña conditions in 1999, the Sierra Nevada had a well-above-average snowpack; in 2022, most mountain ranges were below average, except for a few basins in the Southwest, which looked more like an El Niño year. Similar to strong and moderate El Niño years, plenty of exceptions can be found in specific mountain ranges in strong and moderate La Niña years.
The El Niño Southern Oscillation plays an important role in setting the winter weather patterns for North America. Yet, the atmosphere is a wild, unpredictable place, and sometimes even expected weather patterns, like this winter’s high-pressure system in the Pacific, can lead to unexpected results. So, keep an eye on those weather forecasts, but maybe don’t put those powder boards up for sale quite yet.
More From Zach Armstrong:
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