The world is currently experiencing a La Niña cycle, which means that water temperatures in the Central Pacific Ocean are lower than average for at least three months in a row. The threshold to be in La Niña is at least 0.5 degrees Celsius lower than average, or about one degree Fahrenheit. An El Niño period is the opposite, with water temperatures at least 0.5 degrees Celsius higher-than-average. A La Niña period gets categorized as weak, moderate or strong, but can’t be categorized until it’s over and temperature data can be analyzed. The most recent La Niña was 2017-18. Over the last 50 years, time has been more or less evenly split between La Niña, El Niño, or nothing — water temperatures close to average.
La Niña periods typically peak in strength in the winter, making them relevant to winter forecasting. According to Joel Gratz, founding meteorologist and CEO of Open Snow, “With a decently strong La Niña, the Pacific Northwest — so Washington, British Columbia, those areas — has nearly a lock on above average snowfall.” La Niña affects weather because changes in Pacific Ocean water temperatures ultimately influence storm tracks that move through the Pacific and into the United States and Canada. How a La Niña impacts winter in Colorado is less predictable.
Gratz explains, “On the balance, if you look at all La Niña over the last 50 to 100 years, usually they result in average or above average snowfall [for Colorado] and the storm tracks are generally more out of the west and northwest, which does pretty well for this area of Colorado. So, overall, it’s good news. But every La Niña is different, and La Niña is not the only factor that tells storms where to go. So, in any given La Niña cycle, we could have an above average season, an average season or even a poor season.”
When Gratz refers to storm tracks out of the west and northwest doing well for this area of Colorado, he’s referring to “orographic precipitation.” Orographic precipitation is induced by the presence of mountains. Moist air runs into a mountain and is forced to rise, where it cools and forms clouds that can result in rain or snow. The direction a storm comes from, and the orientation of mountains in relation to that storm track, can impact what kind of snowfall local resorts receive. The taller the mountains, the bigger the impact. Again, Gratz: “Aspen is ringed on three sides by higher mountains, so the best way to get good snow at Aspen is a wind from the west or northwest.” Wind direction is less predictive of snowfall for Sunlight Mountain Resort, because the mountains are not as tall.
According to Gratz, the last strong La Niña period, winter of 2010-11, was much above average for snowpack, but the most recent moderate La Niña, 2011-12, was below average. The most recent weak La Niña, 2017-18, was far below average.
Currently, the snowpack in the Upper Colorado River Basin is about 70 percent of average. Much of western Colorado is currently experiencing “exceptional drought,” the most extreme drought category used by the U.S. Drought Monitor.
According to Gratz, the upcoming forecast is “a little snow on Thursday, though the most snow will be north, [and] some snow on Saturday. At least a couple of inches likely. A potential storm around Jan. 18 or 19, a potential storm around Jan. 21 and a potentially-stormy time around Jan. 25 through the end of the month. All of this is reasonably good news, but it doesn’t count until there are flakes on the ground.”