The El Niño-Southern Oscillation (ENSO) is a major climate pattern characterized by a cyclical shift between warm and cool phases in the tropical Pacific Ocean and the atmosphere above it. La Niña represents the cool phase, marked by lower-than-average sea surface temperatures in the central and eastern equatorial Pacific. This large-scale cooling influences weather patterns across the globe, with effects strongest over North America during the winter months. Whether this phenomenon translates into a snowier season depends heavily on geographic location.
Defining the La Niña Mechanism
La Niña begins with the strengthening of Pacific trade winds. These stronger easterly winds push warm surface water westward toward Indonesia and Australia. This action allows cooler, deeper water to rise to the surface in the central and eastern Pacific, a process called upwelling, leading to the characteristic cooling.
The shift in ocean temperatures profoundly affects atmospheric circulation, specifically intensifying the east-west atmospheric pattern known as the Walker Circulation. The cooler waters suppress cloud formation and rainfall in the central Pacific, while the warmer western Pacific sees increased moisture and convection. This redistribution of atmospheric heat and moisture triggers a series of global atmospheric waves, or teleconnections, that propagate from the tropics to the middle latitudes.
For North America, the most significant result is a change in the path of the mid-latitude jet stream, the fast-moving current of air high in the atmosphere that steers weather systems. During a La Niña winter, the jet stream typically shifts northward and west, entering North America closer to the northwestern United States and southwestern Canada. This pattern frequently causes the jet stream to amplify and take on a wave-like flow across the continent. The repositioning of this powerful wind current is the primary cause for the predictable regional shifts in temperature and precipitation, including snowfall.
Geographical Snowfall Predictions Across North America
The northward deflection of the jet stream during La Niña creates a distinct split in winter weather conditions across the United States. This pattern generally favors colder, stormier conditions in the northern half of the country and warmer, drier conditions across the southern tier. Above-average snowfall is highly concentrated in specific northern regions.
The Pacific Northwest and the Northern Rockies typically see the most consistent increase in snowfall during La Niña events. With the jet stream entering the continent high in this region, it frequently brings cold air and Pacific moisture ashore, leading to conditions that favor heavy winter precipitation. Historically, La Niña winters have been “banner years” for snow across western Canada and into the northern Rockies.
Farther east, the North Central United States, including the Dakotas and Minnesota, also often experiences above-average snowfall. The storm track steered by the northern jet stream frequently brings increased storminess and precipitation to the Upper Mississippi River and Ohio Valleys. This means regions like northern New England and areas susceptible to lake-effect snow along the Great Lakes can also see snowier-than-average conditions.
Conversely, the southern United States, from California across the Southwest and to the Carolinas, tends to experience less storminess and reduced precipitation. This southern tier often sees warmer and drier winters, resulting in below-average snowfall amounts. The lack of a reliable storm track in this region is a defining feature of the La Niña winter pattern.
Secondary Climate Factors That Influence Snowfall
While La Niña provides a strong seasonal forecast signal, it does not guarantee a specific outcome, as other atmospheric factors can temporarily override its influence. These short-term atmospheric oscillations introduce variability that can disrupt the expected La Niña pattern.
The Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) are two such factors that greatly affect the delivery of cold air masses. These oscillations describe changing atmospheric pressure patterns in the high latitudes, influencing the strength and direction of the polar jet stream. When the AO or NAO enters a negative phase, the polar jet stream weakens and buckles, allowing frigid Arctic air to plunge southward into the central and eastern United States.
If a negative phase of the AO or NAO coincides with a storm system steered by the La Niña-influenced jet stream, it can produce a significant snowstorm in an area typically predicted to be warm or dry. These factors are difficult to predict more than two weeks in advance, leading to unexpected cold snaps and snow events that deviate from the long-term La Niña forecast.