Alaska Snowfall Trends

Another Case of Changing Seasonality

Annual snowfall trends over the past 50 years for most of Alaska and vicinity are weak, but there are clear geographic patterns, as since in Fig 1. From northern Chukotka, across the Alaska North Slope into the northwest Northwest Territories; that is, areas near the Chukchi and Beaufort Seas, snowfall is trending higher. In contrast, in the central and southern Bering Sea and Gulf of Alaska and adjacent coastal areas. there is a trend toward less snow. In between, most areas in Alaska and the Yukon show little change. However, that’s not the whole story.

Fig 1. Regression-based percent change in average September through May snowfall based on ERA5 reanalysis.

In fact, there are dramatic changes in the seasonality of snowfall happening that have important societal and environmental impacts. For many area away from the Bering Sea and Pacific Oceans, autumn (especially October and November) was historically snowy and contributed a significant portion of the annual total snowfall. However, that is changing, especially across western and southern Alaska. In most areas this is due to more of the autumn precipitation is nowadays falling as rain rather than snow.

Fig 2. Regression-based percent change in average September through November snowfall based on ERA5 reanalysis.

The situation is completely different in mid-winter, December through February. Almost everywhere from Chukotka across mainland Alaska and into the Northwest Territories is trending toward more snow. The main exceptions are along the Gulf of Alaska coast (especially the outer coast of Southeast Alaska) and the Aleutian Islands, where again more precipitation is falling as rain these days.

Fig. 3 Regression-based percent change in average December-February snowfall based on ERA5 reanalysis.

Both of these observed trends are exactly what we expect in a warming world. As ocean temperatures rise, there is more water vapor evaporating into the atmosphere. In the fall, the warm oceans are keeping low elevation temperatures high enough for longer into the season to reduce snowfall. In winter, temperatures are still low enough that the extra precipitation is falling as snow, even though at least in some areas (like Southcentral and Interior Alaska), there is more rain-in-winter than there used to be. But that’s a story for another post.

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Technical details: This analysis is based on the ERA5 reanalysis data from ECMWF/Copernicus and used the most current version of the reanalysis available as of this writing (Nov 2022). Theil-Sen regression is used in because it is less influenced by extremes in the data as ordinary least squares regression and so provides a more robust trend estimate.