The western United States is undergoing a major shift in precipitation patterns. Large swaths of the West that have historically been dominated by snow in the winter months are starting to see a lot more rain instead. A new study that maps out the predominant form of precipitation shows that this trend could result in an average reduction in snow-dominated area of around 30 percent by the middle of this century.
The western US depends heavily on snowpack to sustain its water supply through the dry summertime, but the new research, published in Geophysical Research Letters in July suggests this may have to change.
The map above shows the current extent of snow-dominated (shown in white and grey) and rain-dominated (shown in blue) areas during the winter months of December, January and February based on data from 1979–2012. Red and pink areas typically have a mix of rain and snow during these months, with redder areas being wetter. The grey areas are projected to transition from 100 percent snow to a mix of rain and snow over the next few decades.
Even more striking is the projection for the shoulder months in the spring and fall as shown on the series of maps to the right. These maps show the probability of snow and rain for each month based on climate models, with 100 percent probability that any precipitation will be rain shown in blue, mixed rain and snow shown in red and pink, and 100 percent snow shown in white. Huge areas that typically have a lot of snow during October and November and March and April could have only rain during those months in the future.
The local details are even more depressing. Areas that could see a complete loss of area that is dominated by snow during the winter months include: the northern Rockies, the Cascades, Puget Sound, the Blue Mountains, the northern Basin and Range, and the Colorado Plateau. And other snowy areas will see drastic reductions, including the Sierra Nevada, the Wasatch and Uinta Mountains in Utah and the Snake River plain.
Overall, the study projects the West could see an average monthly reduction of snow dominated area from historical levels of 53 percent down to 24 percent by mid-century, as shown in the graph below. Much of the strongly snow-dominated area of the West could go from five snowy months down to three.
For more details and data check out the website of lead author Zion Klos of the University of Idaho.
Reference: Klos, P. Z., T. E. Link, and J. T. Abatzoglou (2014), Extent of the rain-snow transition zone in the western U.S. under historic and projected climate, Geophys. Res. Lett., 41, doi:10.1002/2014GL060500.
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