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Research Report by Climate Central
As the world warms, the meaning of winter is changing. In the U.S., a greater percentage of winter precipitation is falling as rain, with potentially severe consequences in western states where industries and cities depend on snowpack for water, and across the country wherever there is a winter sports economy.
A Climate Central analysis of 65 years of winter precipitation data from more than 2,000 weather stations in 42 states, found a decrease in the percent of precipitation falling as snow in winter months for every region of the country. Winter months were defined as the snow season for each station, from the month with the first consistently significant snow, to the last.
In western states where snowpack is critical, we found decreases in the percent of winter precipitation falling as snow at elevations between sea level and 5,000 feet. Above 5,000 feet there is clear regional variation. In California, Colorado, Wyoming, Nevada, Utah, and New Mexico there was either no trend toward rain or a slight trend toward more snow at elevations 5,000 feet and higher. In stark contrast, between 5,000 to 8,000 feet in Montana, Idaho, and Arizona, from 75 to 78 percent of all stations report an increase in rain as a percentage of total winter precipitation. Oregon has only one station above 5,000 feet, but it too reported a strong increase in rain vs snow as winter precipitation. Washington has no stations at this elevation.
These very different results at elevations above 5,000 feet may stem from the different underlying climate and weather patterns in the two regions that has delayed the shift toward more rain above 5,000 feet in Rocky Mountain states, but accelerated it in the Northwest.
In virtually all states with stations below 2,000 feet, the data show a trend toward a higher percentage of rain during the winter precipitation season.
The Northwest - A Region at Risk
- The Pacific Northwest has been the hardest hit, with low elevation snow on a clear path toward oblivion: 81 and 91 percent of stations under 2,000 feet in Washington and Oregon, respectively, show a trend toward a lower percentage of winter precipitation falling as snow over the 65 years analyzed.
- At 2,000 to 5,000 feet in Washington and Oregon the effect is similar, with 63 and 77 percent of stations recording lower percentage of precipitation falling as snow during the winter snow season, over the period analyzed. In the interior Northwest, Montana and Idaho showed the same basic trend between 2,000 to 5,000 feet, with 64 and 82 percent of stations reporting a shift to more rain as a higher percentage of precipitation than snow.
- At higher elevations in Montana and Idaho, the shift was equally strong, with 75 and 78 percent of stations reporting a decrease in the percentage of precipitation falling as snow at 5,000 to 8,000 feet. Oregon has only one station above 5,000 feet, but it too reported a strong increase.
- In the Southwest, California and Arizona showed the same trend from 2,000 and 5,000 feet, with 68 and 83 percent of stations, respectively, registering a lower percentage of winter precipitation falling as snow. From 5,000 to 8,000 feet 76 percent of stations in Arizona showed a shift to a lower percentage of precipitation falling as snow during the winter precipitation season. In California there was no trend at that elevation. New Mexico showed a slight trend toward more snow.
- The Great Plains states, Nebraska, and Kansas saw particularly dramatic shifts, with between 69 and 81 percent of stations experiencing a lower percentage of winter precipitation falling as snow, from 2,000 to 5,000 feet. South Dakota showed a trend toward a lower percentage of precipitation falling as snow at 2,000 to 5,000 feet, with 67 percent of stations reporting the trend.
- Notably, in the central Rocky Mountain States, Colorado and Wyoming, where the states are almost entirely above 5,000 feet, as well as Utah, there was no clear trend toward more rain as a percentage of all precipitation during the winter, at any elevation.
The Midwest and the East
- East of the Great Plains the trend toward a lower percentage of winter precipitation falling as snow is equally strong. Between 60 and 82 percent of stations at elevations below 2,000 feet, in 16 states, showed a sizable shift toward a lower percentage of precipitation falling as snow.
- In 6 of these states; Iowa, Michigan, Minnesota, Missouri, Virginia and West Virginia, more than 70 percent of stations showed this trend.
A majority of stations across the entire continental U.S., have an increasing percentage of winter precipitation falling as rain, with the notable exception of regions above 5,000 feet in Central Rocky Mountains and Sierra Nevada in California. These findings suggest that:
- Serious water reliability issues based on an inconsistent and declining snowpack may be just around the corner in the Pacific Northwest;
- The core Rocky Mountain States of Colorado, Wyoming and Utah, as well as New Mexico, Nevada, and high elevations in California, have yet to experience the shift. It seems unlikely, however, given the strong trend toward rain in other high elevation states, and rising temperatures across the region, that these states will resist forever the trend toward more rain as a percentage of total winter precipitation. When the shift begins, it could have serious consequences for water availability across the West, particularly in Southwestern states and California, and:
- In Midwestern and Northeastern states the strong trend toward a decrease in the percentage of winter precipitation falling as snow in winter months portends dramatic impacts on the winter sports economy across the entire region.