The Great Western Snow Drought of 2015

The winter of 2015 brought unusually warm temperatures to the western United States and serves as a classic example of how warm temperatures can cause snow drought. By April 1, not a single basin in the West was above 86% of median snow water equivalent—and most basins in Washington, Oregon, California, Nevada, Utah, and Arizona were below 40%.

USDA Natural Resources Conservation Service map showing snow water equivalent for the Western U.S. for April 2015. — Figure 1. Snow water equivalent values for the Western U.S. as a percent of the 1981-2010 median, as of April 1, 2015. Source: USDA Natural Resources Conservation Service.

The hardest hit regions were the Cascades and the Sierra Nevada, with snow water equivalent ranging only from about 5% to 20% of median. This was the lowest snowpack in the observational record for the Sierra Nevada. Precipitation in the Sierra Nevada, on the other hand, was quite dry, at 50%–60% of median. More remarkable were the Cascades, where seasonal precipitation was near normal—about 90%–100% of median. Looking only at precipitation, the Cascades would not be designated as being in drought.

USDA Natural Resources Conservation Service map of the Western U.S. showing water year to date precipitation as a percent of the 1981-2010 median — Figure 2. Water year to date precipitation for the Western U.S. as a percent of the 1981-2010 median. Source: USDA Natural Resources Conservation Service.

How does such a sharp contrast between snow water equivalent and precipitation manifest throughout the winter? Above-normal temperatures persisted throughout almost the entire season, with many locations in the West setting all-time October through March temperature records, which date back to 1895.