TRENDS IN THE TEMPERATURE DEPENDENCE OF SNOW VS RAIN IN THE WESTERN
US, 1949-2001
DETTINGER, MICHAEL (1); KNOWLES, NOAH (2); AND CAYAN, DANIEL (1)
(1) USGS, Scripps Institution of Oceanography, La Jolla, CA 92093-0224
(2) USGS, 345 Middlefield Road, Menlo Park, CA 92045
Precipitation can be either a boon to western water resources, flood, and land management or a flood-generating disaster, depending on whether it falls as rain or snow. Logistic regressions of daily occurrences of snow from 1949-2001 at 172 cooperative weather stations with concurrent temperatures and with year provide new measures of how snow occurrence varies with temperature and altitude across the region, and how the relations between snowfall and temperature have trended during the past 50 years.
Both temperatures and the temperature dependence of snowfall have trended in ways that increased the fraction of rainy (vs snowy) days in the Pacific Northwest and Sierra Nevada
(PNW/SN) since the 1950s, and both temperatures and temperature dependences have changed in ways that have muddied rain vs snow trends in the southern Rockies (SR) of Colorado and
New Mexico. Not surprisingly, across the western US, snow falls most often on days with minimum temperatures below freezing, but when minimum-temperature thresholds--“even-odds temperatures,” Teo , at which half of the wet days yield snow and half yield only rain--are determined for each station, the Teo range from about -8ºC to 0ºC. Snowfall frequencies in the
PNW/SN change abruptly from all snowy days at temperatures just colder than Teo to all rainy days at slightly warmer temperatures. However, stations in the SR have broader ranges of temperatures under which some days yield snow and others rain. The well-documented warming in the region since the 1950s has yielded, at 86% of the stations, trends for more wet days (+6% overall) to have had minimum temperatures warmer than Teo recently. The temperature dependence of snowfall also has trended significantly at 62% of the stations, with snowfall in
PNW/SN requiring colder minimum temperatures (at every probability level) now than in the
1950s and a converse trend in SR.