The unrecognized climate change signal: during-storm humidity increases in mountain regions of the western USA.

Precipitation phase depends on the temperature and humidity conditions during storms. In the mountainous western U.S, much of the precipitation falls when surface dew point temperatures are close to 0 °C. Therefore small changes in humidity conditions may have large impacts on the seasonal snow cover. Spatially distributed precipitation volume, air temperature, and relative humidity data provide the foundation for basin-integrated values of precipitation, phase, and humidity. Thirty-one years of hourly precipitation, temperature, and humidity data over the Reynolds Creek Experimental Watershed and CZO are used to track changes over the period of record. Our analysis shows that the proportion of the watershed that is snow-dominated has declined, the rain snow transition zone has increased in elevation, and while RCEW was initially snow-dominated, it is now dominated by mixed phase precipitation and rain.