Late onset of extreme snow drought in the western US in 2021: Origins and impacts

At the start of the 2020/2021 winter, much of the Western US was experiencing drought due to antecedent dryness and warmth from the previous year. Snowpack accumulation throughout the winter was below normal with the exception of the Pacific Northwest. By April 1, the assumed timing of peak snow water equivalent (SWE) in the West, snow drought conditions were widespread but not necessarily extreme. Snow melted at an alarming rate throughout April and by May 1, hydrologic conditions were severely degraded and summer water supply forecasts were shrinking leaving a bleak outlook across the already parched West. What drove the rapid acceleration of snow loss and how did it contribute to the intensifying 2021 Western US drought? We summarize the atmospheric and hydrologic conditions that put the late onset extreme snow drought into motion using gridded and station-based data and atmospheric reanalysis. Observations from the Snow Telemetry (SNOTEL) network will be used to examine SWE melt rates and timing of melt throughout the event in the context of historical melt rates. Initial findings suggest record SWE melt rates during the event scattered throughout the West. Reduced spring and summer streamflows were already inevitable due to the low snowpack and early melt out, however dry underlying soils, as indicated by soil moisture observations, exacerbated the situation by reducing runoff efficiency. Soil moisture observations and model output will be used to examine the hydrologic response to the rapid snow melt event. We will present a new Western Regional Climate Center tool developed to track snow droughts on a daily basis using bivariate (SWE and precipitation) phase diagrams. This study highlights the importance of real-time tracking of the seasonal progression of snow droughts and provides another example of April 1 SWE being a poor metric to inform summer water supply and drought impacts.