Monitoring of drought in the western United States: metrics suitable for tracking drought historically and throughout the 21st century

Drought is a complex phenomenon in the western United States due to intersecting meteorological, physiographic and societal facets. These complexities have prompted stakeholders to express an increasing need for local scale information on drought. The WestWide Drought Tracker (WWDT) is an effort to create a unifying framework for historical and real-time drought monitoring that serves as an interface between scientific research datasets and stakeholders. The WWDT provides intuitive visualization and application tools to disseminate the complex spatio-temporal nature of drought across the western US. The web-based product provides user-friendly access to historical and real-time local drought information by incorporating climate and hydrologic data from both gridded and station-based data sets, accessible through the Western Regional Climate Center. Drought impacts are further complicated by both their duration and their sensitivity to temperature, both of which historically exhibit stark gradients across the western US and are hypothesized to respond in a non-stationary fashion due to climate change. In order to accommodate these challenges, indices better suited to incorporate the impacts of climate change on drought, such as changes in mountain snowpack storage efficiency and evapotranspiration, should be considered for applied use both in present day monitoring and climate change adaptation measures. Examples of retrospective and projected drought indices highlight the constraints of current drought indices and the importance of novel drought metrics that can be used both today and for future impact assessment.