Temporal changes in drought indices used to provide early warning of drought development over sub-seasonal time scales

In this study, the potential utility of using rapid temporal changes in drought indices to provide early warning of an elevated risk for drought development over sub-seasonal time scales is assessed. Standardized change anomalies were computed each week during the 2000-2013 growing seasons for drought indices depicting anomalies in evapotranspiration, precipitation, and soil moisture derived from satellite remote sensing observations and land surface model output. A new metric, known as the Rapid Change Index (RCI), was created to encapsulate the accumulated magnitude of rapid changes in the weekly anomalies for each dataset, and is designed to highlight areas undergoing either rapid increases or rapid decreases in moisture stress. Case study analyses revealed that the initial appearance of negative RCI values indicative of rapid increases in moisture stress often occur several weeks prior to the introduction of severe drought conditions in the United States Drought Monitor. Statistical analyses showed that drought intensification probabilities derived from the RCI datasets have some reliability and forecast skill over the central and eastern United States in regions most susceptible to rapid drought development. Taken together, the results suggest that tools used to identify areas experiencing rapid changes in drought indices may be useful components of future drought early warning systems.