Prediction of Areas Susceptible to Flash Drought Development over the Contiguous United States

Flash drought is a type of drought that develops quickly (usually within 2–4 weeks) in contrast to conventional, slowly evolving drought. Due to its sudden onset, flash drought is more difficult to predict and can cause major agricultural losses if it is not forecasted in a timely manner. To improve the predictive capability of flash drought, a subseasonal tool was developed to predict areas susceptible to flash drought development using the Phase 2 of the North American Land Data Assimilation System (NLDAS-2) data. The tool calculates the rapid change index (RCI) using 7-day mean evapotranspiration anomalies. RCI is the accumulated magnitude of moisture stress changes (standardized differences) occurring over multiple weeks, and drought is likely to develop when RCI is negative. Since RCI changes with time, like all drought variables, it is difficult to capture drought development signals by monitoring RCI maps. In order to create an intuitive drought prediction map that directly depicts drought tendency, a threshold method was employed to identify grid points with large decreases of 7-day mean evapotranspiration anomaly (i.e., RCI less than −0.5) in the last 30 days and under the condition that 3-month standardized precipitation index is less than −0.4. The prediction is made by assuming that antecedent conditions persist into the next 30 days. The performance of the tool is evaluated using both retrospective and real-time predictions, by comparing to areas with 2+ categories of drought degradation in a four-week period identified from the U.S. Drought Monitor. The Heidke Skill Score (HSS) of the predictions from 2000 to 2020 shows high scores during major flash drought events, such as the 2007 Southeast drought, 2011 Texas drought, 2012 Central Great Plains drought, and 2017 Northern Plains flash drought. HSS generally is low during the cold season due to less evapotranspiration activities in the season. Large false alarm rate of evapotranspiration-based tools and the interplay between precipitation and high temperatures remain challenges for flash drought prediction.