Subseasonal Skill Assessment and Integration of the Standardized Evaporative Stress Ratio (SESR) to support real-time flash drought forecasts

Rapid onset flash droughts are associated with quickly developing drought conditions driven by low precipitation and increased evaporative demand which increase heat and water stress thereby negatively impacting ecological health and agricultural productivity. Uncertainty remains in subseasonal forecast models for predicting flash drought onset and identifying its windows of opportunity, where the latter provides increased predictability and subsequently increases economic resilience. To improve flash drought forecasts and identify these windows of opportunity, this study focuses on the standardized evaporative stress ratio (SESR), a well-established flash drought early warning index. Specific goals of the study are to assess seasonal variability of SESR, conduct a forecast skill assessment of SESR in GEFSv12 hindcasts, connect the forecast skill to contributions from recurring modes of subseasonal-to-seasonal climate variability, and produce and integrate SESR into NOAA Climate Prediction Center (CPC) operational subseasonal drought outlook products. To evaluate SESR forecasts, observational SESR was estimated using evapotranspiration (ET) and potential evapotranspiration (PET) from the NLDAS Noah land surface model across climatological periods 1981-2020 and 2000-2019. Flash drought accuracy was assessed with the Heidke skill score for drought intensification categories with results indicating large uncertainties in flash drought representation between different climatological periods and across different regions. We applied the SESR methodology on GEFSv12 five-week ensemble hindcasts over the period 2000-2019 and evaluated deterministic and probabilistic skill. Forecast accuracy was assessed seasonally and during enhanced modes of subseasonal predictability including the MJO and ENSO. Final deliverables will include a trend- and skill-assessment and the integration of SESR into the CPC operational pipeline to increase early-warning predictability and societal resilience against rapid onset flash droughts.