Evaluation of Subseasonal Drought Forecast Skill over the Coastal Western U.S.

February 10, 2023

Lu Su, a Ph.D. student from UCLA, with her advisor and CW3E collaborator, Professor Dennis Lettenmaier, alongside co-authors Qian Cao (CW3E), Shraddhanand Shukla (UCSB), Ming Pan (CW3E) have recently published a paper titled “Evaluation of Subseasonal Drought Forecast Skill over the Coastal Western U.S.” in the Journal of Hydrometeorology (Su et al., 2023; https://journals.ametsoc.org/view/journals/hydr/aop/JHM-D-22-0103.1/JHM-D-22-0103.1.xml). The research aligns with the Subseasonal to Seasonal Prediction of Extreme Weather Area and the Modeling Capabilities for the Western United States Area within CW3E’s 2019-2024 Strategic Plan because it evaluates the subseasonal drought forecast skill using the Noah Multi-parameterization (Noah-MP) model.

Predictions of drought onset and termination at subseasonal (from two weeks to one month) lead times could provide a foundation for more effective and proactive drought management. This study used reforecasts archived in NOAA’s Subseasonal Experiment (SubX) to force the Noah Multi-parameterization (Noah-MP), which produced forecasts of soil moisture from which we identified drought levels D0-D4. It evaluated forecast skill of major and more modest droughts, with leads from one to four weeks, and with particular attention to drought termination and onset. Usable drought termination and onset forecast skill was found at leads one and two weeks for major D0 -D2 droughts; and limited skill at week three for major D0-D1 droughts, with essentially no skill at week four regardless of drought severity (see Figure 1 and Figure 2). Furthermore, for both major and more modest droughts, limited skill or no skill was found for D3 -D4 droughts. Skill is generally higher for drought termination than for onset for all drought events. In addition, drought prediction skill generally decreases from north to south for all drought events. Evaluation of the subseasonal drought forecast skill can better support water resource and emergency management decisions.

This work used the COMET supercomputer, which was made available by the Atmospheric River Program Phase 2 and 3 supported by the California Department of Water Resources (awards 4600013361 and 4600014294 respectively) and the Forecast Informed Reservoir Operations Program supported by the U.S. Army Corps of Engineers Engineer Research and Development Center (award USACE W912HZ-15-2-0019). The research was funded in part by NOAA Regional Integrated Sciences and Assessments (RISA) support through the California–Nevada Applications Program (Grant NA17OAR4310284).

Figure 1: SubX-based debiased Brier skill score (BSS) for lead weeks 1-4 for drought termination. The columns show results for drought levels D0-D4; the rows show leads from week1 to week4. Blank areas denote no drought at this level in this location.

Figure 2: SubX-based debiased Brier skill score (BSS) for lead weeks 1-4 for drought onset. Columns show drought levels D0-D4; rows show leads from week1 to week4.

Su, L., Q. Cao, S. Shukla, M. Pan, and D. P. Lettenmaier, 2023: Evaluation of Subseasonal Drought Forecast Skill over the Coastal Western U.S. J. Hydrometeor., doi: https://doi.org/10.1175/JHM-D-22-0103.1