Hydroclimatic Whiplash Across the Contiguous United States: Characterizing Wet–Dry Transitions in PDSI and PHDI

Hydroclimatic whiplash, characterized by rapid transitions between wet and dry conditions, poses a growing challenge for water resource management and ecosystem stability. However, the spatial and statistical characteristics of wet–dry transitions across the contiguous United States are not fully understood. This study analyzed Palmer Drought Severity Index (PDSI) and Palmer Hydrological Drought Index (PHDI) records from 344 NOAA climate divisions spanning 1895–2024. Whiplash was defined as a transition between severe drought and severe wetness within 12-month rolling windows. Statistical descriptors including lag-1 autocorrelation, variance, and skewness were evaluated for whiplash and non-whiplash windows to assess differences between them. Whiplash hotspots were identified in Texas, Southern California, northern Colorado, western New Mexico, and parts of the Midwest, with PDSI detecting higher whiplash frequency than PHDI. This difference reflects PDSI’s greater short-term sensitivity to near-surface soil moisture compared to PHDI’s longer-term hydrological persistence. Whiplash windows showed elevated variance and autocorrelation relative to baseline conditions across both indices. These findings demonstrate that hydroclimatic whiplash is strongly dependent on index selection and threshold definition, and that elevated variance and autocorrelation may serve as statistical indicators of transition periods with important implications for drought monitoring and climate adaptation planning.