Precipitation Whiplash Events Across the Continental United States: An Event-Based Climatology, Impacts, and Future Projections

Abstract

Precipitation whiplash events, in which one precipitation extreme immediately follows the opposite extreme, have received increasing attention over the past decade, yet existing studies overlook spatial coherence across grid points. Incorporating spatial coherence is critical for filtering out small-scale variability and noise, thereby improving the robustness and reliability of identified events. Moreover, spatially extensive and coherent whiplash events are hypothesized to have greater potential to produce large-scale impacts, such as flooding and wildfires, and are better suited for linking to broader atmospheric drivers and sources of predictability. Additionally, to date, impact-focused research on precipitation whiplash events has largely focused on the western United States, with comparatively little attention to regions east of the Rocky Mountains. Therefore, this dissertation: (1) uniquely incorporates the spatial continuity of grid points to develop a comprehensive database of spatially coherent precipitation whiplash events on subseasonal-to-seasonal timescales across the continental United States and characterizes their climatology; (2) evaluates the relationships between whiplash events and wildfire and flood impacts relative to drought-only and pluvial-only conditions; and (3) assesses how these spatially coherent whiplash events are projected to change under future climate conditions. A new methodology incorporating spatial continuity was developed to identify precipitation whiplash events across the continental United States (1915-2020) on subseasonal-to-seasonal timescales. Events were grouped by region, and comprehensive databases were created for both large (>175,000 km2) and all event sizes, including drought-to-pluvial, pluvial-to-drought, drought-only, and pluvial-only events, and are publicly available online. These datasets were co-located with wildfire and flood records from the Monitoring Trends in Burn Severity program and the National Centers forEnvironmental Information Storm Events database. Finally, the event identification framework was applied to all 100 members of the Community Earth System Model Version 2 Large Ensemble to assess historical variability and future projections. The frequency of both drought-to-pluvial and pluvial-to-drought events has increased across all event sizes. Drought-to-pluvial events also show greater variability in event size, driven by the growth of larger events. In contrast, pluvial-to-drought eventsexhibit increases in the size of larger events but little change in overall averages. Together, these changes are expanding the total area impacted by precipitation whiplash events over time. Antecedent conditions also play an important role: longer preceding droughts amplify the magnitude of drought-to-pluvial whiplash events, whereas longer preceding pluvial periods tend to dampen the magnitude of pluvial-to-drought whiplash events. These evolving characteristics have important implications for hazard risk. Pluvial-to-drought events are associated with increased fire occurrence, while prolonged drought-only conditions tend to produce larger and more numerous fires once ignitions occur. In contrast, flood risk is more strongly linked to pluvial-only events, which generate more frequent and numerous storm reports. Whiplash events also influence the timing and persistence of hazards. Drought-to-pluvial whiplash events promote rapid-onset flooding, concentrating risk immediately following the shift to pluvial conditions, while drought-only conditions also concentrate fire activity early. In contrast, pluvial-to-drought events sustain elevated fire risk over longer periods due to fuel buildup and subsequent drying, while pluvial-only conditions distribute flood impacts more evenlythroughout pluvial periods. As precipitation whiplash events are projected to increase in frequency, spatial extent, and severity, the compounding risks associated with these transitions are also expected to intensify. Consequently, precipitation whiplash represents a growing challenge for risk management, requiring integrated approaches that account for both the magnitude of individual extremes and the rapid transitions between them in future hazard planning.