Could a Brief Wet Spell Accelerate Drought Onset? Climate-Dependent Mechanisms Behind Onset Speed

Flash droughts are defined by their unusually rapid onset, yet what controls how fast they develop remains unclear and may differ fundamentally across climate regimes. Here we deliver a global, process-oriented assessment of flash drought onset speed using satellite-derived evaporative stress to characterize land-surface water–energy limitations, and SHAP (Shapley Additive Explanations) to diagnose the dominant hydrometeorological drivers of acceleration. We find that while humid regions experience flash droughts more frequently, events in drylands intensify more rapidly. This contrast reflects differences in energy and water constrains: net radiation plays a greater role in humid regions, whereas surface drying dominates in drylands. Moreover, short-term antecedent moisture recovery followed by rapid drying accelerates onset, with soil moisture depths and timescales exerting region-specific influences. These results reveal climate-dependent mechanisms underlying flash drought intensification and highlight the need for tailored monitoring strategies in diverse hydroclimatic contexts.