An Overview of Current Research on Rapidly Intensifying Flash Droughts

Drought is often thought of as a slowly evolving climate phenomenon that takes many months or even years to reach its peak intensity; however, recent events across the U.S. have shown that droughts can develop very rapidly if extreme weather anomalies persist over the same region for several weeks to months. Here, we provide an overview of recent research into these rapid onset “flash drought” events that has contributed to a deeper understanding of their characteristics and the processes that control their evolution and predictability over sub-seasonal time scales.

Flash droughts should be viewed as a subset of all droughts that are distinguished from more conventional slowly developing droughts by their unusually rapid rate of intensification. Recent studies have shown that though a deficit in precipitation over some time period is a basic requirement for drought to develop, the speed with which it develops and its ultimate severity are influenced by other environmental anomalies. Flash drought events signified by a rapid reduction in root zone soil moisture content and the rapid emergence of vegetation moisture stress are more likely to develop when large rainfall deficits occur at the same time as other extreme weather anomalies that enhance evaporative demand. For example, unusually high temperatures, large vapor pressure deficits, strong winds, and sunny skies can work together to quickly deplete root zone soil moisture reserves due to increased evapotranspiration (ET). If rainfall is insufficient to compensate for the increased evaporative demand, vegetation stress can rapidly develop over sub-seasonal time scales. This scenario is most likely to occur during the growing season when evaporative demand is highest, which means that flash droughts are most closely associated with agricultural and ecological droughts and can have a large impact on crop yields, livestock forage production, and the health of natural ecosystems. Moreover, their rapid intensification presents unique challenges for vulnerable stakeholders who have less time to prepare for drought when it develops so quickly.

To capture the onset and evolution of a flash drought, it is necessary to use a variety of drought monitoring tools depicting anomalies in soil moisture, ET, evaporative demand, and vegetation health. A typical progression during a flash drought event given initially adequate-to-surplus soil moisture (i.e., energy-limited regime) is for an extended period of enhanced evaporative demand to initially cause an increase in ET as vegetation responds to the anomalous weather conditions, subsequently followed by a period of rapidly decreasing soil moisture content, a sudden transition to water-limited conditions, reduced ET, and the subsequent emergence of visible signs of vegetation moisture stress. This transition from energy-limited to water-limited conditions is often necessary for soil moisture-atmosphere feedbacks to occur. It also exemplifies the complex relationship between evaporative demand, soil moisture, ET, and vegetation health. The intensification rate and final severity of a flash drought will be strongly influenced by the strength and persistence of the atmospheric anomalies forcing its evolution, the magnitude of the precipitation deficits, and the overall vulnerability of the vegetation to drought.