FLASH DROUGHTS IN THE UNITED STATES: DEFINITIONS, DRIVERS, PATTERNS, AND TRENDS

In recent years the term "flash drought" has risen to prominence in the climate impacts literature. Flash droughts can be relatively short in duration, but they are characterized by extremely rapid onset, and they have been implicated in some of the most damaging drought events of recent years. The sudden appearance of a flash drought, often without any perceived warning, creates a challenge to farmers, ranchers, and stakeholders. This has led to a broad call for research to understand the flash drought phenomenon so that these droughts can be more effectively predicted and managed. However, there is still no standard and fully agreed quantitative definition that encompasses all flash drought characteristics, classes, drivers and pathways. To address this gap in the foundations of flash drought research, we proposed a novel Soil Moisture Volatility Index (SMVI) that allows for flash droughts to be defined on the basis of the rapidity of soil drying during the onset of a drought. The first study in this dissertation introduces this method and demonstrates that it compares favorably with other proposed definitions of flash drought when applied to inventory events across the Contiguous United States (CONUS). Building on this work, the second study in this dissertation expands the SMVI approach to flash drought definition to include a metric for flash drought severity. SMVI-based inventory and severity assessment of CONUS flash droughts is then used for a careful study of meteorological variables associated with these events. We find that there are distinct classes of flash drought associated with different meteorological drivers. Specifically, two leading classes of flash droughts are "hot and high evaporative demand" events and "sunny and high pressure" events. These results strongly indicate that "flash drought" represents a composite class of events, with several possible pathways all leading to rapidly intensifying drought conditions. Appreciating this diversity of flash droughts can inform process-based studies and the development of forecast tools. Finally, recognizing the importance of high temperatures to the leading class of flash drought, the dissertation presents a third study that explores the role of teleconnections in setting the stage for flash drought onset. Analysis of the North Atlantic Oscillation (NAO) and the two pressure centers associated with this index shows that these centers of actions of the North Atlantic Oscillation may have some weak associations with elevated temperature in the eastern CONUS on short timescales, but that these associations are more evident at seasonal scale