Meteorological conditions associated with the onset of flash drought in the Eastern United States

Rapid onset droughts, termed "flash droughts", present a series of unique challenges for drought monitoring, forecasting, and mitigation. Due to the rapid onset and lack of early warning systems, stakeholders can be caught off-guard by flash droughts and suffer disproportionate impacts. Despite these impacts, little is known about the physical drivers of flash droughts. The purpose of this study is to determine antecedent meteorological conditions prior to the onset of flash drought in the Eastern United States. Emphasizing the agricultural impacts, flash droughts were defined as periods when the pentad-average 0-40 cm volumetric water content declines from at least the 40th percentile to below the 20th percentile in 4 pentads or less. Meteorological variables from 125 stations in the Eastern U.S. from March - October 1979 - 2010 were analyzed for their relationships with flash drought onset. Consistent with previous findings, flash drought was associated with decreased precipitation and humidity, increased solar radiation, and elevated temperatures. However logistic regression results suggest variables that accounted for surface moisture balance and/or atmospheric evaporative demand were more closely linked with the likelihood of flash drought than temperature and/or precipitation. Associated surface conditions are likely driven by ridging in the mid to upper level troposphere, which is shown to be more persistent leading up the flash droughts in the northern half of the study region. Our results elucidate the meteorological conditions immediately prior to the onset of one type or "flavor" of flash drought, defined by characteristic rapid intensification. Arguably, one could define flash drought with soil moisture thresholds varying from those used in this study and/or different time scales of soil moisture depletion. Therefore, we additionally argue that absences of both a standard flash drought definition and consistent precedent for identifying flash drought complicates monitoring and predicting these events.