2017 Flash Drought in Montana—A Case Study of Methods and Metrics for the Detection and Monitoring of Flash Droughts

The 2017 flash drought in the Northern Great Plains was as surreptitious as it was devastating. Few anticipated the onset, duration, or impacts. In Montana, antecedent conditions in late 2016 and early 2017 offered little warning. Soil moisture across much of the state in the fall of 2016 exceeded the 90^th percentile, and snow water equivalent in most basins in the spring of 2017 exceeded 100% of normal. Despite a positive outlook in mid-April, the combination of unusually warm temperatures, above average winds, and the failure of the reliable spring rainy season triggered the rapid onset of an acute terrestrial flash drought in the spring of 2017. Many have argued that this drought was a rare anomaly few could predict. However, given the clarity of hindsight and the application of less traditional indices, the warning signs of the 2017 drought were apparent well in advance of the onset.

The swift degradation of conditions in the spring of 2017 despite above average water year precipitation illustrates the danger of an overreliance on precipitation as a drought indicator. Drought detection and monitoring in an era of extreme weather variability must pivot from a reliance on precipitation data and surface water availability to a multi-pronged evaluation that accounts for precipitation, temperature, precipitation timing and evaporative demand in the assessment of drought conditions. A large percentage of Montana’s precipitation comes as snow in the winter months and as rain in April, May, June, and July. In late 2016 and early 2017, Montana experienced a shift in precipitation timing to October, December, January and February. With respect to the water year, amounts accumulated in 2017 were mostly average to above average. However, this shift in precipitation timing in 2017 served as a primary catalyst in the evolution of the flash drought.

The flash drought of 2017 offers important lessons and insight that are critical to the identification and evaluation of the next flash drought. This presentation will explore the hazards of the reliance on water-year precipitation, and more specifically, accumulated snowpack as reliable indicators for both soil moisture and water supply; the inadequacy of traditional drought metrics like the Surface Water Supply Index and the Palmer Drought Severity Index as indicators for flash drought; the role of both temperature and precipitation timing as important drought indicators; and the importance of atmospheric demand and its role in the evolution and detection of flash drought.