Attribution of the 2017 Northern High Plains Drought
INTRODUCTION. The northern High Plains, particularly much of Montana and the Dakotas, had extreme to exceptional drought conditions develop during the summer of 2017. When the months of May, June, and July are combined, the year 2017 received 60% of normal precipitation and is ranked as the second driest (only after 1936) of the period 1901–2017 (Fig. 1a; see also Fig. ES1a in the online supplemental material). It also had anomalously warm temperatures relative to the 1901–2017 record (Figs. 1b and ES1b). The exceptional dryness combined with the unusual heat over central and eastern Montana resulted in a rapidly declining Palmer Severity Drought Index, which, although of short duration, reached extreme drought levels comparable to those of historical droughts (e.g., during the 1930s). The occurrence of the drought over Montana and the Dakotas, an important spring wheat–growing region in the country, during the crop growing season caused agricultural losses of $2.5 billion and contributed to one of Montana’s worst wildfire seasons on record (NOAA/NCEI 2018). This study investigates the causes of the 2017 northern High Plains drought, particularly the roles of the 2017 sea surface temperature (SST) anomalies and atmospheric internal variability. It also assesses the impact of the post-1901 long-term warming trend on the frequency of drought occurrence in the area.