The Midwest of the United States is a region extensively utilized for agriculture and livestock production despite great susceptibility to widespread and persistent drought. While the location and duration of droughts are related to dynamic meteorological factors, pinpointing when and where a drought will commence, how long it will persist, and when the drought will end, remains a challenge. This investigation examines significant Midwest drought events from a synoptic meteorological perspective through an assessment of air mass frequency over the past decade. A synoptic approach is useful since air masses characteristically describe multiple weather and climate parameters at the same time across wide areas. The daily air mass conditions in the Spatial Synoptic Classification that are dominant during extreme droughts are examined across the region and compared to “normal” periods without substantial or extensive drought. Extreme episodes are established with new criteria expanded from United States Drought Monitor information, normal average decadal and seasonal baselines are calculated, and the air mass frequency departures from these periods are examined for statistical and practical significance. Results indicate that the Dry Polar, Dry Tropical and Moist Tropical air masses exhibit important and statistically significant changes in frequency during drought. Tendencies for substantial increases in warm and dry types, regardless of season, and moist air mass declines are detected. The exact air masses with significant changes are unique for different sub-regions, particularly in the northwest and south. These patterns are consistent with changing upper-air flows such as southerly, meridional flow to more southwesterly, zonal flow.



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