Abstract
Synoptic analysis of daily and monthly mean circulation patterns indicates that bimodal warm season precipitation distributions are a frequent occurrence in the upper Middle West. Normal seasonal changes in the positions of ridges and troughs comprising the circumpolar westerlies are associated with a June maximum, a mid-summer decline, and a second peak in September. Warm season depressions (WSDs) in the precipitation profile occur most frequently during periods of northwesterly flow or amplified ridging in the mid-continent. Low frequencies of WSDs are associated with southwesterly flow and/or trough conditions. Labeling this phenomenon an “aberration” (Trewartha, 1981) is contrary to the known relationships between mid-latitude precipitation, atmospheric vertical motions, and the modal North American summer season circulation pattern.
Published Version
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