A Synoptic Climatology of the Bimodal Precipitation Distribution in the Upper Midwest

Abstract

This paper presents an investigation of the synoptic climatology of the precipitation regime in the Upper Midwest. The annual march of precipitation is characterized by a bimodal distribution, with maxima occurring during the months of June and September. Midsummers are noticeably drier. Using daily precipitation and twice-daily 700 mb height data for the period 1950–1979, composite difference maps of the 700 mb height field corresponding to wet and dry Junes, Augusts and Septembers were constructed to identify the nature of the difference in midtropospheric circulation associated with the midsummer minima and the September maxima. Correlation of monthly total precipitation with monthly precipitation event frequency was undertaken to determine whether precipitation during the summer months is the result of a small number of high-intensity storms or in fact due to the frequency of less intense rain events. The influence of North Atlantic tropical cyclones on the late summer precipitation was investigated using tropical cyclone tracks and satellite photography. Results indicate that 1) the monthly precipitation in the region primarily depends upon the frequency of rain events as opposed to a small number of intense storms; 2) the midsummer minima is a result of increased northerly flow aloft of continental origin in response to the building of a ridge to the west; 3) the secondary maxima is associated with a southwestward shift in position of the mean summer trough, producing an increased southwesterly flow aloft with increased water vapor advection from the Gulf of Mexico; and 4) remnant North Atlantic tropical cyclones entering the region can significantly influence the precipitation receipt for a given month, but there is no evidence in the record to indicate that tropical cyclones are the sole mechanism responsible for the September maxima.