Atmospheric Circulation Associated with the Midwest Floods of 1993

This paper presents an observational analysis of the large-scale atmospheric circulation prior to and during the Midwest floods of June–July 1993. The floods developed and persisted in association with three major circulation features, none of which alone would likely have produced such intense and prolonged flooding. First, a persistent, positive phase of the North Pacific teleconnection pattern was observed throughout the Pacific sector for four months prior to the onset of the floods. This anomalous circulation was associated with much above-normal cyclone activity over the middle latitudes of the North Pacific and with below-normal cyclone activity over the western and central United States. Second, a major change in this pattern occurred over the western United States in late May, which established very strong zonal flow from the western Pacific to the eastern United States. This flow provided a “duct” for the intense cyclones to propagate directly into the Midwest throughout the month of June. These storms triggered a series of intense convective complexes over the Midwest, resulting in major flooding. Third, during July a persistent wave pattern with highly amplified southwesterly flow became established over the western and central United States. This circulation, in conjunction with a quasi-stationary frontal boundary and sustained moisture transport into the central United States, was associated with a continuation of excessive rainfall and flooding in the Midwest.