ATMOSPHERIC CIRCULATION ASSOCIATED WITH THE MIDWEST FLOODS OF 1993

This paper presents an observational analysis of the large-scale atmos pheric circulation prior to and during the Midwest floods of June-July 1993. The floods developed and persisted in association with three ma jor circulation features, none of which alone would likely have produc ed such intense and prolonged flooding. First, a persistent, positive phase of the North Pacific teleconnection pattern was observed through out the Pacific sector for four months prior to the onset of the flood s. This anomalous circulation was associated with much above-normal cy clone activity over the middle latitudes of the North Pacific and with below-normal cyclone activity over the western and central United Sta tes. Second, a major change in this pattern occurred over the western United States in late May, which established very strong zonal flow fr om the western Pacific to the eastern United States. This flow provide d a ''duct'' for the intense cyclones to propagate directly into the M idwest throughout the month of June. These storms triggered a series o f intense convective complexes over the Midwest, resulting in major fl ooding. Third, during July a persistent wave pattern with highly ampli fied southwesterly flow became established over the western and centra l United States. This circulation, in conjunction with a quasi-station ary frontal boundary and sustained moisture transport into the central United States, was associated with a continuation of excessive rainfa ll and flooding in the Midwest.