Atmospheric aspects of the 2008 Midwest floods: a repeat of 1993?

This paper identifies and documents the major large-scale atmospheric circulation characteristics that preceded and facilitated the devastating floods across the Midwestern United States along the upper Mississippi, Missouri, and Wabash River basins in the late spring/early summer of 2008. These circulation features were also placed in the context of the 1993 floods that occurred within a similar temporal and spatial domain. This process included investigating the relationship between various atmospheric conditions and the timing of flood-producing rainfall events at monthly and sub-monthly timescales. Unlike in 1993, the 2008 flood event took place 1 month earlier in the year (May), lasted over a much shorter time period, and extended eastwards into Indiana. The comparison confirmed much previous work regarding the factors associated with Midwest flooding during the early warm-season and revealed the possible influence of the state of the North Atlantic Ocean and the North Atlantic oscillation (NAO) on warm-season flooding across the Midwest. Both the 2008 and 1993 events were preceded by an NAO positive phase; the pattern switched phase about 2 months before the onset of the flooding and remained negative for the duration of the floods. While the major atmospheric conditions known to be associated with major flooding across the Midwest were present in 2008 (e. g. a strong Great Plains low-level jet), the level of their development, persistence, and geographic orientation/position was very distinct from those observed in 1993. These conditions were also embedded in different state of the remote hemispheric circulation over the Pacific North American region. Together, these variations contributed to the 2008 Midwest floods that occurred earlier in the warm season, were shorter-lived, and had an impact on a slightly different geographic area. Copyright (C) 2009 Royal Meteorological Society