Historic U.S. Midwest Floods of 1993 & 2008

Bodner et al. (2011) compared atmospheric circulation patterns over the continental U.S. during two historic Midwest floods in the summers of 1993 and 2008. The U.S. Geological Survey classified both the 1993 and 2008 floods as being 500-year floods. The 1993 event was the most devastating flood along the Upper Mississippi River Basin during modern times and set numerous records of high river levels. During the 2008 flood, the Cedar River (at Cedar City, Iowa) set a flood record which was more than 3m (~11 feet) above the previous record and extensive flooding was also reported in Illinois, Wisconsin, Nebraska and South Dakota. It is of interest to analyze the large-scale atmospheric flow patterns which produced such extensive flooding, especially in the context of present (summer 2012) ongoing drought over the U.S. southeast and Midwest region.

In their paper, Bodner et al. identified an extreme 13-day rainfall period during 1993 (29 June-11July), and again in 2008 (June 2-June 14), which periods were responsible for the extensive flooding. During both events, anomalously high precipitable water and moisture flux were carried from the Gulf of Mexico around the periphery of the sub-tropical high which intersected a quasi-stationary baroclinic zone, thereby supporting the development of frequent MCSs (Mesoscale Convective Systems, e.g., Schumacher and Johnson, 2005) over the Midwest region. Such MCSs are a climatological feature of summer Midwest climate as shown by Carbone et al. (2002). Composites of 850-hpa normalized anomalies of moisture flux and winds for the two 13-day periods show a low-level atmospheric jet stream entering the U.S. Southwest, carrying the Gulf moisture into the Midwest all the way from the corn belt to the Lower Great Lakes region of the conterminous USA. It was this moisture source from the Gulf which helped produce extensive and record-breaking rainfall over many locales at that time.

The larger (continental) scale atmospheric pattern and tropical forcing regimes, however, were markedly different during the winter and spring of 1993 and 2008. In the tropics, El Niño conditions were present during 1993, while La Niña conditions dominated during 2008. Despite these differences, above normal rain and snowfall occurred in both years and this resulted in above normal soil moisture heading into summer in both years. This abundant soil moisture during the winter and spring season helped produce the historic summer floods in 1993 and 2008.

In the last 25 years, the U.S. Midwest suffered a severe drought in the summer of 1988, while extensive flooding was produced in 1993 and 2008, as discussed above. These and other studies help us understand the mechanics of atmospheric flow patterns that can result in extensive floods as well a severe drought, like this year's drought which is affecting the US corn belt in particular, and what relationship they have - if any - to global warming.

Additional References
Carbone, R.E., Tuttle, J.D., Ahijevych, D.A. and Trier, S.B. 2002. Inferences of predictability associated with warm season precipitation episodes. Journal of the Atmospheric Sciences 29: 2033-2056.

Schumacher, R.S. and Johnson, R.H. 2006. Characteristics of US extreme rain events during 1999-2003. Weather & Forecasting 21: 69-85.