Conditions Leading to the Flood (Excerpt from SIR 2008-5235)
The June flooding in Wisconsin was caused by heavy rain falling on saturated soils at a time when streamflows already were much above normal. Wet conditions for the southwestern part of the state started on August 18, 2007, with intense, 10- to 12-in. rains that caused flooding and mudslides and resulted in disaster declarations for three southwestern counties. These rains followed a moderate to severe drought in the early summer months of 2007. In Madison, in south-central Wisconsin, an August 2007 rainfall total of 15.18 in. was the all-time highest rainfall total for any month since records had been kept starting in 1897.
Over the winter of 2007–08, heavy snowfall resulted in Federal disaster declarations for 10 counties in south-central and southeastern Wisconsin (Wisconsin Recovery Task Force, 2008). Snowfall in Madison set a record of 101.4 in. Precipitation for the following spring was well above normal (National Climatic Data Center, 2008). By late spring, streamflow and water tables were above normal across the region.
On the weekend of June 7 and 8, a stalled frontal boundary of an extremely moist air mass produced heavy rains that set a 48-hour rainfall record in Milwaukee of 7.18 in. and produced greater than 4 in. of rain in 24 hours in Milwaukee and Madison (National Weather Service, 2008). All-time daily rainfall records were set at four climate stations in southern Wisconsin (table 2). The 100-year 48-hour rainfall for southern Wisconsin is about 7 in. (Huff and Angel, 1992).
Another bout of intensive rain, funnel clouds, and flash floods occurred on June 12, resulting in a 7-day rainfall of 12–15 in. in south-central Wisconsin (National Weather Service, 2008). For southern Wisconsin, the 100‑year 5-day rainfall for southern Wisconsin is 9–10 in. and the 100‑year 10-day rainfall is 10–11 in. (Huff and Angel, 1992). Extensive flash flooding occurred in Sauk, Columbia, Jefferson, Waukesha, Milwaukee, and Racine Counties. At many locations, more than 70 percent of the rain fell on June 7, 8, and 12. By June 13, total precipitation exceeded 10 in. throughout south-central Wisconsin, with 12–16 in. concentrated in a corridor from northern Sauk County into northwest Dodge County (fig. 1). As of June 16, Milwaukee had 10.96 in. of rain for June, which is the record rainfall for any month. The Midwestern Regional Climate Center (2008) reported that precipitation amounts in southern Wisconsin were greater than 400 percent of normal values. For June 2008, new monthly records were set at several climate stations in southern Wisconsin (table 2).
Figure 1. Cumulative precipitation (top) in the Midwest for June 1 through June 15, 2008, and (bottom) in southern Wisconsin for June 5 through June 13, 2008. (images from National Weather Service, 2008)
Table 2. Daily rainfall estimates for June 8 and 9, 2008, and rainfall probabilities for a 24-hour duration at National Weather Service (NWS) climate stations.
[Data from National Climatic Data Center (2008) and Huff and Angel (1992). See figure 1 for station locations. Rainfall probability is the probability or odds of having rainfall amounts equaled or exceeded for any given duration. For example, a probability of 0.01 means there is a 1 percent chance of that daily rainfall being equaled or exceeded. The probabilities of 0.01, 0.02, and 0.04 correspond to 100-, 50-, and 25-year rainfalls, respectively]
|Record for 24-hour duration (in.)
|Rainfall probability for|
24-hour duration (in.)
|West Allis, Wis.
|Cottage Grove, Wis.