1Urban Flash Flooding in Downtown Madison, WI and Southern Wisconsin, 27 July 2006 Rebecca Westby Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, Wisconsin AOS 453 Final Case Study (Manuscript received 5 May 2009) Abstract The afternoon hours of 27 July 2006 proved to be quite an eventful one as torrential rains drenched the downtown area of Madison, WI with 3 to 5 inches of rain within a 90-minute time frame, causing damage to almost every building on the University of Wisconsin-Madison campus. This case study sought to determine the synoptic and mesoscale forcings that set up and sustained the heavy rainfall that led to flash flood conditions. It was hypothesized that the flash flooding was caused by multicellular convective cells that moved roughly parallel to a slow-moving or stationary surface frontal boundary and was sustained by a warm, moist low-level southerly flow, allowing the rainfall to be focused over one particular area and causing localized flash flooding. Additionally, it was suspected that the forcing mechanisms primarily responsible were surface and low-level forcings, including daytime heating as a mesoscale forcing, because upper-level forcings such as jet maximums are typically rare during the summer months when the jet stream retracts further north. In the end, this hypothesis was supported by the data provided in the synoptic and mesoscale analyses. The storm system of interest did consist of multicellular convective cells that traversed along a stationary frontal boundary and was sustained by warm, moist low-level southerly flow. It was also determined that the synoptic convective forcings were provided by the stationary surface frontal boundary in addition to cooling in the mid-levels of the atmosphere, while the mesoscale forcing was provided by daytime heating. As expected, there was also no upper level forcing due to the lack of presence of a jet maximum. The combination of these forcings along with the moist, southerly flow allowed for deep, moist convection to occur along the surface frontal boundary and led to the observed flash flood conditions ___________________________________________ I. Introduction The afternoon hours of 27 July 2006 proved to be quite an eventful one, as torrential rains drenched the downtown area of Madison, WI with 3 to 5 inches of rain within a 90-minute time frame. Areas most affected by these heavy rains reached from the west side of Madison to the Capital Square, and almost every building on the University of Wisconsin-Madison campus sustained flood damage1. This particular flooding event was later categorized as a “100-year flood.” The term “100-year flood” 1 NOAA Satellite and Information Service. [http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~640572] is a statistical designation indicating that a flood of this magnitude has a 1-in-100, or one percent chance, of occurring in any given year.2 The probability occurrence for such floods is estimated with the use of statistical techniques, through a process called frequency analysis, and is based on the probability that the given event will be equaled or exceeded in any given year.3 The cause of flooding is often, if not always, associated with the interaction 2 USGS. The “100-Year Flood.” Fact Sheet 229-96. [http://pubs.usgs.gov/fs/FS-229-96/] 3 USGS. Water Science for Schools. Floods: Recurrence Intervals and 100-Year Floods. [http://ga.water.usgs.gov/edu/100yearflood.html]2between meteorological conditions and hydrological conditions.4 In terms of the meteorological conditions, there are two primary types of flooding that are characteristic of the upper-Midwest region: flash floods and river floods. Flash floods are caused by intense, high to extremely high rainfall rates over a relatively short period of time,5 while river floods are associated with moderate to high rainfall rates spread over days and sometimes even months.6 Nonetheless, meteorological conditions play an important, if not primary, role in flood occurrence because they provide the most necessary ingredient: rainfall. As for the hydrological conditions, a given event’s chances to produce a flood are dramatically affected by factors such as antecedent precipitation, the size of the drainage basin, the topography of the basin, and the amount of urban use within the basin, to name a few. As land is converted from fields and forested land to roads and parking lots, it loses the majority of its ability to absorb rainfall.7 Thus, urban areas are more susceptible to floods, particularly flash floods, because a high percentage of the surface area is composed of streets, roofs, parking lots, and other materials that cause runoff to occur very rapidly.8 4 Doswell, C.A., H.E. Brooks, and R.A. Maddox. June 1996. Flash Flood Forecasting: An Ingredients-Based Methodology. Weather and Forecasting. Volume 11. p. 560-581. 5 USGS. Water Science for Schools. Questions and Answers about Floods. [http://ga.water.usgs.gov/edu/qafloods.html] 6 NOAA Satellite and Information Service. [http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~640572] 7 USGS. Water Science for Schools. Questions and Answers about Floods. [http://ga.water.usgs.gov/edu/qafloods.html] 8 USGS. U.S. Department of the Interior. Significant Floods in the United States During the 20th Century – USGS Measures a Century of Floods. USGS Fact Sheet 024-00. March 2000. [http://ks.water.usgs.gov/pubs/fact-sheets/fs.024-00.pdf] With the given information in the previous paragraphs, the flooding event of 27 July 2006 can easily be categorized as a flash flood. The first indication is the high rainfall amounts in a 90-minute window. This implies a high rainfall rate over a relatively short period of time, a characteristic typical of flash floods as described above. In addition, the hydrological conditions of the Madison area exacerbated the situation and helped lead to flash flood conditions. The hilly terrain and the urban settings of the downtown area and the University of Wisconsin-Madison campus, which contain high amounts of non-absorbing materials such as concrete and asphalt, acted to enable the runoff water to quickly overwhelm the storm sewers and concentrate in the low-lying areas.9 Thus, both meteorological and hydrological conditions played a role. However, only the meteorological conditions will be explored in depth in this case study. Interestingly,