JULY 2011 AMERICAN METEOROLOGICAL SOCIETY | 1 AFFILIATIONS: Shepherd, Mote, Dowd, and Roden—University of Georgia, Athens, Georgia; Knox—Office of the State Clima- tologist, Athens, Georgia; McCutcheon— U.S. Environmental Protection Agency, Athens, Georgia; Nelson—National Weather Service, Peachtree City, Georgia CORRESPONDING AUTHOR: Dr. J. Marshall Shepherd, Univer- sity of Georgia, Climatological Research Laboratory, Department of Geography, Athens, Georgia 30602 E-mail: marshgeo@uga.edu DOI:10.1175/2010BAMS3003.1 ©2011 American Meteorological Society to levels exceeding the estimated 500-yr lood. he Yellow River stream gauges in Gwinnett, DeKalb, and Rockdale Counties measured lows that submerged the 100-yr loodplains but failed to reach the 200-yr lood level, which has a 0.5% chance of occurring in any given year (www.usgs.gov/newsroom/ar- ticle.asp?ID=2316). he 100-yr lood level with a 1% chance of occurrence in any given year is one of the standards that the Federal Emergency Management Agency (FEMA) uses to set lood insurance rates and prevent lood plain development. he USGS recorded 100-yr flood levels on the Chattahoochee River at Vinings in Atlanta (Fig. 2), where stage heights An Overview of Synoptic and Mesoscale Factors Contributing to the Disastrous Atlanta Flood of 2009 BY MARSHALL SHEPHERD, T HOMAS MOTE, JOHN DOWD, MIKE RODEN, P AMELA KNOX, STEVEN C. MCCUTCHEON, AND STEVEN E. NELSON R ecent literature suggests that damage, loss of life, and costs from looding have risen in recent decades (Ashley and Ashley 2008; Brissette et al. 2003). In a 2009 Journal of Climate article, Seager et al. noted that regions of the southeastern United States face increasing vulnerability to hydroclimatic ex- tremes because of population growth and increasing population density. In 2008, a majority of the popula- tion lived in urban areas, and by 2030 this number is expected to reach 81%. he unsustainable, modiied water cycle will afect the ecosystem, infrastructure, and societal activities, thereby requiring revolution- ary designs, management, and policies. Burian and Shepherd (2005) and Reynolds et al. (2008) represent a sample of recent literature that has reconsidered implications of precipitation on urban drainage and hydrological processes. In September 2009, the metropolitan area of Atlanta and surrounding areas in northern Georgia experienced disastrous urban looding that inundated major transportation arteries, closed several major school systems, submerged the popular Six Flags theme park, and contributed to at least 10 deaths as of October 2009 (Fig. 1). he United States Geo- logical Survey (USGS) measured the largest low ever recorded on Sweetwater Creek near Austell, which has a streamlow record dating back to August 1904. Parts of Cobb and Douglas Counties were inundated FIG. 1. (top) Flooding on U.S. Interstate 285 loop around Atlanta and (bottom) Six Flags theme park. (Images courtesy of the Atlanta Journal Constitution, www.ajc.com).