The storm system was as a result of a low pressure zone that persisted in the Southeast. Warm, moist air from the Gulf of Mexico was pulled in to create a long-lasting rain event. Ground saturation was at a high level due to previous rain events meaning the fresh rains had nowhere to go. The rain water caused runoff into streams, rivers and creeks in and around Atlanta. Interestingly, the rains were not a welcome site even after the droughts that have been associated with the region for the last 2 years.
Aerial photos from the USGS show the devastation in the region. Homes, businesses, and even the Six Flags theme park have been flooded by dirty and murky flood waters. Other states experiencing high river flood stages include Mississippi, Alabama, Louisiana, and Florida.
Sources:
A Climatological Assessment of Flood Events in Georgia
NCDC Storm Data
USGS Newsroom
National Weather Service, Peachtree City, Georgia
National Weather Service Peachtree City, GA
Other Helpful Georgia Flood Resources