The image shows estimates of rainfall for the southeastern United States from
September 14–21. The estimates, acquired by multiple satellites, are calibrated
with rainfall measurements from NASA’s Tropical Rainfall Measuring Mission
(TRMM)satellite. The highest rainfall amounts—more than 300 millimeters
(11.8 inches)—appear in blue. The lightest amounts appear in pale green.
Credit: NASA/Jesse Allen
For the past four years, drought has parched the soil of north Georgia farms, nearly drained Atlanta’s 38,000-acre reservoir, and left area lawns brown. Then in just eight days in September 2009, the region’s weather took a turn that likely had residents asking if they were on a climate seesaw.
As millions of Georgians watched and as many fled for higher ground, meteorological forces coalesced to deliver heavy rains and flooding not seen in the southeastern U.S. in more than 100 years, according to the National Weather Service.
Millions of dollars of property were destroyed, and ten lives were lost. The flooding dominated local TV news and compelled the governor to declare a state of emergency across 17 counties.
Even before the rains ended, research meteorologist, Marshall Shepherd a professor of Atmospheric Sciences at the University of Georgia, and colleagues began piecing together rainfall and soil moisture data from NASA satellites, Doppler radar, weather reports and ground-level rain gauges to assemble a clearer picture of the climatological factors that fueled the flooding.
Though not yet peer reviewed, Shepherd’s initial findings suggest what may have prompted the downpours and floods. Moisture from the Gulf of Mexico was drawn into the southeast by a stalled low pressure system from the Mississippi Valley. The moist air and a series of meteorological disturbances merged with a key ingredient – a dramatic increase in urban land cover – to bring this historic weather event to the region for the second time in as many years.
“We had days and days of downpours and an extraordinary 24-hour rainfall event at the end of that period. With soil moisture already at a high, the rain could no longer infiltrate the soil and we reached a tipping point for flooding,” explained Shepherd, a native of metropolitan Atlanta who NASA has funded to investigate how urban land cover and pollutants affect rainfall and surface water changes.
“The rain and the soil moisture content combined to overwhelm rivers and streams,” he said. “Add to that Atlanta’s impenetrable roads and sidewalks, which increase the volume of runoff, and you get the event of record we witnessed.”
According to Shepherd, the long drought in the southeast – which caused job losses in agriculture and lawncare and other water use hardships across many sectors of society — is now over.
“In fact, October is normally the driest month in North Georgia,” Shepherd said. “But this year, sea surface temperature data from NASA and NOAA satellites tell us a moderate El Nino in the Pacific appears likely to lead to a cooler and wetter fall and winter in the Southeast.”
Curiously, drought may be the last of Atlanta residents’ weather worries in the next season or two to come.
–Gretchen Cook-Anderson, NASA’s Earth Science News Team