A vigorous cold front is pushing across the Southeast U.S. This system has plenty of upper level support and low level lift. Deep-seeded moisture is streaming north ahead of the cold front producing areas of showers and storms.
Here is an image showing Precipitable Water (green/red)(PW), low-level moisture transport (arrows), radar, and pressure (black lines).
The red-shaded spots near the gulf coast represent PW values of 2.0 inches. This represents very high levels of atmospheric moisture. Note the red arrows. These indicate low-level flow. Moisture is streaming quickly northward across all of Mississippi, Alabama, and Tennessee. Low pressure over Louisiana and Mississippi will add to the lift.
Here is a short-range model showing anticipated accumulated rainfall around midnight. Heaviest rainfall amounts (at least according to this model) will occur over a large part of Mississippi, Alabama, and North Georgia. Widespread rainfall amounts of 0.50 to 2.50 inches are indicated. The red streaks indicate rainfall potential of 3 inches or more, with some purple spots indicating rainfall of 4 inches or more.
With the ample upward lift, and the deep moisture, these rainfall amounts seem entirely possibly to me.
The upper system will lift northeast over the next 24 hours. This will spread heavy rain north, but decrease intensity further east. While the core of drought in Georgia will receive some rainfall, it will be much less than areas further north and west.
Finally, here is another model showing the probabilities of 1 inch of rain or more. The red-shaded spots have a probability of 80 percent or more of receiving at least an inch of rainfall.