Hourly Forecast Detailed. 1  0am:The Grantville, GA forecast for Mar 25 is 63 degrees and Partly cloudy. There is 82 percentage chance of rain and 8 mph winds from the Southeast. 2  3am:The Grantville, GA forecast for Mar 25 is 59 degrees and Clear. There is 84 percentage chance of rain and 9 mph winds from the South-Southeast.