It warned that road closures were likely and some communities may be temporarily isolated due to rising creeks and localised flooding.
“Rainfall totals are forecast to be in the range of 40 to 80 millimetres over the far northern parts of the North West Pastoral district and the North East Pastoral district north of Marree,” the flood watch stated.
“At this stage, most of the rain is expected between Saturday afternoon and late Sunday. Some localised higher falls are possible.”
Adelaide is expected to receive between 20-50 millimetres of rain, and higher falls are expected in the Adelaide Hills.
Rainfall totals for over the Mount Lofty Ranges are expected to be between 50 to 70 millimetres.
Meanwhile, a section of a popular scenic route from Adelaide into the Barossa Valley will be closed from noon on Saturday as a precautionary measure due to the predicted rain.
SA Planning, Transport and Infrastructure said Gorge Road would be closed between Playford Bridge and Torrens Hill Road and is anticipated to re-open on Tuesday.
A detour will be in place along Lower North East Road, North East Road, and Tippett Road to allow access to Cudlee Creek businesses and attractions.
SES chief officer Chris Beattie has warned people to not to drive, ride or walk through floodwater.
A flood watch update is expected to be issued at 11:00am on Saturday.