A severe weather warning has been issued for parts of South Australia as the downgraded Cyclone Yasi sweeps into the state.

State Emergency Service (SES) duty officer Darryl Wright said reports were coming in on Friday afternoon of heavy rains in many parts of the Flinders Ranges, north of Adelaide.

The Bureau of Meteorology was forecasting rainfall of between 50 and 100mm in some areas, increasing to up 200mm in the northwest pastoral district, north of Coober Pedy.

The bureau said a severe thunderstorm warning and a flood watch was current for some districts.

"We have received reports that a number of creeks in the northern Flinders Ranges, including the Bandioota Creek near Blinman, have started to flow," Mr Wright said.

"Along with the road closures there could be fast flowing water in the creeks so motorists driving through these districts should continually check for any closures."

The bureau said a band of rain with local heavy falls and local flooding would develop over areas of SA by early Saturday morning, easing to scattered showers during the afternoon.

It said showers and thunderstorms could extend as far south as Adelaide's northern suburbs.