South Australia will bake on Thursday as a tongue of hot air from central Australia drifts south, sending temperatures soaring more than 15 degrees above average in some areas and raising fire danger ratings across the state.

Adelaide had already exceeded 30 degrees by 8:30am on Thursday and is forecast to reach a top of 39C in the afternoon. This would be the city's warmest day since February and 12 degrees above average for a December day.

Elsewhere, the mercury is tipped to reach 43 degrees at Ceduna and 42 degrees at Nullarbor in the state's west. These forecasts are a whopping 16 and 15 degrees above average for this time of year, respectively.

The burst of heat will raise the risk of bushfires across the state today, with Very High to Severe fire danger ratings forecast for 12 of the state's 15 districts. Total fire bans are in place until midnight on Thursday for the West Coast, Lower Eyre Peninsula, Yorke Peninsula, Kangaroo Island and Mt Lofty Ranges districts.

The hot air mass will liger over most of SA on Friday, before a cool change sweeps across the state's west during the morning and reaches other southern districts during the afternoon and evening. Fire danger ratings will be elevated across most central and eastern districts on Friday.

The highest December temperature measured at Adelaide's West Terrace weather station was 44.2 degrees in 1904. This is unlikely to be challenged this week.