After several days of searing heat, Adelaide has been hit by a thunderstorm that created a spectacular, late-night light show over parts of the city.

According to the Bureau of Meteorology, there were about 8,500 lightning strikes across South Australia and over the coastal waters.

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Most of those strikes were concentrated in the south, over the agricultural area and the city — particularly the southern suburbs.

"We had a mid-level cloud band develop over the southern part of the state late yesterday," senior forecaster Tom Boeck said.

"Thunderstorm activity started to develop over the southern part of Eyre Peninsula and the southern Mount Lofty Ranges and moved northwards overnight, resulting in a significant amount of lightning over the Adelaide metropolitan area.

"Those thunderstorms have now moved east into the Murraylands, more northern parts of the Mount Lofty Ranges and Riverland."

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Other than several small grassfires, the system did not cause much trouble, with the State Emergency Service reporting only five call-outs since midnight.

The storms followed several days of extreme heat, with Adelaide peaking at 42.2 degrees Celsius on Friday, and were accompanied by rain falls of up to about 10 millimetres.

"The highest fall was 9.6mm at Sturt Creek, just south of Blackwood, so the higher falls [were] in the south and there was also 9mm at Longwood," Mr Boeck said.

"The remainder of the metropolitan area received between about 1-3mm."

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But the milder weather will not last long. Adelaide is forecast to reach 31C today before tops start to climb again mid-week.

More very hot weather is expected over the long weekend, with 40C on Australia Day and 42C on Saturday and Sunday.