Thunder prompts SA earthquake fears

Thunder so loud it shook windows and set off alarms has rolled through Adelaide, prompting fears of an earthquake or an explosion.

The storm has also dumped half the city's May rainfall in one night, with some regional centres posting their best May falls for more than 20 years.

The peal of thunder at about 2.30am woke residents from the southern to the northern suburbs as it rolled across the city.

Callers to local radio stations said it was the loudest thunder they had ever heard while posts on Twitter indicated some thought the city had been hit by a quake or a major explosion.

Businesses reported alarms going off with alarms also triggered in cars parked in the open.

The weather bureau said Adelaide had 32mm of rain in the 24 hours to 9am on Tuesday, half the May average.

Good falls were also reported in various regional centres, including Cleve, on Eyre Peninsula, which had 22mm, making it the wettest May day in 22 years.

Forecasting group Weatherzone said more than 6000 lightning ground-strikes were detected during the storm.

"The culprit was a surface trough which was enhanced by another trough high in the atmosphere, both acting to trigger the convection," Weatherzone said.

"Moisture was available in an air-stream stretching back to northern WA and the warm waters of the Indian Ocean."