FORECASTERS are warning that severe thunderstorms could sweep across many parts of Western Australia, including Perth.

Residents from Newman in the Pilbara to Bridgetown in the southwest and Southern Cross in the Goldfields-Esperance region have been urged to prepare for the storms, which are expected to develop into the evening.

Heavy rainfall could lead to flash flooding, large hail and damaging winds, according to the Bureau of Meteorology.

"Perth is at risk if temperatures climb to 28C," forecaster Neil Bennett said.

"It's a risk, and one we can't afford to ignore."

The temperature in Perth was 22C at 12.20pm (WST).

It is feared thunderstorms could test containment lines on the southern boundary of the bushfire currently burning in the Bremer Bay area, in the Great Southern region.

The fire, which was started by lightning, is not fully contained or controlled and authorities believe there is a possible threat to lives and homes.

It has crossed the Bremer River and Gordon Inlet Road, and Doubtful Islands Road in parts, and also entered the Fitzgerald River National Park.

Meanwhile, residents of Nullagine, in the Shire of East Pilbara, have been warned to prepare for flooding expected today and tomorrow.

There is said to be no immediate danger but the Nullagine River is currently above the minor flood level.