Severe thunderstorms have begun developing in Queensland's south with large hail falling on the Granite Belt.

Forecaster Michael Knepp says one rain gauge at Stanthorpe has measured about 15 millimetres of rain in as many minutes.

He says the thunderstorms are slow moving - and that means they could produce large hail and flash flooding.

"It's really started to explode on our radar in the last 10 to 20 minutes," Mr Knepp said.

"We probably can expect towards Toowoomba and along the ranges activity in the next few hours - maybe continuing into the evening."

Hail the size of marbles has been reported at Stanthorpe.

The Weather Bureau says that because the cells are moving very slowly, it may be hours before they reach the coast.

Emergency Management Queensland says the quiet start to the storm season may change this afternoon.

Director of Disaster Management Mike Shapland says this may have caused people to become complacent and not be as well prepared as they should.

"People can still take a number of steps today to prepare for the possibility of a storm this afternoon or this evening," he said.

"It is particularly important that residents ensure their gutters are clear to avoid water backing up in their drains.

"They should make sure all outdoor furniture is secured so that it can not become a projectile if the winds pick up.

The weather bureau says places that may be affected over the next few hours include Warwick, Toowoomba, Stanthorpe, Boonah, Oakey, Millmerran, Crows Nest, Allora, Clifton, Pittsworth, Jondaryan and Yarraman.