A PERFECT combination of climatic conditions are joining together and could see dangerous supercell storms form over the coming days, meteorologists have warned.

Large parts of Queensland and New South Wales, including Sydney and Brisbane, could be affected by storms and heavy rain on Thursday and into Friday.

Sky News meteorologist Tom Saunders cautioned the east coast could see "potentially some of the most powerful thunderstorms so far this season".

It's bad news for Queenslanders who have already been plagued by storms. Earlier this week, hail the size of 50c coins fell on the Gympie region while landspouts - small tornado-like swirls of wind and rain - had been seen on the Gold Coast, reported the Courier Mail.

In Toowoomba on Tuesday, the hail was so intense it looked like it had snowed in parts of the city.

Even outside the potential supercell storm zone, rain will be a feature in many of Australia's capitals. But in other areas, mostly inland, the highs could nudge 40C. Birdsville, in western Queensland, is predicted to hit 42C on Friday.

While the Bureau of Meteorology has not issued any severe weather warnings, that could change over the next 24 hours. Thunderstorms have begun to develop in the Tasman Sea.

Mr Saunders said thunderstorms required three ingredients to form: a trigger to initiate uplift, instability to maintain uplift through the troposphere and moisture so the rising air can condense into cloud.

"Thursday will deliver the trifecta along the east coast and ranges from Queensland's Wide Bay down to southern NSW."

Mr Saunders said accurately predicting major storms was notoriously difficult. However, he said what had really caught his eye was the unstable conditions over Sydney and Brisbane and the higher levels of wind shear.

Wind shear is a rapid change in wind speed and direction over a small area.

"Wind shear is not a requirement for storms to develop but it's critical for severe thunderstorms. Without wind shear a storm simply collapses on itself before it can mature to a level of intensity which would bring severe weather.

"Instability and shear levels on Thursday look sufficient for supercell development across south-east Queensland, and along the northern and central NSW coast and adjacent ranges."

Mr Saunders said when wind shear reaches high levels, storms can begin to rotate. That's when they become the powerful and dangerous supercell storms.

"This process can last for hours and allows a supercell to maintain a strong updraft and downdraft couplet - leading to torrential rain, damaging winds, hail and sometimes tornadoes."

He stressed this did not guarantee a supercell - just because all the ingredients are in place doesn't mean the meal will be cooked to perfection.

For instance, a south-easterly wind change due to push through Sydney on Thursday could suck in a blanket of low cloud off the Tasman Sea with light showers which would drop the surface temperature and prevent uplift.

"Settled weather will return on Friday to eastern Australia but as we approach the peak of the storm season these events will become more frequent," said Mr Saunders.

"Indeed the next potential for storms will arrive from Saturday to Monday."

The weather Bureau has put Australia on La Nina "watch" after a prolonged stretch of neutral conditions. As oppose to El Ninos, La Ninas tend to bring wetter weather to Australia's east coast.

END OF THE WEEK WEATHER IN THE CAPITALS

Brisbane

A mostly sunny Wednesday with a max of 29C will slide into an unsettled Thursday with a chance of thunderstorms in the afternoon and evening, possibly severe. And just like that, Friday will be sunny and clear with a high of 31C stretching into the weekend.

Sydney

Rain will be a key feature all the way to the weekend interrupting sunny spells. Highs or around 25C and possible storms on Wednesday evening and Thursday. The weekend will be partly cloudy.

Canberra

Similar to Sydney with highs of 25C but a higher chance of rain and possible thunderstorms.

Melbourne

Rain will ease over the coming days with the sun peeking through the cloud at times. A high of 17C on Thursday will rise to 26C on Friday before unsettled conditions on the weekend.

Hobart

A high of 19C on Thursday will rise to 24C for the weekend. Mostly cloudy with the odd shower, the likelihood of rain will increase as we head into the weekend.

Adelaide

Mainly settled. A high of 22C on Thursday will give way to 30C on Friday before dipping down again on the weekend. Possible showers here and there.

Perth

A high chance of showers for the next few days with highs of around 20C.

Darwin

34C and mostly sunny. Storms for the weekend.