NSW To Be Smashed By Another Giant Hail And Thunder Storm

Giant hail and damaging winds are predicted as NSW braces for another summer storm.

Residents in Blackheath, Katoomba and Lake Burragorang, in the NSW Blue Mountains region, were put on alert by the Bureau of Meteorology for heavy rain and damaging winds on Wednesday afternoon.

Hail stones, about the size of grapes, were reported in the Blackheath area, and the BOM has warned "giant" hail stones can be expected.

The storm  is moving east towards the Sydney metro area.

The storm is expected to stay in the western suburbs of Sydney, and there is a low chance of hail in these areas, a BOM spokesperson told 10 daily.

Unusual lumpy, puffy clouds -- known as mammatus clouds -- usually associated with severe storms, formed over Sydney on Wednesday afternoon.

"Their presence merely indicates descending pockets of small droplets or ice crystals from an anvil surface," the Bureau said on its website.

Extreme weather conditions are expected to continue in NSW, with a severe heatwave predicted for the rest of the week.

The BOM said NSW can expect hot days and warm nights as the heatwave lingers over most of the state until at least Sunday.

The heatwave is being caused by a high pressure system over the Tasman Sea that is directing a ridge towards the Queensland coast, a Bureau spokesperson told 10 daily.

"This is bringing north to north-westerly winds into NSW, dragging warm air in from central Australia," he said.

Feature Image: Karla Garry/ Supplied