Thunderstorms Sydney
An emergency worker chainsaws a tree that destroyed a wall following a tornado in the Sydney suburb of Kurnell, December 16, 2015. Reuters/Jason Reed

Sydney, the Hawkesbury region and the Blue Mountains have been hit by severe thunderstorms in the next few hours, a warning issued by Bureau of Meteorology (BoM) says. According to the forecast, residents have been warned of destructive winds, heavy rain, and large hailstones. Heavy rains can also cause potential flash-floods.

BoM’s thunderstorm warning also included the Wingecarribee and Wollondilly areas. Sarah Chadwick, BoM forecaster, says a supercell storm is moving towards Moree in northern NSW.

At 11:30 a.m., the BoM detected severe thunderstorms on weather radar near the Grose Valley in the Blue Mountains. The thunderstorms were moving south east and were expected at Cranebrook, Hazelbrook and Penrith by 12:00pm and Liverpool, Camden and Warragamba by 12:30pm.

“We could expect to see quite intense bursts of rainfall associated with these storms. These particular storms, which are classified as more dangerous ones, can live a bit longer and travel along larger areas,” said Chadwick.

Many regions, including Central and Southern Tablelands, Illawarra, Sydney, Hunter, the South Coast and several others will be impacted by the storm, reports the ABC. Around 10:14 a.m., wind speed of 93 kilometres per hour was recorded at Thredbo Top station.

“It's a very big system, with widespread instability and the potential for severity along the whole way,” said Rob Sharpe, a meteorologist with Weatherzone.

Humidity levels were as high as 98 percent in Badgerys Creek in Sydney's west earlier on Friday morning. The bureau radar picked up hailstones within the storms, 2cm in diameter.

“They are plenty of storms building back behind this one ... We could see waves of them coming off [the ranges],” said bureau meteorologist Zac Porter.

There is a lot of moisture in the atmosphere. Conditions are extremely unstable and there is a fair amount of heat.