It could be a very weekend for large parts of the eastern seaboard. Picture: BoM.

Some parts of the state could be drenched with up to 200mm of rain, enough to lead to flash flooding in swollen rivers.

It doesn’t bode well for this Saturday’s Sydney Gay and Lesbian Mardi Gras, which could see rain falling on its parade.

Christian Democrat NSW upper house MP Fred Nile would often “pray for rain” to dampen spirits during the famous festival. But God may not be entirely on his side with Darlinghurst and the city’s CBD possibly escaping the worst of the weather.

In contrast, most of Australia’s other capitals are likely to experience a pleasant weekend with highs around the 30C mark. In Perth, the mercury could get up to a scorching 39C.

The Bureau of Meteorology’s rainfall forecast has at least 100mm — possibly up to 200mm — falling in southern NSW. Picture: BoMSource:Supplied

NSW

NSW’s woes are being caused by a weather system in the Tasman Sea.

At 5pm Thursday, the first barrage of storms was poised to hit, with the Bureau of Meteorology issuing a severe thunderstom warning for the NSW central tablelands and Mid North Coast for “the next several hours”.

“A weak lower pressure system is forming off southern and central parts of NSW and that’s going to bring persistent showers across central and southern parts,” Jordan Notara, a forecaster for the Bureau of Meteorology, told news.com.au.

Ground zero for the deluge on Friday will be the NSW south coast, including Wollongong.

“We may see 100mm of rain across the Illawarra and south and, if heavier showers associated with thunderstorms come through, up to 200mm.

“The rain will be persistent over particularly Friday and Saturday and that may lead to flash flooding.”

The Bureau issued the first of its severe weather warnings for the Illawarra, the south coast, and parts of the Southern Tablelands and Snowy Mountains this afternoon, warning of the heavy rainfall and thunderstorms predicted.

A flood watch is in place for Bega and Moruya and further warnings are expected as the weekend nears.

Sydney may just escape the worst of the rain but could still clock up 30mm on Friday, Saturday and Sunday. Temperatures are likely to be a relatively mild 23-24C.

“In the current state, Sydney is on the edge of the rainfall. However, parts of southern Sydney may see higher totals and flooding,” he said.

Along the state’s north coast, the amount of rain will be far less but unending drizzle could be the theme of the weekend.

Inland, away from the system, conditions will be drier and temperatures could easily push into the 30s. Canberra will reach 27C on Sunday with a possible shower.

Rain is likely anywhere along the eastern seaboard but Victorian and SA should remain sunny. Picture: BoM.Source:Supplied

VICTORIA AND TASMANIA

The warmest day for Melbourne will be Saturday with a high of 28C and a possible shower in the afternoon. Hobart will see blue skies and 21C, a milder outlook following high temperatures well above average — reaching almost 27C — earlier this week.

WESTERN AUSTRALIA AND SOUTH AUSTRALIA

Perth will be the hottest capital this weekend, with the mercury soaring as high as 39C on Sunday. It will be only slightly less baking on Saturday. Any showers will pass through on Friday, leaving the weekend hot and sunny.

Adelaide will also push past 30C but it will be more manageable than Perth with a high of 33C on Saturday.

QUEENSLAND AND THE NT

Brisbane will likely catch the tail end of the system bearing down on coastal NSW. There is a possibility of showers across the weekend and a maximum of 31C on Saturday. Light rain can be expected anywhere on the coast with heavy showers and possible storms in the Far North of the state.

Darwin will be humid and stormy with a high of 33C.

Despite the rain currently barging into NSW, the Bureau of Meteorology has warned that autumn’s weather could be much like summer. With the La Nina weather system failing to form there is little to stop westerly winds bringing the hot Central Australia air to the east coast over the coming months.

There is also the possibility of El Nino waking early than anticipated from its slumber. It wasn’t due back for some time but sea surface temperatures have been increasing in the eastern Pacific Ocean and are now warmer than average for the first time since June 2016.

If El Nino does re-emerge during winter, it’s likely to bring below average rainfall and unseasonably warm weather.