IT MAY be sunny and bright out at the moment but get ready Australia because the deluge is coming.

A low pressure system is slowly creeping up on the entire eastern seaboard which could see a month’s rainfall from the skies in just three short, and very soggy, days.

But for one populated pocket of Queensland the drenching could be even more disastrous with up to five times the average rainfall for May falling.

The Bureau of Meteorology has issued a flood watch for Queensland coastal catchments from Cairns to Gladstone, including Townsville, Mackay and Rockhampton.

The watch extends inland to include parts of the Burdekin and Fitzroy catchments.

Bureau of Meteorology Weather Services Manager Richard Wardle said the deluge would hit northern parts of the state late on Tuesday.

“Widespread rainfall totals in the range of 100-200mm can be expected with rainfall decreasing as it moves south,” he said.

“While there is the potential for riverine and flash flooding, rainfall totals and flood levels associated with this system will be generally lower than those seen with the passage of ex-tropical cyclone Debbie.”

The downpours could lead to flash flooding particularly in areas still wet following Cyclone Debbie.

Tom Saunders, chief meteorologist with Sky News Weather, told news.com.au the low pressure system was being bolstered by a mass of cold air in the lower atmosphere aiding heavy rainfall.

“The low is not particularly intense. However, it is slow moving which will allow tropical moisture to feed south and build on its eastern flank,” Mr Saunders said.

The system will move slowly eastwards across Eastern Australia in a wide band with Brisbane, Sydney and Melbourne all in the firing line. Lighter showers will strike Perth and Adelaide as well with only the country’s remote north west likely to escape the showers.

While the downpours could begin as soon as Thursday, state capitals are likely to bear the brunt on Saturday.

“The rain event will peak between Thursday and Saturday and extend from tropical Queensland to northern Tasmania.

“Much of the Murray-Darling Basin should receive about 25 to 50mm of rain — about a month’s worth of rain,” said Mr Saunders.

But it’s coastal areas of Central Queensland, including Bowen, Townsville and Mackay, which will be particularly badly hit.

Residents of Townsville and Moranbah, which can usually expect an average of 32mm of rain in May, could be hit with anything from 50 to 150mm.

Griffith in southern NSW could see 50mm by the weekend, more than 15mm above the average while Swan Hill could get double its monthly rainfall.

“For tropical Queensland, the rain may lead to river flooding as catchments are still wet after Severe Cyclone Debbie,” said Mr Saunders.

“Across southeast Australia flash flooding is possible from thunderstorms on Thursday and Friday, along with other storm threats including damaging winds and hail.”

WHEN WILL THE RAIN HIT YOUR CITY?

Sydney is expected to be sunny with a high of 21C tody before the rain begins in earnest on Thursday, although temperatures should hold up.

Melbourne should bump around highs in the late teens for the next few days with possible showers but ensure to pack a brolley from Friday.

Brisbane was 25C and partly cloudy on Tuesday before the rain hit today. The mercury will dip to 23C on Friday as the rain makes its entrance. Further north the rain will come even sooner with showers appearing in Townsville by this evening.

A high of 15C in Hobart Tuesday will rise to 17C by Saturday with rainfalls here and there but Launceston and the north coast will be wetter.

A sunny Canberra, sitting on 17C, will see showers increasing with the heaviest downpours on Saturday.

Away from the worst of the weather, Adelaide will see intermittent showers all this week and highs of around 20C.

Perth will keep its glorious blue skies and temperatures in the mid-twenties until Friday before scattered showers settle in.

Darwin will be 33 and sunny.

Quick, let’s all book a flight to the Top End.

benedict.brook@news.com.au