Australia Weather News

The stormy weather of recent weeks is expected to continue this summer. (Supplied: Karen Newlyn)

The Bureau of Meteorology has declared a La Niña, the climate driver typically associated with wet conditions for eastern and northern Australia over summer.

BOM head of operational climate services Andrew Watkins said the La Niña was "likely to persist until at least January, possibly through the summer as a whole".

This La Niña is expected to be relatively weak and short-lived, but with many catchments already full following a La Niña last summer, as well as a wet winter and spring, its impacts could still be severe.

The announcement comes as more heavy rain is forecast for the east.

Rain this week

On Tuesday a low pressure system was set to develop over South Australia, bringing thunderstorms and showers for parts of the state as well as inland New South Wales.

"Flood watches have already been issued for SA and we're likely to see flood watches also issued for parts of northern and eastern Victoria and much of inland NSW either today and tomorrow," BOM senior meteorologist Dean Narramore said.

By Wednesday, the focus of the rain and storm activity will be through inland parts of Queensland, Western NSW and eastern SA.

But Thursday is the main day of concern, with areas of heavy rain across northern Victoria, west of the ranges in NSW and much of southern inland Queensland.

On Friday the rain will be focused over eastern Queensland, eastern and central NSW and possibly parts of northern and eastern Victoria, according to Mr Narramore.

Over the next four days widespread falls of 50-100mm in those areas expected.

By Saturday rain and storms should contract to northern NSW and Queensland.

"These rainfall numbers on already saturated soils and, as mentioned, is likely to lead to renewed river level rises on many of our already flooded rivers," Mr Narramore said. 

He identified the Lachlan River, the north-west slopes of NSW and East Gippsland as potential areas of concern.

Keep up with your local warnings at ABC Emergency.

What is a La Niña?

These kinds of widespread rain events are typical when La Niña conditions are in play.

The La Niñaclimate driver occurs when there are cooler than average waters in the eastern Pacific Ocean and warmer waters in the west.

This results in stronger than average trade winds and more moisture over northern and eastern Australia.

"Those weather patterns shift cloud and shift rainfall towards Australia, so we tend to get a wetter spring and also a wetter a summer, at least in earlier parts of summer," Dr Watkins said. 

Temperatures are also typically cooler during a La Niña, but the increased humidity and slow moving high pressure systems mean heatwaves are not off the cards.

"We can still get heatwaves, and the heatwaves we get over summer tend to be longer," Dr Watkins warned.

Conditions have been slowly creeping towards La Niña over the past few months and though they did not quite reach the BOM's official threshold, they still helped fuel the wet weather we've been seeing recently.

But, as Dr Watkins says, "La Niña  is not the only game in town".

The recent wet conditions have been aided by the negative Indian Ocean Dipole (IOD) which acts like La Niña but in the west and the warm waters to the north.

But as summer kicks in the negative IOD is expected to break down and the La Niña is expected to dominate the climate drivers.

The official summer outlook will be issued on Thursday, but Dr Watkins says the indications are that we're looking at wetter than normal conditions, particularly for eastern parts of Queensland, eastern New South Wales, and possibly in parts of eastern Victoria as well.

It's cyclone season

This season's severe weather outlook suggests the risk of flooding is above average for the east and north this year, with a slightly above average risk of tropical cyclones.

The first tropical cyclone of the season was officially named Cyclone Paddy on Tuesday night as it reached category 1 strength over the Indian Ocean, south-east of Christmas Island.

This cyclone comes slightly earlier than normal, as would be expected in a La Niña year.

Cyclone Paddy is expected to head west and drop back below cyclone status in the early hours of Wednesday morning.

ABC