Illawarra residents should prepare a wet spring and summer as well as potential flooding caused by a La Nina event.
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But thankfully sunny and clear weather is expected across the long weekend, with temperatures ranging from 25 to 28 degrees in Wollongong.
This week the Bureau of Meteorology declared that La Nina has developed in the Pacific Ocean.
La Nina is the cool phase of the El Nino Southern Oscillation. It is associated with cooler than average sea surface temperatures in the central and eastern tropical Pacific Ocean.
La Nina typically results in above-average spring rainfall for Australia, particularly across eastern, central and northern regions.
It can also mean cooler days, more tropical cyclones, and an earlier onset of the first rains of the wet season across the north.
The BOM's seasonal spring outlook for the Illawarra and South Coast indicated there will be greater than 70 percent chance of exceeding median rainfall due to La Nina.
Between 200mm and as much as 300mm could fall over the Illawarra and South Coast from October to December.
Temperatures are expected to hit between 21 to 24 degrees, and there is a 45 to 55 percent chance of temperatures exceeding the median maximum temperature.
BOM climate operations manager Dr Andrew Watkins said the outlook was being largely driven by changes in sea surface temperatures in the tropical Pacific and Indian oceans.
"Most long-range forecasts analysed by the Bureau, including from our own climate model, are indicating a La Nina could develop in the spring, which typically results in above-average winter-spring rainfall for Australia, particularly across eastern, central and northern regions," he said.
"A La Nina also typically brings cooler and cloudier days, more tropical cyclones, and an earlier onset of the first rains of the northern wet season."
He said NSW was likely to experience more rainfall, with a chance of widespread flooding, including flash flooding. The rain could fall into water storage catchments too.
Dr Watkins said the La Nina formation reduced the risk of getting extreme heat waves, however the heatwaves would likely be longer in duration and more humid.
He added the bushfire risk was reduced but it would be unlikely if the state escaped any fires over the summer season.
The last La Nina event occurred from 2010-2012 and resulted in one of Australia's wettest two-year periods on record. Widespread flooding occurred in many parts of Australia associated with the record rainfalls.
The BOM said impacts of La Nina can vary significantly between events. It is likely this year will not see the same intensity as the 2010-11 La Nina event, but is still likely to be of moderate strength.
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