La Nina set to rule a wet and wild summer for NSW
After a year that began with catastrophic bushfires, 2020 will end with a big wet summer bringing an increased risk of flooding and the busiest cyclone season in a decade.
Australia is set for its first La Niña-induced summer in nine years, says Sky News Chief Meteorologist Tom Saunders, with the weather pattern expected to trigger heavier rainfall, cooler days and more tropical cyclones.
Bushfires are "guaranteed" not to be as widespread or fierce across NSW as last season due to wetter soil and vegetation and cooler conditions brought by increased cloud cover, he said.
"It's a substantial contrast compared to last year," he said. "2019 for Australia, was the hottest year on record and the driest year on record and we're talking about well over 100 years of data. It really was an unprecedented year.
"We had frequent dust storms, we had for NSW the worst bushfire season on record, and you would say for many parts of the state we were at the end of the worst drought on record."
October to April is the period meteorologists refer to as severe weather season, when Australia experiences extreme events like bushfires, dangerous heatwaves and cyclones.
The last season was one of the hottest on record, and while temperatures this season will still be above average in some regions, Mr Saunders said was unlikely this summer will be as extreme due to increased cloud cover and reduced solar radiation associated with La Niña.
In his long-range forecast for the next seven months, he predicts the severe multi-year drought across the eastern states will be further subdued as above average rainfall replenishes dry land and empty dams in coming months.
Unlike "El Nino" events associated with warmer and drier conditions for eastern Australia, La Nina cycles delivers above average rainfall and cloud cover. There is a 70 per cent chance of a La Nina forming.
The last significant La Nina event in 2010-11 was Australia's wettest two-year period on record, causing widespread flooding including the worst in Brisbane since 1974.
Cyclone Yasi, one of the most intense tropical cyclones to have hit Queensland, occurred during a La Niña in 2011, so did Cyclone Tracy in 1974, which levelled much of Darwin.
The Bureau of Meteorology declared a La Nina alert last month, and Mr Saunders said global modelling indicates it will continue through the summer to become a significant event.
However as it's late forming and not yet supported by another climate driver, the Indian Ocean Dipole, widespread flooding was less likely.
"We're just hoping for good soaking rain for our farmers."
Much of NSW has already seen significant drought relief this year, with pockets of eastern Australia experiencing their wettest January to August period since 1990.
Rain totals in many areas are already three times what was received throughout the whole of 2019 including Dubbo which has had 651mm in 2020 so far, compared 211mm last year.
But 36-month deficits of 500mm to 1000mm still exist across much of eastern Australia, he said, and the La Nina was the "best chance" of redoing those deficits in coming months.
In terms of severe thunderstorms - when hail, strong winds, flash flooding and tornadoes can strike, La Nina conditions generally bring more rain events for northern NSW and less explosive storm events in the afternoon.
Sydney can expect 15 to 19 days of severe thunderstorms within a 150km radius of the CBD which is "bang on average".
In tropical regions, La Nina wet seasons increase cyclone activity and between nine and 13 are forecast over Australian waters, of which four to six should make landfall.
"The long term average is 11 however there's been a reduction in last few years. If we see ten (cyclones) that would represent the busiest season in ten years," he said.
Global warming plays a part in severe weather seasons, and unless a strong La Nina event occurs, temperatures will be warmer than normal over extended periods, he said.
Australia has recorded above average maximum temperatures each year since 2001, except 2011 which was one of the strongest La Niña's on record.