Australia likely to see a rise in cyclone activity after record low season: BoM

Separately, Sydney's run of mild weather has a few days to run after temperatures on Monday reached almost 12 degrees above average.

The Australian region registered only three tropical cyclones in the 2015-16 season, with just Cyclone Stan making landfall – both record low numbers during the nearly 50 years of reliable records, the bureau said.

The coming season, though, is likely to be more stormy, with odds favouring the number of storms to reach at least the 11 tropical cyclones for the region recorded in an average year. Of those, four cross the Australian coast in a typical season, bringing high winds, powerful waves and storm surges to parts of West Australia, Queensland and the Top End.

Last year was "an extremely benign season", Andrew Watkins, manager of Climate Prediction Services for the bureau, said.

"It is highly unlikely Australia will see a cyclone season as quiet this year."

One influence is the abnormally warm waters around Australia, with some regions 2-3 degrees above average. "Certainly, we've seen temperatures approach record levels over the winter," Dr Watkins said.

And unlike last year, conditions in the Pacific have shifted away from one of the strongest El Ninos on record towards a La Nina. During La Nina years, rainfall and storm activity tends to shift westwards towards Australia. (See bureau chart below of the areas of expected cyclone activity.)

'Flood risk'

An increase in cyclones may disrupt economic activities such as iron ore and coal shipments from northern ports.

The storms may also bring environmental impacts, such as more damage to coral reefs recovering from bleaching which was the worst on record this year.

They may also bring more rain to regions that have been soaked by the unusual wet spell of late. September was the country's second-wettest on record going back to 1910, the bureau said last week.

"The soils, particularly in eastern Australia right up into the north, are close to capacity in terms of how much water they can hold," Dr Watkins said.

"The soils are sodden at the moment and our rivers are full, and so if there are any significant or long-lasting rain events, the water doesn't have a lot of places to go," he said. "We have to be very careful about the flood risk at the moment for eastern Australia."

The bureau defines tropical cyclones as low-pressure systems with sustained winds of at least 63 km/h and gusts of 90 km/h or more. Waters need to be at least 26 degrees to generate such events.

Apart from having few such storms last year, none of the three registered in the Australian region reached category-3 strength – another first in the 47 years of records, according to the bureau.

Category-3 storms have winds gusts of at least 165 km/h, according to the bureau's definition.

The relatively quiet season was largely in line with the bureau's outlook issued a year ago.

While coral bleaching is less of a risk for the Great Barrier Reef this summer, other reefs may be at a greater threat from so-called ocean heatwaves. La Nina years typically see a build-up of warm waters in the regions to Australia's north that flow southwards along the WA coast, Dr Watkins said.

(See Bureau chart below showing September sea-surface temperatures.)

Sydney posted its warmest day in six months on Monday but the unusual warmth – almost 12 degrees above average – is unlikely to return for a while.

The mercury touched 33.6 degrees but dropped off steeply with the arrival of a late-afternoon cool change. The temperature sank 7 degrees in just 10 minutes at the airport, Kim Westcott, a meteorologist at Weatherzone, said.

While notable Monday's heat wasn't that exceptional. Observatory Hill has notched a day of 33 degrees or warmer this early in the season 37 times over the site's 158 years of records. That's about once every five years on average,  the Bureau of Meteorology said.

The forecast for the rest of the week is for mostly sunny days with temperatures at or slightly below the city's October average of 22 degrees before conditions warm again for the weekend.

Thursday's predicted top of 18 degrees looks likely to snap the 24-day run of at least 20 degrees.

Sydney has largely been spared of the wild weather bringing floods and cool conditions across most of south-eastern Australia in recent weeks.

The mercury failed to reach 20 degrees on just two days in September, a record for Sydney, the Bureau of Meteorology said. The city's rainfall was about average for the month.

Queensland had a similar rain and temperature mix to NSW last month. It was the state's third-wettest September on record, with the extra cloud cover making it the 10th coolest for maximums.

Night temperatures, though, were mild. Statewide minimums were the sixth warmest on record, the bureau said last week.

This story first appeared on The Sydney Morning Herald