A low over southern Australia “is a very complex system”, generating multiple cold fronts, bureau forecaster Helen Reid said. “It’s quite a cold snap, this one.”
Snow falls will be widespread over the weekend, including over parts of the northern and central tablelands, and reach down to areas above 500 metres sea level, she said.
Warragamba Dam, Sydney’s biggest reservoir, remains at 100 per cent full after recent rains, and the city’s overall storage sits at 98.2 per cent full as of Tuesday, WaterNSW said.
The coming cold fronts, though, are likely to drop most of their rain on the western side of the ranges, with little making to the eastern side, Ms Reid said. Sydney can only expect the odd shower and mostly sunny conditions into next week.
On Tuesday, the bureau also raised the chance of a La Nina event forming in the Pacific in 2020 to 70 per cent, or three times the norm.
“Surface waters are cooler than average [in the tropical Pacific], while sub-surface temperatures have cooled further over the past fortnight,” the bureau said.
During La Ninas, rainfall patterns shift westwards, typically raising the odds of rainfall over much of Australia during spring.
Conditions in the IndiFoutan OceanF have also shifted towards favouring increased rainfall over Australia, with a so-called negative Indian Ocean Dipole likely during September to November.
Those climate drivers are related in the longer term rainfall and temperature outlooks. These show strong odds favouring a wetter than average September to November for all of the eastern Australian mainland and most of Tasmania.
The expected cloudier than usual conditions will likely mean daytime temperatures will be cooler than normal while overnight temperatures will be relatively mild, the bureau said.
Get our Morning & Evening Edition newsletters
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.