It's official; the Bureau of Meteorology has declared a La Niña pattern is established in the tropical Pacific, and that is big news for weather in Australia. Indicators in the Pacific Ocean and the atmosphere above it currently exceed La Niña thresholds. Long-range forecasts show there is a high chance La Niña will last until at least January 2021.
La Niña typically means wetter weather for much of eastern Australia, so it is no surprise that much of the south-eastern mainland away from the coast has high chances (above 80%) of a wetter than average October to December. That includes parts of northern and western Victoria. At the same time, days are likely to be warmer than average south of the Dividing Range and nights are very likely (greater than 80% chance) to be warmer than average across the state, and most of Australia.
Adding to the wet signal for the months ahead is a higher than normal chance of a negative Indian Ocean Dipole. A negative Indian Ocean Dipole is when warmer waters concentrate near Australia's north-west, generating extra cloud and favouring weather patterns that can bring more rainfall to south-eastern Australia. While some of the climate models anticipate a negative Indian Ocean Dipole pattern will last for a couple of months, others show a return to more neutral conditions in November. Indian Ocean Dipole events tend to break down when the monsoon arrives in the southern hemisphere around the end of spring. Last year was a notable exception when a strong positive Indian Ocean Dipole lasted until the last week of December, coinciding with a late arrival of the monsoon over northern Australia, and a very dry October and December for Victoria (eighth-driest and third-driest respectively). This year is looking quite the opposite.
Looking back on September, rainfall has been below average for most of the eastern half of Victoria and very much below average (in the bottom 10% of records) for Central and East Gippsland. Most of Western Victoria has had near average rainfall and parts of the south-west even had above average monthly totals. Despite snow down to low levels and a few very cold days in the last week—maximum temperatures were 6–8 °C below average on 25 September in parts of the west and the North East—averaged over the month, days and nights have been warmer than usual across the state.