Australia's spring weather has deviated significantly from expectations. Instead of the anticipated wetter season, many regions, including New South Wales, Queensland, and the Northern Territory, are experiencing record-high temperatures for October. This unexpected weather pattern can be traced back to a polar warming event in September, which altered weather patterns across the Southern Hemisphere. However, there is optimism for a shift in conditions as a developing La Niña may bring much-needed rain and storms in the coming weeks.

Two significant cloud bands are expected to deliver rain to about 70% of Australia. Although the effects of last month's polar warming are still present, an influx of tropical moisture from the Indian and Pacific Oceans is anticipated to bring relief. The interaction of humid air from the north with low-pressure systems will lead to widespread showers and thunderstorms. The first band of rain formed over the western interior and is expected to move across South Australia, Victoria, and southern New South Wales over the weekend. Severe storms, including hail and flash flooding, are likely in South Australia, particularly in the west.

In Adelaide, light showers are expected in the morning, increasing in frequency by the afternoon. If a storm passes directly over the city, it could receive up to 25 millimeters of rain by midnight. Patchy rain and isolated thunderstorms are also forecasted to move into western Victoria and southwestern New South Wales on Saturday, advancing toward the east coast on Sunday. Rainfall totals will vary, with most regions expecting between 10 to 20 millimeters, but some areas could see totals ranging from a few millimeters to 40 millimeters.

Melbourne could experience rainfall between 5 to 35 millimeters, with the higher end potentially marking the city's heaviest rainfall in over a year. A second band of rain is expected to form on Monday over central Northern Territory and tropical Queensland, spreading to southeastern Queensland and northern New South Wales by Tuesday. Some severe storms may impact Brisbane and surrounding areas as early as Sunday.

This second wave of showers will also bring thunderstorms, with forecasts suggesting an average of 20 to 50 millimeters of rain along the coast and ranges from Townsville to Newcastle by Wednesday. The arrival of rain on Tuesday will end Brisbane's unusually dry and hot spell, which has seen October temperatures averaging above 29 degrees Celsius, three degrees higher than the long-term average.

Despite the relatively dry spring, experts note that polar warming events significantly influence Australia's weather. An atmospheric scientist recently highlighted how a cooling stratosphere contributed to the unexpected wet summer of 2023-24, despite predictions of a dry season due to El Niño. Although temperatures high above Antarctica stabilized earlier this month, the effects at the surface may persist into November.

The developing La Niña, which typically increases rainfall across Australia, is also a factor. While the Bureau of Meteorology has not yet confirmed its arrival, the National Oceanic and Atmospheric Administration in the U.S. has declared it. Observations indicate that sea surface temperatures in the central-east Pacific have cooled, trade winds are stronger than normal, and cloud cover is below average near the International Date Line. The Southern Oscillation Index is also trending toward La Niña levels.

Although this La Niña event has emerged late in the year and may be short-lived and weak, it combines with a negative Indian Ocean Dipole and high water temperatures around Australia to create favorable conditions for increased rainfall in November. The Bureau of Meteorology's latest outlook suggests a 60% to 80% chance of above-median rainfall across much of eastern Australia, South Australia, and the Northern Territory. Global models show a less certain range of 50% to 80%.

Looking ahead, the influence of the stratospheric warming, negative IOD, and La Niña is expected to diminish by January. Consequently, the summer outlook shows no clear signal for rainfall, indicating an equal chance of below or above median rain for most areas. However, the temperature outlook is more definitive, with climate change contributing to expectations of another record-hot summer, similar to those experienced over the past decade.