Key Takeaways:
- A major shift in the weather pattern is expected to bring a week-long drenching to Australia’s eastern seaboard, reducing the short-term fire threat.
- Severe thunderstorms and flash flooding are likely in NSW and VIC, with the greatest risk in East Gippsland, south-east NSW, and northern slopes and ranges.
- The storm threat will extend across central east and north-east NSW, with a burst of strong winds and heavy rain predicted to shift slowly up the coast through the weekend.
- River flooding is possible, particularly in the Illawarra and south coast, but minor flooding is currently the favored outcome due to relatively dry catchments.
- Ex-Tropical Cyclone Koji continues to dump rain over tropical Queensland, with a new tropical low and area of flooding expected to emerge over the Top End this weekend.
Introduction to the Weather Shift
The extreme heatwave and devastating bushfires that affected Australia last week are expected to give way to a major shift in the weather pattern, bringing a week-long drenching to the eastern seaboard. This change in weather is expected to reduce the short-term fire threat, but it also has the potential to generate both flash and river flooding. The rain and thunderstorms will be a welcome relief for firefighters, but it will also bring its own set of challenges, including the risk of flooding and landslides.
Severe Thunderstorms and Flash Flooding
The most significant drenching in months is ahead for eastern parts of New South Wales and Victoria, with severe thunderstorms and flash flooding likely in all three states, including the ACT. The storm outbreak will intensify and expand on Thursday as an inland trough deepens, stretching from south-west Queensland, through eastern NSW to most of Victoria. East Gippsland, the south-east corner of NSW, and along the state’s northern slopes and ranges are at the greatest risk. The biggest thunderstorm threat on Friday will extend across central east and north-east NSW, with a bit of steady rain and the odd storm also developing along the south coast and through Gippsland, Victoria.
River Flooding and Fire Threat
While flash flooding is possible in multiple districts, the risk zone for river flooding is mostly confined to the Illawarra and south coast, and thankfully, minor flooding is currently the favored outcome due to relatively dry catchments. The sudden shift in the weather is due to the location of high-pressure systems, with a high-pressure cell tracking below Tasmania and directing humid onshore winds onto the east coast, providing the moisture to fuel the rain and thunderstorms. The short-term fire threat will be reduced, but it is far from over, as rainfall reduces the intensity and spread of a bushfire, but it takes a significant amount of precipitation for a large fire to be completely extinguished.
Tropical Cyclone Koji and New Flood Threats
Ex-Tropical Cyclone Koji continues to dump rain over tropical Queensland, with a new tropical low and area of flooding expected to emerge over the Top End this weekend. The system has already dumped well over 200mm over parts of the Central Highlands, Central Coast, and Capricornia since the weekend, in some cases well over double the monthly average. The remnants of Koji will still trigger localized heavy rain and flash flooding from the Gulf Country to the Central Coast on Thursday, with heavy downpours from thunderstorms persisting over a similar region on Friday. The BOM’s ensemble rain forecast for the next week shows the potential for more than 100mm over north-east Queensland and the western Top End.
Long-term Weather Outlook
The combination of the initial thunderstorms, the rain to follow, and further showers next week should boost weekly totals to above 50mm along the coast and eastern ranges from the Queensland border to West and South Gippsland. Totals from Sydney to East Gippsland may average in the range of 100 to 200mm and could even hit the 200 to 300mm range on pockets of the NSW south coast. The most widespread rain since August is likely on the NSW coast during the next seven days, along with possible heavy falls in Gippsland, Victoria. However, the long-term weather outlook is still uncertain, with the potential for another heatwave to develop in the coming weeks, and the fire threat is expected to remain high until the summer months come to an end.
