News

Melbourne activates its 'rain shield', dodging moisture from the northwest

Source Weatherzone Tue 16 Sep 2025

Melbourne locals hoping for the first decent soaking of the 2025 spring would have been disappointed after rain dodged most suburbs overnight, with no rain recorded at the city’s main Olympic Park gauge – or in most other locations across the city – in the 24 hours to 9am Tuesday. Melbourne has received just 5.4mm of rainfall in the first half of September, with no more than 1.2mm on any day. The long-term monthly average for September is 58mm. The prospects for rain looked promising overnight as bands of moisture could be seen streaming across Victoria on the radar – so how did Melbourne dodge the rain? Image: Combined radar and satellite loop in the 12 hours from 8:30pm to 8:30am this Tuesday, September 16, 2025. The answer lies in Melbourne's 'rain shield' against moisture from the northwest. 'Rain shield' is not an official meteorological term. It’s a phrase we’ve coined to help you visualise the blocking effect of the Macedon Ranges, about an hour’s drive north of the Melbourne CBD, which act as a barrier to rain reaching the city when it comes from the northwest. Melbourne’s most reliable rainfall comes from the southwest, when showers stream across Port Phillip Bay and douse the city. Often this rain comes with cold fronts with their chilly, blustery winds – and as any Melburnian knows, these weather systems can strike even in midsummer. Two of Melbourne’s four heaviest rain days of 2025 to date came from southwesterly systems. On January 6, a bitterly cold system for that time of year delivered 24.6mm (and a maximum of just 18.9°C). On March 16, 18mm was recorded in another cold southwesterly, with an unseasonable maximum of just 19.1°C on the 17th. But not all significant Melbourne rain events originate in the southwest. Melbourne’s heaviest rain day for 2025 to date was January 12, when 31mm fell. That was a day of severe thunderstorms across a wide area – and winds in Melbourne that day blew from just about every direction (except the northwest). And of course, there are exceptions to every rule. Melbourne’s heaviest day of rainfall in the 2025 winter was June 8, when 15.2mm was recorded under consistent northwesterly winds. What happened to the 'rain shield' that day? A strong low pressure system centred over Victoria pulled moisture from the Southern Ocean up into central Victoria, then slung it back towards Melbourne as air rotated clockwise around the low. So that June 8 system wasn’t your typical northwesterly stream where air flows towards the southeast from the interior of the continent. That more typical pattern is illustrated well on today’s synoptic chart. Image: Synoptic chart for Tuesday, September 16, 2025. In summary, significant rain can and does occasionally fall in Melbourne when it comes from the northwest. But on days like today and yesterday with a broad-scale northwesterly stream across southeastern Australia, it’s less likely due to the blocking effect of the Macedon Ranges – which are only about half as high as the Victorian Alps to Melbourne’s northeast, but still hefty enough to influence Melbourne’s weather. Meanwhile the chance of showers persists in Melbourne this Tuesday and Wednesday, with a higher chance of the first meaningful spring rain from Friday through to Monday, as winds shift to the west and southwest. Our Melbourne forecast is here. - Weatherzone © Weatherzone 2025