Australian Weather

01 Jul 2025, 6:18AM UTC

The terms 'East Coast Low' and 'bomb cyclone' are making news headlines in Australia this week as severe weather batters parts of eastern NSW. So, what are these weather systems and what makes them so dangerous for people being impacted by their ferocious weather? What is an East Coast Low? Australia’s east coast is affected by various types of low pressure systems every year. However, the most dangerous weather typically comes from a particularly intense breed of low pressure system called an East Coast Low. The term East Coast Low refers to an intense non-tropical low pressure system that remains in close proximity to Australia’s east coast for a prolonged period of time, causing multiple modes of severe weather. There are numerous technical aspects of East Coast Lows that distinguish them from other types of low pressure systems in eastern Australia. In general, East Coast Lows: Sit close to Australia's east coast (within 200km) for more than 12 hours. Have a steep pressure gradient surrounding their centre. Cause severe weather, usually including damaging winds, heavy rain and dangerous surf. What is a ‘bomb cyclone’? The term 'bomb cyclone' refers to a low pressure system that intensifies rapidly over a relatively short period of time. This process is called ‘explosive cyclogenesis’, also known as ‘explosive bombogenesis’ and sometimes referred to as a ‘bomb low’, ‘bombing low’, or ‘bomb cyclone’. Explosive cyclogenesis happens when the central pressure of a low drops by a defined threshold within 24 hours. The pressure threshold varies depending on latitude (how far north or south the low is located). Around Sydney's latitudes, a pressure drop of at least 15 hectopascals (hPa) in 24 hours is required for explosive cyclogenesis. The maps below show that a low pressure system underwent explosive cyclogenesis near the NSW coast this week, with its central pressure dropping by 24 hPa in 24 hours.  Image: Mean sea level pressure analysis chart for 4pm AEST on June 30, showing a low pressure system near the NSW coast with a central pressure of 1015 hPa. Source: Bureau of Meteorology. Image: Mean sea level pressure analysis chart for 4pm AEST on July 1, showing a low pressure system near the NSW coast with a central pressure of 991 hPa. Source: Bureau of Meteorology. Are all East Coast Lows bomb cyclones? Not always but they often are. Some East Coast Lows form when tropical cyclones move south and transition from tropical to extra-tropical low pressure systems. In these cases, the central pressure may not drop enough to be classified as bomb cyclone, but they are still intense and dangerous. When a low pressure trough forms near the coast of NSW or southeast Qld and rapidly deepens into an ECL, this usually involves explosive cyclogenesis. East Coast Lows that are also bomb cyclones typically cause a more sudden onset of severe weather. Are bomb cyclones rare? Not really. Bomb cyclones can be found in many parts of our planet from the tropics to the polar regions. Australia has already seen about six to eight bomb cyclones since March this year, but most of these have occurred out over sea away from the coast. It is rarer for explosive cyclogenesis to occur within close proximity to Australia’s coast and when it does, the severe weather can be particularly dangerous because it intensifies quickly. Image: Enhanced water vapour satellite images showing a low pressure system intensifying near the NSW coast on Tuesday, July 1, 2025. How can I stay safe? If you are in an area that is impacted by an East Coast Low or bomb cyclone, you should follow advice from local emergency services and monitor severe weather warnings for information on the expected impacts. The main weather threats from these intense low pressure systems are heavy rain, flooding, damaging to destructive winds and dangerous surf.

01 Jul 2025, 1:01AM UTC

Heavy rain, destructive winds and damaging surf will batter eastern NSW in the coming days following the explosive development of an East Coast Low. A coastal trough located off the southeast Queensland and northeast NSW coast rapidly intensified into a low pressure system overnight into Tuesday morning. This ferocious intensification, which meteorologists refer to as explosive cyclogenesis, can be seen playing out in the video below. Video: Satellite imagery across eastern NSW and the Tasman Sea showing the East Coast Low rapidly developing on the morning of Tuesday, July 1, 2025. Widespread 24-hour rainfall accumulations to 9am on Tuesday of 20-40 mm have been recorded between Noosa, in southeast Queensland and Ulladulla on the NSW’s South Coast. Heavier falls of 70 to 120 mm were also recorded around Jervis Bay and north of Newcastle, with notable falls of: 120mm at Jervis Bay Airfield 101mm at Currarong 84mm at Vincentia 80mm at Meldrum East Coast Low forming through ‘explosive cyclogenesis’ The coastal trough that intensified overnight into Tuesday morning had a minimum pressure of about 1014 hPa on Monday night. However, once the low started to rapidly deepen, its central pressure had already dropped by 8 hPa 6 hours later at 4am on Tuesday. The central pressure of the developing East Coast Low was down to about 1002 hPa by 10am on Tuesday. With further deepening expected on Tuesday, the system is expected to meet the criteria for it to be classified as a ‘bombing cyclonic low’ at this latitude, sometimes referred to as a 'bomb cyclone'. This process of rapid and explosive intensification is a necessary condition in the formation of a high impact East Coast Low as we're seeing forming near the NSW coast. The Tasman Sea has seen several "bomb cyclones" in the past months through the formation of significant Tasman Lows meeting the rapid deepening threshold. This low will however be notable due to its proximity to the coastline, making it the first East Coast Low since July 2022. East Coast Low reaches peak intensity near Sydney On Tuesday morning, the East Coast Low was located east of the NSW Central Coast. It is forecast to edge slowly closer to Sydney and the Central coast during Tuesday, possibly coming within 100 kilometres of the coast. The central pressure of the system should reach its lowest value on Tuesday evening or Wednesday morning, possibly dropping to a central pressure value below 990 hPa. Image: forecast MSLP and 22-hour accumulated rainfall to 2am on Wednesday, July 2, 2025. The proximity to the coast means that intense rain, damaging winds and hazardous surf are all being generated and sent into nearby areas to the south and west of the low. This includes most areas between Newcastle and Moruya, including Sydney and Wollongong. Heaviest rain shifting south of Sydney Heavy rain spread south across the Hunter on Monday night and into the Sydney region on Tuesday morning. Periods of heavy rainfall are still expected, most likely on Tuesday afternoon and again on Wednesday morning. However, the heaviest rainfalls are most likely further south into the Illawarra and South Coast. Accumulated falls of 40-60mm are still expected across most of the Sydney and Hunter region in the coming 24 hours or so into Wednesday, with isolated locations receiving up to 100mm. Image: Forecast 24-hour rainfall to 10am on Wednesday, July 2, 2025, across central NSW. Heavy, intense and flooding rainfall for Illawarra and South Coast Areas north from Ulladulla saw rainfall intensify late on Monday night, amounting to 20-40mm of rainfall since, with the Jervis Bay area recording 70-120mm to 9am on Tuesday. Flooding rainfall is set to intensify on Tuesday afternoon, with a deep stream of moist southeasterly winds around the low converging with the local topography and opposing wind flow. As seen below, most of the Illawarra and South Coast regions and parts of the Southern Tablelands are expecting rainfall accumulations of 100-150mm over the next 24 hours, with areas between Jervis Bay and Bermagui seeing the potential for up to 300mm. Image: 24-hour rainfall and MSLP forecast to 10am on Wednesday, July 2, 2025, across the NSW South Coast region. Powerful and damaging-to-destructive winds batter the coast As the East Coast Low reaches peak intensity on Tuesday and Wednesday, strong and powerful winds will impact much of eastern NSW. Wind gusts reaching 130 km/h are possible along the coastline south of about Seal Rocks, with gusts of 80-100 km/h also possible across a more widespread swath of NSW on and east of the Great Dividing Range. Image: 10m wind gusts at 1pm on Tuesday, July 1, 2025. The strong winds are whipping up large and unruly surf across the NSW coastline with a Coastal Hazard Warning in place for the coast between Moruya Heads and Seal Rocks. Severe Weather Warnings for heavy rain and damaging winds are likely to remain in place for parts of eastern NSW over the coming days. Check the latest warnings for the most up-to-date information.