Canal

• Now we get flash floods much further inland, such as Broken Hill in 2012 and 2022 and Cobar, Bourke and Nyngan in 2022.
• Flash floods are those beginning between one and six hours after rainfall, while riverine floods take longer to build.
• Instead, we’re seeing warm, moist air pushed down from the Coral Sea, leading to thunderstorms and floods much further inland.

What’s changing?

• Inland, flash floods occur when intense rain hits small urban catchments, runs off roads and concrete, and flows into low-lying areas.
• Early this month, the subtropical jet stream changed its course, triggering a cyclonic circulation higher in the atmosphere over inland eastern Australia.
• The result was localised extremely heavy rain, which led to the Warragamba Dam spilling and flood plain inundation in western Sydney.
• These are characterised by a deepening coastal trough and upper-level low pressure systems further west, over inland eastern Australia.
• Instead, flash floods occurred when slow-moving upper-level low pressure circulations encountered air masses laden with moisture evaporating off the oceans.

Haven’t there always been flash floods?

• Previously, inland floods tended to come after long periods of widespread rain saturated large river catchments.
• Inland flash floods were not so common and powerful as in recent decades.
• What about the famous inland floods which move through Queensland’s Channel Country and fill Kati Thanda/Lake Eyre?
• These are slow moving riverine floods, not flash floods.

Read more:
Changes in the jet stream are steering autumn rain away from southeast Australia

Short, intense rain bursts are going global

• Dubai this week had a year’s rain (152 mm) in a single day, which triggered flash floods and caused widespread disruption of air travel.
• Other parts of the United Arab Emirates got even more rain, with up to 250 mm.
• In Western Australia’s remote southern reaches, the isolated community of Rawlinna recently had 155 mm of rain in a day.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.