November 28, 2024
At 3:05pm WST on November 28, Tropical Cyclone Robyn, the first tropical cyclone of the season in Australian waters, was officially named.
Fortunately, Robyn is not expected to impact Australia or any of its territories. Currently influenced by an upper trough to its west and an upper anti-cyclone to its east, Robyn is tracking south and may adopt a southeast path before eventually being steered westward, out of Australian waters, by a broad high-pressure system to its south.
Track map for Tropical Cyclone Robyn. Source: BoM
Did Robyn Really “Weaken” Before Being Named?
Eagle-eyed weather enthusiasts may notice something odd. At 9am WST, six hours before Robyn’s official classification, the system was a tropical low with a central pressure of 993mb, gusts of 100km/h, and sustained winds of 75km/h. By 3pm WST, Robyn was classified as a tropical cyclone, but its central pressure had risen to 994mb, while wind speeds remained the same.
So, why the change?
The answer lies in the official definition of a tropical cyclone in Australia. To be classified as a tropical cyclone, the system must have gales extending more than 50% around its centre. At 9am, Robyn did not meet this criterion, despite having cyclonic winds. By 3pm, the gales had wrapped around more than half of the centre, allowing it to meet the definition of a tropical cyclone.
The BoM tropical cyclone bulletin sheds light on why Robyn wasn't a tropical cyclone this morning - the 34 knot wind radius only occurred in two quadrants, before expanding to all quadrants in the afternoon (meeting the definition of a tropical cyclone).
This distinction is also why ex-tropical cyclones are often no longer classified as tropical cyclones. While parts of these systems may still exhibit Category 1 or 2 strength, they lose their classification if gales do not extend more than halfway around the centre.
Cyclone Season Outlook
Tropical Cyclone Robyn marks the beginning of what is expected to be a slightly more active cyclone season in Australia, driven by warm ocean temperatures around the continent. For those living in cyclone-prone areas, now is the time to review your cyclone preparation plans and ensure you’re ready for the season ahead.
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