Tropical Cyclone Kirrily is not your standard Australian cyclone, and that will have a major impact on the weather experienced during and after landfall.
The most unusual feature of the storm is its irregular shape – or more specifically its distinguished cloud pattern where the majority of the convective cloud and rain is positioned to the north of the eye.
This is the reverse of the typical pattern for Queensland cyclones and will result in a heightened flood risk for regions north from Townsville and a lower risk across areas to the south.
Why Kirrily’s rain and flooding won’t follow a normal pattern
In the Southern Hemisphere winds spin clockwise around tropical cyclones, meaning when they hit Queensland’s east coast they produce easterly winds to the south and westerly winds to the north.
This is clear when observing the wind field around Kirrily below – onshore winds below the eye and offshore winds on the northern flank.
Cyclones impacting eastern facing coasts will therefore normally produce heavier rain to the south where the winds are blowing off a moist tropical sea, and less rain on the northern flank where winds are drier, blowing offshore from land to sea.
Kirrily has not followed this typical trend and satellite imagery overnight and through Thursday has clearly showed the majority of cloud development is north of the centre.
A result of the atypical cloud configuration is on impact the heaviest rain from Kirrily will fall north from about Townsville and will rapidly taper off down the coast towards Mackay.
This trend will continue as the system decays over the inland and lead to heavy rain spreading to north-west Queensland through Friday while lighter rain falls over the state’s central districts.
So how much rain will fall? Thankfully we won’t see a repeat of Jasper’s record flooding because Kirrily is accelerating, and this mobility will ensure heavy rain quickly moves to western Queensland from Friday rather than lingering for days near the coastline.
Despite its fast movement the storm will still drop hundreds of millimetres near the storm’s path including the risk of some major flooding.
One model’s rain forecast from Kirrily through the next 5 days is below and it shows a band of 100 to 300 millimetres from the coast to the NT.
Will the strongest winds and storm surge also be north?
While Kirrily’s rain is displaced north, the risk of destructive winds and a storm surge is most certainly still in the conventional area.
Cyclones in the Southern Heliosphere typically produce higher wind speeds and the largest storm surge in the front left quadrant (south of the eye for cyclones moving west) because winds in this region are travelling in the same direction as the system itself – much like how a ball thrown from a moving vehicle will travel faster if directed forward.
So while areas south of Townsville may be spared from the storm’s most intense rain, the threat from wind is still substantial.