Issued Wednesday, November 29th 2017. It looks like a rare event is expected to take place in order to give Australia its first cyclone for Season 2017-18. Above image via WindyTV showing the Fujiwhara effect which is being forecast for Friday.
Tropical Cyclone Cempaka (soon to be Ex-TC Cempaka) and Invest 96S are currently situated over Indonesian waters North-East and North-West of Christmas Island respectively. These storms are expected to linger over this region for the next 24hrs before drifting into Ausrtalian waters later on Thursday. Its during this time, and across Friday, that the 2 systems are expected to near each day and enter what is called the Fujiwhara effect.
The simplest way of explaining the Fujiwhara effect is an effect or occurrence named by Sakuhei Fujiwhara (a meteorologist in Japan who discovered the occurrence) where 2 low pressure regions or cyclones near each other, close the gap between each other and the orbit around each other. The two systems will orbit around a centre point which is approximately half way between the 2 centres of the 2 systems in a clockwise direction. The larger or stronger system of the 2 will dominate the interaction and become the parent low / cyclone.. causing the other to weaken or become part of the stronger systems overall rotation.
Fujiwhara effect and how the systems interact with each other
This effect is expected to take place as stated above during late Thursday and across Friday. By late Friday, the dominating system (which is expected to be the current Invest 96S) is forecast to take over and a single rotation is expected to reach cyclone strength over Australian waters. Its a fine line at this stage for what the system may be named as it is expected to become a cyclone prior to the Fujiwhara influence which would be over Indonesian waters (thus named Dahlia), but if its a low and then is named by Australia it will be named Hilda.
From there, the system is expected to maintain cyclone strength and track South to South-East towards the WA Coast where it may reach as high as Category 3 strength. Impacts on the WA Coast are hard to determine this far out, but the system “should” impact the Coast to some degree early next week.