Tropical Cyclones Seroja and Odette – The Fujiwhara Effect – 10/4/2021

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There are two tropical cyclones off the north west coast of Western Australia with Tropical Cyclone Seroja being the stronger of the two while the smaller weaker storm has been named Odette.

Both storms now appear to be circling one another and it appears that the smaller and weaker storm is poised to merge with the larger stronger storm - Seroja.

This is a very rare event to see within Australian waters.

According to Wikipedia, it was Sakuhei Fujiwhara who described the effects of two tropical cyclones being close to one another in 1921. The rare phenomena is termed the “Fujiwhara Effect” described as when 2 nearby cyclonic vortices start to move around each other, then close their distance between their corresponding low pressure areas. This can cause the two circulations to merge.

The storms engage in binary circulations when they are less than 1,400 km apart. The two vortices will be attracted to each other and spiral into the centre point then merge. If the two storms have unequal strength, the larger storm will dominate over the weaker storm.

This is what appears to be occurring as Odette closes the distance with Seroja. Tropical Cyclone Seroja is expected to dominate the system. However, this makes forecasting challenging and difficult.

It is noted that Tropical Cyclone Seroja may not reach a category 3 storm when compared to earlier models but is still expected to reach a strength towards the higher end of Category 1 or lower end of Category 2 during its final life span with peak wind gusts of 80 knots or approximately 150 km / hour. The storms have stayed offshore but a 1,000 km stretch of the western Australia coast is at risk from the storm as landfall is forecast between Coral Bay and Jurien Bay and there is a threat to Carnarvon, Denham, Kalbarri and even Geraldton.

As seen in the Worldview image (NASA) taken the 9 April 2021, there are three storms in close proximity of one another however one has since decayed leaving Odette and Seroja as the remaining systems.

Whatever occurs now, Tropical Cyclone Seroja will not last too much longer because it will either make landfall or if not, it will decay over open ocean because ocean waters are significantly colder further south of its position and too cold to sustain a tropical cyclone.

Two tropical storms off NW Western Australia – A rare event – 7 April 2021

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Rarely seen in Australian waters, there are two tropical storms within close proximity of one another. The southernmost storm is the weaker of the two and is named Tropical Storm Twenty Seven while the northernmost storm is Tropical Cyclone Seroja.

It appears that both storms will come close enough to one another to influence each other’s wind fields and it is possible that both storms may start to rotate around each other. At the present time, both storms will remain offshore but because there are two storms, forecasting what they do will be more difficult than usual.

It appears that both will remain offshore but there is still a risk that one being Tropical Cyclone Seroja could approach the Pilbara Coast or Gascoyne over coming days.

Tropical Cyclone Seroja is still expected to reach peak intensity as a Category three storm in coming days but once it starts to cross over colder waters close to the Tropic of Capricorn, this storm will commence its weakening phase.

Having two such storms within close proximity to one another within Australian water is a rare event and forecasting what they do will become a challenge in coming days.

Tropical Cyclone Seroja – NW Western Australia – 5 to 9 April 2021

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The EOSDIS Worldview image (NASA 5/4/2021) is showing two tropical storms to the south of Indonesia as follows:-

1 - At the south west corner of the satellite picture - Tropical Storm Twenty Seven which may briefly reach tropical cyclone status but given location, it does not pose any threat to any land mass or population centre. This storm should decay over open ocean.

2 - Tropical Storm Seroja which is the stronger of the two storms and situated at the north east corner of the picture. This storm is south west of East Timor and weather models clearly show this storm developing into a significant tropical cyclone over coming days. The CIMSS model of the 5/4/2021 suggests that this storm may reach Category Three on the Saffir Simpson Scale.

Tropical Storm Seroja is expected to track to the south west but remaining off the coast of Western Australia. After 96 hours, the storm is forecast to track more to the south. This storm is within an environment where ocean temperatures are within the range of 30C to 31C which would favour a strong storm of Category three strength or greater.

CIMSS models suggest a storm with maximum winds at the core reaching 105 knots or 195 km/h within 96 hours.  The Model as shown would suggest a threat to the north west coast of Western Australia within the next 96 hours as the storm tracks more to the south.

Over the next four days, the storm will impact shipping lanes and flights between Australia and Indonesia but not impact any population centre.

The storm will be watched to ascertain what occurs over coming days especially at it approaches the north west coast of Western Australia.