Forecaster Blog: Tropical Cyclone Yasa and its distant cousin, Tropical Cyclone Zazu

15 Dec 2020 0 Share

Ben Macartney

Chief Surf Forecaster

For the latest update on TC Yasu's emerging east-swell potential, click here for the detailed forecast.

The abrupt onset of our easterly swell-season over the past few days probably hasn’t exactly produced the surfing goods you might have been dreaming of when the deepening trough appeared on the forecast charts earlier this month.

Instead, we’ve seen heavy, way-out-of-control, storm-swell hammering southern Queensland and northern NSW coasts over the last three days, while spreading further south to produce consistent, mid to large ENE swell, with persistent onshore winds keeping a lid on quality all the while.


Will TC Yasa deliver in time for Christmas? Time will tell. Photo: CW.

Will TC Yasa deliver in time for Christmas? Time will tell. Photo: CW.

It’s hardly amounted to an epic start to the 2020/21 tropical swell season, but there’s plenty of light on our long-range east-swell horizon leading up to the Christmas period.


In tandem with monsoonal onset across Australasian longitudes, we’ve seen an extensive monsoon trough set up across the tropical Pacific Ocean in the past week, spawning two tropical cyclones situated to the east and west of Fiji.

Two tropical cyclones to the east and west of Fiji highlight a peak in tropical activity over the Southwest Pacific for the month of December. Source: RSMC Nadi.

Two tropical cyclones to the east and west of Fiji highlight a peak in tropical activity over the Southwest Pacific for the month of December. Source: RSMC Nadi.

The former and more remote, category one Tropical Cyclone Zazu is unlikely to have a notable impact on our swell window. The storm will briefly contribute to a long belt of ESE tradewinds, tending deep into the Southwest Pacific, well east of the dateline. The system is, however, widely forecast to track away to the southeast and weaken over the next 48 hours, thereby spelling the end of its influence.

TC Zazu is forecast to adopt a southeastward track, taking it quickly out of effective swell-producing range of the East Coast. Source: RSMC Nadi.

TC Zazu is forecast to adopt a southeastward track, taking it quickly out of effective swell-producing range of the East Coast. Source: RSMC Nadi.

Tropical Cyclone Yasa: The category three cyclone is quickly emerging as a likely source of long-range easterly groundswell for the East Coast, but the arrival of said swell is still a good week away. TC Yasa is currently situated in a complex steering environment; one that should see it tracking eastward on a collision course with Fiji on Friday.

TC Yasa, on the other hand, is projected to move south, into our east-swell window this weekend. Source: RSMC Nadi.

TC Yasa, on the other hand, is projected to move south, into our east-swell window this weekend. Source: RSMC Nadi.

TC Yasa is then forecast to curve south this weekend – and it’s at this point we may see wave-growth accelerating across our long-range swell window. The storm is projected to set up a broad easterly fetch in conjunction with the subtropical ridge to the south, mostly exhibiting speeds of 20 to 35 knots. Within that fetch are stronger core winds surrounding TC Yasa’s eye-wall, peaking at sustained speeds of 100 knots and gusts reaching 130 knots later this week. Those core winds are, however, confined to a relatively small surface area about 80 to 200 nautical miles in length.

Fiji stands to cop a hiding at TC Yasa approaches from the west later this week, bringing core winds of 100 to 130 knots to the island nation. Source: Surfline.

Fiji stands to cop a hiding at TC Yasa approaches from the west later this week, bringing core winds of 100 to 130 knots to the island nation. Source: Surfline.

Given the vast distance of TC Yasa from the Eastern Seaboard, it’s the broader ESE tradewind flow we’re eyeing as a notable source of east swell. So although the models are already picking up a leading, longer-period east pulse arriving off the cyclone on Monday, it probably won’t be until Tuesday 22nd or Wednesday 23rd that we really see the bulk of swell making landfall across the region.

 TC Yasa falls into range of the East Coast as it moves south of Fiji this weekend. Source: Wave Tracker.

TC Yasa falls into range of the East Coast as it moves south of Fiji this weekend. Source: Wave Tracker.

Of further interest is the later stages in TC Yasa’s lifecycle. Recent GFS model runs indicate the system will turn westward, on a trajectory bringing it incrementally closer to the East Coast over the course of next week. The big upside to this possibility is accelerated wave growth propagating towards the coast as the storm and it’s swell producing winds move with the swell being produced. That’s keeping the door open to another solid easterly pulse arriving right in time for Christmas, but for the time being confidence on these later stage developments is low, with major shifts in TC Yasa’s forecast track likely over the next few days.

If long-range model guidance is anything to go by, we could be in for a very merry Christmas, compliments of TC Yasa's late-stage developments. Source: Wave Tracker.

If long-range model guidance is anything to go by, we could be in for a very merry Christmas, compliments of TC Yasa's late-stage developments. Source: Wave Tracker.


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