Forecaster Blog: Tropical Cyclones Ava and Irving to Merge Over the Indian Ocean
COASTALWATCH | FORECASTER BLOG
Issued Tuesday, 9 January 2018
The convergence of two ex-tropical cyclones over a remote part of the southern Indian Ocean looms as the catalyst for a major swell-event for Australian coasts next week. This event is projected to impact both Western Australian and the Southern Australian coasts; taking the form of a high-energy SW groundswell arriving at long peak periods in excess of 20 seconds. The Tropical cyclones (TC’s) in question are TC Irving and the now ex TC Ava – both positioned over disparate parts of the Indian Ocean.
At face value, the development of a tropical cyclone usually holds fairly straightforward swell-potential. Along the Eastern Seaboard that usually means some kind of NE or E swell-event aimed at most directly at southern Queensland and northern NSW coasts. On the West Coast a cyclone can, once in a blue moon, produce in a freak NW swell; but otherwise the vast bulk of swell is easterly – and therefore aimed westward at Madagascar and Africa’s eastern coast.
What’s usually less-apparent is the latent swell-potential associated with the final stages in a tropical cyclone’s lifecycle; better known as extratropical transition. As TC’s move south into the mid latitudes (usually between 30S and 40S), they encounter the baroclinic zone, where increasing vertical wind-shear and lower sea surface temperatures invariably lead to a rapid weakening and even complete dissipation of the system within a few days. However, as an ex TC move into this environment they also encounter larger and much colder, drier air-masses – and if conditions are right the collision of the systems can culminate in the development of a much larger, supercharged extratropical (cold core) low.
That’s exactly what’s forecast to occur as the two ex TC’s merge into a deep, mid-latitude storm later this week. Latest model guidance exhibits strong agreement on this scenario; indicating the storm’s central pressure will bomb into the 930’s or 920’s just east of the Kerguelen Islands on Friday and Saturday.
A resulting, severe gale to storm-force westerly fetch generated by the storm is projected to drive deepwater seas and swell to peaks of 40 to 50 knots over the Southern Ocean later Friday through early Saturday; in turn spawning a long-period SW swell-event across Australian shores. Western Australia will be the groundswell’s first port of call; filling on Monday and Tuesday to produce large, powerful surf across the Southwest.
A smaller version of the groundswell is then projected to hit South Australia and Victoria on Wednesday and Thursday. Given the two storms are yet to merge as they undergo extratropical transition there’s still plenty of scope for changes to the projected size and timing of the swell, so stay tuned to the detailed forecasts and keep an eye out for further updates as the week progresses.
Winter keeps on keeping on this weekend.
Indonesia's Supersuck turning on in a big way for Luke Hynd, Andrew Mooney and Riley Laing
Grom on the rise
The waiting period has started
The waiting period has started
With five special QnA screenings with Nathan and Hollywood director Michael Oblowitz from this Sunday
Some of the best beach break waves in recent memory.
Check the highlights from a rad event
Matt Meola Lands This Crazy Air, Ross Clarke Jones Is Going to Be on Survivor, and TourNotes From Margies
This Week In Surfing: Ten Things From Surfing & the Internet on the Week That Was June 15, 2019
See the session that landed Ace a cover of Surfing World
A new round of Easterly swell could see good to great conditions across the NSW coast next week.
Forget the World Title
Bring on winter.