Forecast: Rare north-west cyclone swell inbound for West Oz
There’s a couple of fundamental rules in surf forecasting:
1) be cautious with long range model guidance,
2) be extra cautious whenever there’s a tropical cyclone in the region (they’re prone to tipping the models way off balance).
And last of all...
3) be extra, extra cautious if you’re looking at a combination of points 1) and 2), especially if the models are steering said tropical cyclone away from the standard climatic tracks.
Every year we see many tropical cyclones develop across the northern part of Australia, as well as the Indian Ocean, Coral Sea and South Pacific basins.
Most go unnoticed by surfers, because they rarely influence our surf outlook. This is especially the case in Western Australia, where tropical cyclones form in the state’s far north-west, well away from major surfing regions, usually pushing away from the coast in an unfavourable direction for Australian swell production.
However, the waters off Western Australia usually account for the greatest percentage of regional Tropical Cyclones per year: this season, the Bureau of Meteorology predicted 12 Tropical Cyclones would form across the Western region and Northwestern sub-region, compared to 4 Tropical Cyclones in the Eastern Region (Coral Sea) and 3 Tropical Cyclones in the Northern Region.
So it is with great interest (and trepidation) over the last few days that we’ve watched Tropical Cyclone Marcus develop in the Arafura Sea, move through the Van Diemen Gulf and across Darwin.
The early stages of this cyclone were of little interest to most surfers, excluding a small bunch of hardy Darwinites willing to brave the croc-infested waters around Nightcliffe Beach.
TC Marcus is expected to continue on a W/SW track for the next few days, and once it eventually moves out into more open waters it’ll intensify - and could reach Category 4 or 5 by the middle of the week. But again, all of this will occur off the Kimberley Coast, tracking outside our primary swell windows.
Some time around Wednesday is where things start to get much more interesting. As TC Marcus starts to push west of WA longitudes, it’ll encounter a large high pressure system in the Southern Indian Ocean. Most of the models are in agreement that this will recurve TC Marcus to the south, though there are some differing scenarios on whether it be more S then S/SE (towards the SW WA coast), or S then S/SW (further into the Indian Ocean).
The models also differ on whether TC Marcus will main intensity throughout this southern track.
But that’s not the most crucial factor regarding our surf prospects, because a southward-tracking Tropical Cyclone pushing against a ridge to the south-west will ordinarily strengthen SE winds on its southern flank, aimed away from the WA coast - with little surf prospects as a result.
What we need is a secondary continental ridge to the east, helping to maintain a strong N’ly fetch about its eastern flank.
And incredibly, this is what the models are suggesting may very well happen. Storm to hurricane force N’ly winds are currently modeled to maintain strength as TC Marcus pushes south, and in doing so should generate a long period NW swell that’s expected to push down the WA coast later next week and into the weekend. This scenario is not too dissimilar to Severe Tropical Cyclone Bianca, which delivered incredible waves across many parts of the West Oz coastline in 2011. Remember this?
At this stage we’ll see N’ly swells building across Gnaraloo later Thursday, tending NW into Friday and reaching a peak on Saturday. It’ll be delayed by almost half a day further south at Kalbarri and Geraldton, building Friday and peaking later Saturday and overnight into Sunday.
Across southern regions, the new NW swell will possibly make landfall late Friday afternoon in Perth and Mandurah, building Saturday and peaking Sunday. Margaret River will see the new swell build Saturday and peak Sunday.
Some of the forecast data is pretty impressive - Margaret River (being located the farthest from the primary swell source) is likely to see the longest swell periods, which could reach 19 seconds on Saturday as the leading edge arrives. It’s difficult to get an estimation on potential surf size, as cyclones are very hard to pin down at long range - but some exposed coasts (particularly those further north, closer to the swell source) could be looking at surf size in excess of 6-8ft.
In fact, the raw data alone suggests much more than this however we need more time to assess the mid-week track of TC Marcus, to have any confidence in what might eventuate. And local winds will play a key role in whether any part of this swell is surfable. If TC Marcus pushes close to the mainland we may see periods of onshore winds, however it remains well offshore we’re more likely to see gusty offshores.
And remember: NW groundswells are very rare events in Western Australia, and thus result in unusual surf conditions just about everywhere. Reefbreaks that are usually slow and fat can light up with lengthy hollow sections, and lonely beachbreaks normally sheltered from large south-west swells can fire with perfect A-frames if their alignment and bathymetry is bang on with the incoming energy. These kinds of swells are the events to go searching for new waves across novelty coasts.
One final note in predicted surf size: our ‘surf height’ forecast model for WA locations has been fine-tuned based on climatic swell conditions (i.e. predominant swells from the S thru’ W), and is therefore under-calling surf size for this event. As such, keep an eye on Craig’s detailed Western Australian Forecaster Notes throughout the week for a more precise update.