Forecaster Blog: Tropical Cyclone Forming Near Vanuatu Fuels Rise in SE, ESE Swell For Southern QLD This Weekend
Updated Wednesday, February 13, 2019
It’s been a pretty slow start to the tropical-swell season, but at long last there’s every reason to think we’re about to enjoy a more substantial run of swell under favourable SSE winds across southern Queensland’s points this weekend.
Category one Tropical Cyclone Oma, located about 235 nautical miles northwest of Vanuatu, has emerged as the focal point of an active Monsoon trough spanning the Southwest Pacific Ocean. As discussed in my recent blog, the cyclone looms as a major driver of energetic SE to ESE swell as it interacts with a building subtropical ridge to its southwest over the next few days. Over the last 48 hours we’ve seen the various models tightening up on the projected strength and track of TC Oma leading into the end of this week, this weekend and through to early next week – and this in turn is lending increasing confidence to projected surf emanating from the system.
But just how is this event likely to stack up, compared to say, the E groundswell generated by Tropical Cyclone Winston, back in 2015? Or even TC Gita in 2018?
The early stages of this event commenced with the arrival of a southerly change across the Eastern Seaboard on Wednesday, leading in a strengthening sub-tropical ridge that’s projected to build over the Tasman Sea into the end of the week - in turn setting up a broadening SSE to SE fetch inside northern NSW and southern Queensland’s short-range swell window. In the first instance that means rising SSE/SE windswell for these more northern venues on Friday, characterised by a steady build in wave height and lengthening of wave period.
As TC Oma further strengthens it’s forecast to curve south and then southwest, on a trajectory that will take it directly across New Caledonia this weekend. As this occurs, a broad SE fetch will go to work across the northern Tasman and southern Coral Sea; generating a larger component of ESE swell fanning out towards the QLD/ northern NSW coast. Going on recent WW3 runs, this will culminate in larger, mid-period ESE swell building to peaks of 4 to 6ft plus across southern QLD and northern NSW coasts this weekend – perhaps even bigger as it reaches peak size later Saturday/ early Sunday.
At this early stage it’s worth keeping in mind that we could see changes to these projections in light of TC Oma’s development over the coming days – but however it pans out it now looks clear that the coming weekend will see no shortage of surf.
By early next week we should initially see a gradual easing trend in easterly swell across the region, levelling off in the vicinity of 3 to 5ft Monday/ Tuesday under moderating SE tradewinds. Beyond that, the models point to a range of scenario’s. The EC model has ex-TC Oma going extratropical as it moves poleward across the Tasman Sea from Tuesday to Friday next week. This could see a renewed push in sizeable, mid-period easterly swell setting in across the entire Eastern Seaboard – but at this stage confidence on this possibility is still at the low end of the scale.
Sowing the Seeds of Doubt
While this all, prima facie, looks somewhere between really good and straight up epic, there are a few key reasons to view these projections with a critical eye. In the wind-graph below, we can see how the coastal fringe is only exposed to the south-western boundary of the fetch area, with the bulk of sea-state being generated by the fetch aimed further north. That points to a more directional event, quite possibly of shorter peak periods than what current runs are showing; translating into good, but far from epic conditions for the points. To further elaborate, the stronger peak periods of 10 to 11 seconds showing for Sunday are linked to sustained SE winds of 30 to 35 knots over the northern Tasman Sea on Friday through early Saturday, triggering a strong component of ESE swell spreading radially off the primary, deepwater SE swell-train.
In short, there are competing scenarios indicating the fetch won’t quite live up to these projections – and hence we may well see projected wave height and period dropping back a notch or two as the week progresses. Having said all that, the vagaries of tropical cyclone development means there’s still plenty of wiggle room for revisions to the projected swell and the timing of its arrival, so stay tuned for more on how it will play out later in the week.
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