East Coast Swell Alert: Tropical Cyclone Tuni

1 Dec 2015 1

Ben Macartney

Chief Surf Forecaster

Issued Tuesday, 1 December 2015.

Sure, it’s not the kind of easterly swell-event that requires jet-skis and big-wave boards to access, but it’s the first tropical cyclone swell event of the 2015/16 season and it’s already in the water -  inexorably closing the distance on Australia’s East Coast.

As far as easterly swell sources go, Tropical Cyclone Tuni is about as remote and innocuous as you’re likely to see; being located over 2,000 nautical miles offshore – and that’s a distance on par with southern Indian Ocean swell sources that deliver SW swells to Indonesia. Although a significant dilution of both swell-size and consistency inevitably follows as a result of wave erosion and dispersion, it’s still shaping up as an excellent run of mid-sized easterly groundswell to kick off the start of summer across the entire Eastern Seaboard. Even better, local winds are set to swing in our favour just in the nick of time.

Track This Swell On The Coastalwatch Wave Tracker

The prospect of mid-sized easterly groundswell and light offshore winds is always appealing and its about to become a reality across the East Coast this weekend. Photo: Randy Sparrow.

The prospect of mid-sized easterly groundswell and light offshore winds is always appealing and its about to become a reality across the East Coast this weekend. Photo: Randy Sparrow.

  • Category 1 Tropical Cyclone Tuni formed over the Southwest Pacific on Sunday and tracked southeast below Samoa on Monday before being downgraded to a tropical depression by Tuesday morning...
  • A large, slow moving high pressure system cradling TC Tuni from the south has supported a broad easterly tradewind belt spanning much of the Southwest and south-central Pacific Ocean over the past week.
  • A resulting mid-period easterly swell begins to build in across the East Coast from Wednesday onwards.
  • Expect the bulk of swell to show across southern Queensland and far northern NSW coasts at 3 to 5ft between Thursday and Sunday under a predominant SSE to SE airflow.
  • The leading edge of the groundswell arrives at low-levels across Sydney and the South Coast on Friday ahead of the bulk of swell arriving this weekend at about 3 to 4ft under early light offshore winds tending NE during the afternoons.
  • A slow easing trend in E swell follows early to mid next week.

If you’ve been scouring latest Mean Sea Level Pressure charts from the Bureau of Meteorology (BOM) wondering where Tropical Cyclone Tuni might be hiding, you’d be hard pressed to find it. The BOM’s sphere of influence generally only extends as far east as the 160E meridian – and hence swell sources forming out near Fiji or further east tends to fly under the radar.

As mentioned in the Tropical Cyclone 2015/ 2016 Seasonal Outlook, the strong El Nino currently in effect over the tropical Pacific Ocean means there’s a higher probability we’ll see more tropical cyclone activity (and hence swell sources) taking place deeper in the Pacific - specifically near and east of the dateline. That’s precisely what we’ve seen this week with the formation of Tropical Cyclone Tuni.

The system commenced its lifecycle as a slow moving tropical depression meandering slowly across the South Pacific Ocean in a region 450nm north of Fiji between 24 and 27 November. The system gradually intensified as it adopted an east to south-eastward track last Thursday and Friday, and it continued on a southeast trajectory with further intensification on Saturday. The storm was subsequently named Tropical Cyclone Tuni (TC Tuni) as it moved below Samoa early on Sunday 29 November. The system maintained category 1 strength as it continued on its southeastward track on Monday before being downgraded to a tropical depression during the early hours of Tuesday morning.

Monday's MSLP chart from Fiji's Meteorological Service depicts ex TC Tuni tracking south-eastward, deeper into the south-central Pacific Ocean. The broad pressure gradient south of the cyclone represents a vast easterly fetch aimed at Australia's East Coast. Source: Met Fiji.

Monday's MSLP chart from Fiji's Meteorological Service depicts ex TC Tuni tracking south-eastward, deeper into the south-central Pacific Ocean. The broad pressure gradient south of the cyclone represents a vast easterly fetch aimed at Australia's East Coast. Source: Met Fiji.

This Wave Tracker image depicts significant wave height, showing the bulk of swell associated with TC Tuni located well east of Fiji. The swell undergoes ignificant erosion and dispersion as it covers the distance to the Eastern Seaboard over the next few days.

This Wave Tracker image depicts significant wave height, showing the bulk of swell associated with TC Tuni located well east of Fiji. The swell undergoes ignificant erosion and dispersion as it covers the distance to the Eastern Seaboard over the next few days.

Tropical cyclones are catagorised by their highest sustained wind-strength and category 1 is the minimum - denoting gale to strong gale force winds of 30 to 50 knots. TC Tuni only briefly supported these thresholds before weakening and transitioning into an extratropical low on Tuesday morning. Given ex-TC Tuni is now positioned just of 2,000 nautical miles east of Australia’s eastern seaboard, it’s fair to say it doesn’t amount to a significant source of swell in its own right. However, the cyclone’s formation coincided with the evolution of a huge, stable high pressure system over the South Pacific that’s been cradling TC Tuni’s slow development since November 24.

Over the last five days an expanding pressure gradient squeezed between the high and the cyclone gave rise to a vast fetch area encompassing much of the Southwest Pacific. Although core wind speeds associated with TC Tuni only peaking around 35 to 50 knots within a confined surface area near the cyclone’s eye, it continues to work in conjunction with the high to the south to support a broader, 20 to 30 knot easterly fetch extending some 1,500 nautical miles across the South Pacific.

This Wave Tracker image depicts peak wave period, showing the leading edge of the groundswell arriving at peak intervals of13 to 14 seconds on Friday morning ahead of the bulk of swell filling in during Friday afternoon and over the weekend.

This Wave Tracker image depicts peak wave period, showing the leading edge of the groundswell arriving at peak intervals of13 to 14 seconds on Friday morning ahead of the bulk of swell filling in during Friday afternoon and over the weekend.

It’s the impressive length and duration of this moderate-strength fetch  responsible for the mid period easterly groundswell inbound across the region later this week. The slow evolution of the system also translates into a gradual unfolding of the swell; slowly picking up in size into the end of the week ahead of an equally slow decline in wave energy early to mid next week.

The bulk of E groundswell associated with TC Tuni is on track to fill in from Thursday to Saturday, producing solid surf ranging from 3 to 5ft plus across the more exposed open breaks across southern Queensland and far northern NSW. Although the vast distance travelled by the groundswell will translate into long waits between the larger sets, conditions are looking excellent for Queensland’s points through the height of this episode. The arrival of a vigorous southerly change on Wednesday night occurs just in time for the peak of the swell setting in on Thursday and Friday – maintain strong SSE winds well suited to the point waves on both days.

FOR A DETAILED ANALYSIS OF CONDITIONS FOR SOUTHERN QUEENSLAND AND FAR NORTHERN NSW CLICK HERE

Although the southern half of the NSW coast lies a little out of the direct path of the swell, the region will also benefit from several days of mid-sized ENE swell – producing inconsistent sets in the 3 to 4ft plus across the most exposed east facing beaches as the swell reaches a peak between Friday afternoon and Sunday morning. Likewise, local conditions come to the party just in time. A new high-pressure system building in across the NSW coast on Thursday leads in a pattern of early light winds tending moderate NE – producing clean conditions through the morning hours from Friday to Sunday. By Sunday, the tail end of the E swell will have settled a notch or two and going on long-range model guidance the downward trend in E swell will continue unabated early to mid next week. 

FOR A DETAILED ANALYSIS OF CONDITIONS FOR SYDNEY AND THE SOUTH COAST CLICK HERE

Ex-TC Tuni is forecast to continue moving away to the southeast as it gradually weakens. The low is expected to substantially dissipate over the central Pacific this weekend. Source: Met Fiji.

Ex-TC Tuni is forecast to continue moving away to the southeast as it gradually weakens. The low is expected to substantially dissipate over the central Pacific this weekend. Source: Met Fiji.

Tags: swell , alert , queensland , tropical , cyclone (create Alert from these tags)

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