East Coast Swell Alert: September 30, 2014

30 Sep 2014 14 Share

Ben Macartney

Chief Surf Forecaster

Swell Alert issued 3.30pm, Tuesday, 30 September 2014.

A building trend in wave heights across the NSW coast through the second half of the working week culminates in a strong round of long period groundswell, arriving just in time for the the Labour Day long weekend.

Also See: Sydney Surf Forecast Chart

A period of big, clean surf looms for southern exposures on Saturday - and there should be still fun-sized leftovers throughout Sunday.

A period of big, clean surf looms for southern exposures on Saturday - and there should be still fun-sized leftovers throughout Sunday.

BASIC SUMMARY

- Strong pulse of S groundswell for the NSW coast, peaking on Saturday morning.

- South facing beaches to expect the brunt of inconsistent 5 to 6ft+ sets, easing to 4 to 6ft during the afternoon.

- Wide variations in surf heights across east facing beaches.

- A drop in size on Sunday, leftover SSE swell running at 2 to 4ft across the region.

OVERVIEW
If you were thinking of kicking off the long weekend with a tootle around on a stand-up-paddleboard or a quick leisurely swim in the ocean before breakfast, think again. Come Saturday morning the NSW coast will be under the influence of a powerful, long interval south groundswell – characterised by big lulls that could easily catch the unwary off-guard. The origin of this episode is a deep polar low forecast to intensify deep below the continent over the next few days. The evolution of this system follows on the coattails of large low pressure system currently weakening as it moves slowly eastward below Victoria. Although this anteceding low is situated too far west to have an impact on our swell window, it leaves an energetic sea-state in its wake, providing our synoptic protagonist with a head-start on wave growth as it tracks deep below Tasmania on Wednesday.

As the low migrates below Tasmania it drives maximum seas and swell to peaks of 40ft over the Southern Ocean; the raw material from which a long S groundswell is born.

As the low migrates below Tasmania it drives maximum seas and swell to peaks of 40ft over the Southern Ocean; the raw material from which a long S groundswell is born.

The low is projected to stall and further intensify deep below the Tasman Sea later this week.

The low is projected to stall and further intensify deep below the Tasman Sea later this week.

Along with this initial head-start in wave generation, this impending storm system fulfils much the criteria required to produce a large, long interval S groundswell for the Eastern Seaboard. The low is forecast to support a long, 35 to 55 knot WSW fetch across latitudes bounded by 50S to 60S as it migrates slowly below Victoria and Tasmania on Wednesday and Thursday. Although wind speeds should ease in strength as the system approaches New Zealand longitudes on Thursday and Friday, it’s forecast to stall over the Southern Ocean, roughly 1,500 nautical miles south-southeast of Sydney – thereby sustaining the duration of the fetch out to several days. These combined parameters are projected to drive maximum significant wave height into the 40ft range deep below the Tasman Sea on Thursday – in turn spawning a long period south groundswell spreading up along the NSW coast – starting late Friday.

The leading edge of the groundswell is forecast to start arriving at some of the longest peak intervals you’re likely to see along the Eastern Seaboard; up to 18 to 20 seconds as it begins to make landfall along the southern half of the NSW coast late Friday. This is likely to see a rapid transformation in conditions – particularly across the South Coast during Friday afternoon, with large 4 to 6ft sets likely to materialise anytime from 4pm onwards. Likewise Sydney’s exposed and south facing beaches should see a significant spike in wave energy as inconsistent sets begin to arrive on Friday evening, preceding the bulk of swell moving in early on Saturday morning.

As is typical of long interval S groundswells it will inevitably result in a wide range of wave heights depending on a given location’s exposure the southerly swell direction – but also upon bathymetric modification and focussing of the swell. Indeed these variances are likely be exacerbated by the long wave periods associated with the groundswell; producing surf as large as 8 to 12ft plus at select deepwater reefs, while only amounting to 5 to 6ft plus across south facing beaches while ranging anywhere from 3 to 6ft across the majority of east facing breaks. As is typical of such refracted groundswells the swell is likely to be sporadic; characterised by sudden pulses punctuated by long lulls between the larger sets. The peak of the swell coincides with early light N winds that will gradually pick up out of the NE during the day, so south facing breaks and sheltered northern corners will be the places to head.

The swell should back off considerably into Saturday afternoon and a steep decline should follow overnight. The tail end is likely to be down to 3 to 4ft across south facing beaches by Sunday morning. These projections remain contingent on how the storm evolves over the next few days so stay tuned for updates later in the week.

This forecast image depicts peak wave period; showing the leading edge of the groundswell arriving along the NSW coast at 18 to 20 second intervals late Friday, leading in the bulk of swell hitting early Saturday.

This forecast image depicts peak wave period; showing the leading edge of the groundswell arriving along the NSW coast at 18 to 20 second intervals late Friday, leading in the bulk of swell hitting early Saturday.


Tags: swell alert , surf forecast (create Alert from these tags)

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