Forecaster Blog: Is a major E swell for the East Coast on the long term horizon?

8 Nov 2017 0 Share

Ben Macartney

Chief Surf Forecaster

COASTALWATCH | FORECASTER BLOG

Issued Wednesday, 8 November 2017

Since we emerged from winter, the state of the climate encompassing the tropical Pacific Ocean has progressively slipped from an ENSO neutral classification to a weak La Nina pattern. These cryptic terms are used by meteorological agencies around the globe in reference to how things like sea surface temperatures, air pressure, cloudiness and tradewinds are behaving across the low latitudes. These ENSO conditions are also the key indicators used to assess tropical cyclone potential for a multitude of Pacific nations – Australia included. The point is, a weak La Nina lends a little bit of weight to slightly above average tropical cyclone numbers inside Australian longitudes – and it can also be associated with an earlier than normal start to the season.

A mid to large E swell into mid to late November is starting to look like a distinct possibility. Photo: Uploaded to CW by Dkimber.

A mid to large E swell into mid to late November is starting to look like a distinct possibility. Photo: Uploaded to CW by Dkimber.

And almost on cue, recent long-range computer modelled forecasts are picking up the development of a deep tropical low/ cyclone; speculatively developing somewhere between New Caledonia and Fiji mid to late next week. For now, the key GFS and ECMF models are both showing impressive E swell potential; albeit in very different guises. The latter, EC prognosis is the most forthright; indicating a the low will move south on Thursday before rapidly intensifying over the Southwest Pacific; setting up north of NZ, roughly 1,000 nautical miles east of the southern Queensland coast. The low is forecast to generate a broad, 20 to 35kt SE/ESE fetch in conjunction with a high to the south – in turn giving rise to a robust ESE swell, aimed most directly at southern Queensland and northern NSW coasts.

This would see a solid round of midperiod ESE inbound; possibly starting to build as early as the weekend of Saturday 18th November, ahead of the bulk of energy arriving around Monday 20th and Tuesday 22nd; speculatively peaking in the 3 to 5ft range.  However, at this early stage there’s clearly plenty of elasticity in projected surf-heights and arrival times. Corresponding GFS runs are lagging on the low’s development; indicating it will develop more gradually, just south of Noumea on Friday 17th before deepening a little as it moves south. Again, this would see a broad easterly fetch of 20 to 30 knots developing throughout our easterly swell window on Friday 17th.

Long term GFS runs show the forecast location of the low on Sunday, 19 November, clearly favouring the NSW coast. Source: NOAA.

Long term GFS runs show the forecast location of the low on Sunday, 19 November, clearly favouring the NSW coast. Source: NOAA.

The key implication of recent GFS runs is a more rapid south to southwestward track taking the low into the northern and central Tasman Sea over the weekend of Saturday 18th. This scenario could see larger E swell impacting the mid and southern NSW coast, rather than locations further north – so at this early stage it’s fair to say E swell potential   is poorly defined. On the upside, the models can be viewed as showing strong agreement given the extended lead time – and from this you can infer that some kind of E swell event is likely for the East Coast during the week of Monday 20th. We’ll be monitoring the models closely over the coming days so stay tuned for updates in the detailed forecast.

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