Forecaster Blog: Huge South Swell?

11 Oct 2016 0

Ben Macartney

Chief Surf Forecaster

COASTALWATCH | FORECASTER BLOG

Tuesday, 11 October 2016

Huge South Swell? Probably Not, But It Will Do Till One Gets Here.

If you religiously track the evolution of swell events from the moment they appear on long-range forecasts, you’re probably familiar with the feeling of being killed softly with each passing day.

The shortcomings inherent in such long-range projections tend to overstate the strength of weather systems – and that’s definitely been the trend this spring– particularly with the GFS model.GFS is the primary source of data that drives the majority of wave models you might check and hence the general trend is for each consecutive model run is to slightly downgrade the size and power of the swell with each passing day. So, by the time the swell’s arrival is nigh, it’s often gone from something like a 20ft storm swell to a typical 4ft day at your local beaches.

FOLLOW THE EVOLUTION OF THE SWELL ON THE WAVETRACKER

In light of the October average, this week's S swell looms large for the NSW coast. Photo: CW user Dan Bielich.

In light of the October average, this week's S swell looms large for the NSW coast. Photo: CW user Dan Bielich.

That almost sums up the S swell that’s about to hit the NSW coast later this week. What was originally shaping up as a large to heavy swell event peaking in the 6 to 8ft+ range came crashing back down to sea-level this week. The source is a low-pressure system forecast to develop along an active cold front tracking across Tasmania and Bass Strait over the next 24 hours. As the front sweeps up the southern half of the NSW coast on Wednesday evening it ushers in a 30kt southerly fetch throughout our mid to short range swell window in conjunction with a large high building in over Tasmania.

SEE ALSO: Bali, Indonesia Surf Forecast 12 - 21 October, 2016 

Latest Wave Tracker model runs display the deepwater SSW swell reaching maximum size southeast of Tasmania on Wednesday. A solid round of mid-period S swell follows later Thursday and throughout Friday.

Latest Wave Tracker model runs display the deepwater SSW swell reaching maximum size southeast of Tasmania on Wednesday. A solid round of mid-period S swell follows later Thursday and throughout Friday.

The inception of the fetch occurs east and south of Tasmania Wednesday before it extends north into the Tasman Sea on Thursday morning. Although the northward movement of the fetch area will exacerbate wave-growth, the strongest fetch area sets up on Wednesday before weakening as it elongates into the Tasman Sea on Thursday.

SEE ALSO: Your Local 7-Day Forecast

The upshot is a two-stage episode; commencing with a first push in S wind swell on Thursday morning, preceding the arrival of a stronger, mid-period S pulse. This should show throughout the afternoon; manifesting as a solid push in surf heights towards 4 to 6ft levels during and it should continue to run at a similar size throughout Friday morning before easing from there.

The downside is an accompanying, 20 to 30 knot southerly flow arriving with the rising swell on Thursday and this will invariably limit clean surfable options to southern corners as the swell builds, so get in earlier to make the most of a short-lived SSW bias in wind direction before it swings SSE during the afternoon. Even at this late-stage there’s still scope for some fine-tuning to the projected size and timing of the swell, so stay tuned to the detailed forecast for an update on Wednesday.

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