East Coast Swell Alert: Wednesday, 16 August 2017

16 Aug 2017 1

Ben Macartney

Chief Surf Forecaster

Swell Alert issued by Ben Macartney 12 Noon Wednesday 16th August 2017.

Just when it started to look like spring had arrived early, winter looks like making a big comeback this weekend. To date we haven’t seen much in the way of classic winter southerly storm-swells this season, so it’s fair to say we’re probably overdue. If latest model runs prove accurate, a late resurgence in the Southern Ocean storm-track will drive a large, stormy S swell-event to unruly levels; big enough to send most of us scurrying for the shelter of a southern corner, while rendering conditions untenable across the majority of exposed open breaks. The good news is conditions are set to improve as the tail end of this event settles to more manageable levels early next week- and with any luck this will coincide with brand new sand-banks, making for great surf state-wide.

WATCH THIS SWELL on the Coastalwatch Wave Tracker

Just when you thought winter was over...Photo: Dan Bielich.

A large, complex low pressure system moving into the southern Tasman Sea on Friday combines with a strong 1030hPa high developing over Victoria to set up a near-gale force southerly fetch inside our swell window.
A steep rise in S swell follows across Sydney and the South Coast on Saturday; building to 6 to 8ft plus across south facing breaks during the day, with chances for a larger 8 to 10ft plus peak during the afternoon. Strong southerly winds and heavy swell limit rideable options to southern corners.
Large SSE swell is likely to persist on Sunday as moderate SW to SE winds gradually ease in strength, making for improving conditions throughout the day.
The bulk of S swell hits northern NSW at similarly heavy levels on Sunday, while wrapping in across southern Queensland at a more manageable size throughout the day.
A slow easing trend in SSE swell follows on Monday and Tuesday, opening up good to excellent conditions across exposed beaches under lighter NW winds.
Revisions to forecast surf heights and the projected timing of its arrival may change over the coming days depending on how the low develops compared to forecast guidance.

Overview
This impending event is linked to a large, complex low pressure system currently moving slowly eastward below the mainland. Wednesday morning saw the early effects of the low as an associated cold front crossing the NSW coast led in a gusty, 20 to 30 knot westerly airflow. The front precedes the passage of the low pressure centre moving below Tasmania on Thursday and out into the southern Tasman Sea on Friday. Unlike the majority of frontal systems moving clear of Tasmania, this one isn’t expected to generate the usual strengthening WSW/ SW fetch out of eastern Bass Strait. Instead, winds will remain straight westerly across its upper flank – and as a result we’re still likely to be looking at near-flat conditions on Thursday and into Friday morning.

The early stages of the low's development on Thursday won't offer much in the way of new S swell on Friday. Source: BOM.

The good new is wind-vectors are widely forecast to turn in our favour as the low moves out across the far southern Tasman Sea during Friday; turning WSW across the low’s upper flank and SSW at 30 to 35 knots off Tasmania, across the low’s western flank. This should occur just in time to nudge directional S swell up into the 1 to 2ft across Sydney’s south facing breaks on Friday arvo, with scope for larger 2 to 3ft sets materialising late in the day.

However Friday afternoon pans out, there’s now little doubt we’re looking at a large to extra-large S swell on Saturday. The low pressure centre is forecast to track north-eastward, towards New Zealand on Saturday – and this equatorward movement will drag the broad southerly fetch area up into the southern Tasman Sea; thereby moving the strongest swell-producing winds in the same direction and at the same speed the with the swell being produced. This phenomenon, known as a ‘captured fetch’, can dramatically compound the size of the swell and often results in surf heights arriving well above forecast levels.

A heavy spike in S storm-swell is currently projected to peak across Sydney, Newcastle and the South Coast during Saturday. This may change depending on how the low evolves compared to forecast guidance. Source: Wave Tracker.

Hence, this is shaping up as a heavy S swell; probably reaching 6 to 8ft plus across south facing beaches during the day, with scope for larger sets during the afternoon. Initially only sheltered southern corners will be making sense of the swell as strong southerly quarter winds write off exposed breaks on Saturday. By Sunday morning expect still large surf along with a SSE slant in swell direction. This will coincide with early WSW winds turning light S to SE before dying off into the afternoon, so expect improving conditions across exposed breaks all day.

If all this sounds a bit out of your league, Monday morning looks like a great opportunity to target an exposed beach. The last of the swell should still be running at a solid 3 to 4ft or more under early NW winds and conditions should remain good as the swell slowly eases throughout the day. Stay tuned for updates on this event as the models better capture its development later in the week.