Issued by Ben Macartney, 4.25 pm AEST, Friday 16th April 2021

Brief Overview:

A strong round of long period S groundswell arrives under light winds this weekend.
A couple of smaller, reinforcing S pulses overlap on Monday 19th and Tuesday 20th.
Scope for another energetic spike in short-range S swell on Wednesday 21st.
Favourable, light offshore/cross-shore winds prevail over the coming week.

Saturday 17th: 4-6', occasional 7' faces AM, rising 6-8'+ faces PM. Light land-breezes tend moderate onshore and then ease throughout the afternoon.
SWELL/SURF: Long-period south groundswell reinforces an easing south windswell. Early wobbly conditions improving as winds fade to variable during the morning.
WIND: Early SW/WSW 5-10 knots, fading to variable in the morning, then giving way to varied E/NE sea breezes in the afternoon.

Sunday 18th: 5-8' faces AM, easing 4-6'+ PM. Clean conditions all morning become textured in the afternoon.
SWELL/SURF: Reinforcing, long-period S groundswell peaks in the morning and slowly eases.
WIND: Light offshore westerlies AM, tending to light, late N/NE sea breezes PM.

Monday 19th: 4-6' faces AM, easing 3-5' PM. Super clean in the morning and inconsistent set waves. Biggest early and slowly backing off during the afternoon.
SWELL/SURF: Small, reinforcing pulse of long-period S groundswell sees sporadic sets. Slight, short-period SSE windswell also in the mix.
WIND: Light WNW all morning, tending light to moderate N/NE in the afternoon.

Tuesday 20th: 4-7' faces AM, easing 4-5' PM. Super clean with inconsistent sets, biggest in the morning and slowly easing through the day.
SWELL/SURF: Reinforcing long-period S groundswell, possibly mixing with small ESE swell.
WIND: Light WNW, tending to light N/NE sea-breezes in the afternoon.

Wednesday 21st: 3-5' faces AM, rising to 4-6'+ early PM. Clean conditions likely. SWELL/SURF: Renewed building trend in directional S swell sets in throughout the day. WIND: Early moderate WSW/SW, tending S during the day, then to light sea-breezes in the afternoon.

Forecast Outlook

Friday afternoon's gradual increase in south swell is a precursor to a reinforcing round of long-period southerly groundswell this weekend. Forerunners are projected to arrive at 17-18 second intervals across the southern half of the NSW coast on Friday night, leading in the bulk of groundswell on Saturday.

The source was a deep Southern Ocean low that migrated through our long-range swell window over the last 48 hours, setting up an expansive, severe gale to storm force (40-50 knot) fetch - in turn generating satellite confirmed wave-heights in the 30-40ft range. The sheer size and strength of the fetch has generated a vast wave-field over a deep latitudinal band. That sees a double-pulse of long-period groundswell: the first arriving late Friday and early Saturday, ahead of a second pulse moving overlapping with the first later Saturday and throughout Sunday. The arrival of the groundswell coincides with favourable winds all weekend.

Following close behind this system we have a couple of vigorous frontal lows set to rejuvenate wave-growth as they migrate eastward below the mainland through Friday and the weekend. The first has already re-established a broad 40 knot WSW fetch, south of the Bight around 50S to 55S. This source will continue to support seas in the 20-30ft range as it weakens below Victoria and Tasmania later Friday through early Saturday.

That's closely followed by a second frontal low tracing the same track, invigorating a slightly weaker, but longer-lasting fetch across the same area of open ocean. Both frontal progressions go to work upon an already energetic sea-state, resulting in two, back-to-back pulses of long-rang southerly groundswell arriving on Monday and Tuesday respectively.

Overall, the storm-track exhibits a stronger zonal (westerly) bias over this period, indicating the size both pulses will fall short of what arrives this weekend. Still, they should be just enough to keep set waves up at slow, waist to shoulder high levels and occasionally bigger on both days.

There's been no definitive resolution surrounding the forecast track and strength of a small surface low forming on a trough, just off the mid-northern NSW coast on Saturday. The GFS model continues to push the low steadily eastward across the central Tasman Sea with slow intensification on Sunday/ Monday - and that's keeping the door open to a modest increase in short period ESE swell on Tuesday.

However, the chances of any notable ESE swell continue to look doubtful. The GFS model remains an outlier. Broad consensus among several other key models favouring a weaker low pressure trough aiming a moderate and short lived SSE/SE fetch at northern NSW coasts. The projected track and strength of the low will become clearer over the next 24 hours.

A new frontal progression moving clear of Bass Strait on Tuesday is now showing improved potential for short-lived spike in south swell on Wednesday. Recent model runs have upgraded associated wind-speeds, hinting at a decent spike peaking in the 4-6'+ range (on the face) during the day or afternoon, before quickly fading as NW winds freshen across the NSW coast on Thursday morning. That continues to point to tiny surf setting in as residual S swell flattens out later Thursday through early Friday 23rd .

The good news is a resurgence in south swell is now firmly on the radar for Friday and the weekend of Saturday 24th. That follows a stronger frontal system sweeping across the southern Tasman Sea later Thursday through early Friday 23 - in turn linked to a large, complex low pressure system moving below the mainland and into our swell window. That presents good potential for a sizeable upswing in south swell on Friday 23rd, speculatively easing Saturday 24th ahead of a second pulse arriving on Sunday 25th. Specifics on the size and timing of this episode will gradually come into focus over the next few days.

Next Update: Saturday afternoon, 17th April.

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