Forecaster Blog: Tasman Low Hammers the East Coast All Week

14 Jul 2020 0 Share

Ben Macartney

Chief Surf Forecaster

The development of a large and intense low pressure system over the central Tasman Sea on Monday saw a rapid transformation in conditions throughout Tuesday. Surf across Sydney and Newcastle picked up to a windblown 4 to 6ft early to mid-morning and by the early afternoon was looking more akin to a 6 to 8ft+ storm-swell across exposed coasts, with more size projected into late arvo and evening.

Strong SW winds opening up potential for big, clean surf across the Mid-north coast over the next couple of days. Photo: Uploaded to CW by Ayedo.

Strong SW winds opening up potential for big, clean surf across the Mid-north coast over the next couple of days. Photo: Uploaded to CW by Ayedo.

That marks the first of several days featuring heavy, mid-period SSE, SE and ESE swell impacting the entire NSW and southern Queensland coasts over the course of this week. For Sydney, Newcastle and locations south, this event looks like coming to a head on Wednesday as a powerful pulse of SE swell peaks in the 10 to 12ft+ range across exposed areas, preceding a slow decline setting in throughout the remainder of the week.

A complex Tasman low goes to work, setting up a full week of large and powerful surf for the Eastern Seaboard. Source: BOM.

A complex Tasman low goes to work, setting up a full week of large and powerful surf for the Eastern Seaboard. Source: BOM.

The impressive size and strength of the low is further compounded by its extended lifecycle over the Tasman Sea; maintaining intensity throughout Wednesday and Thursday before slowly filling as it drifts out towards New Zealand’s North Island on Friday; still supporting swell producing winds across its southern and western flanks all the while.

Following the storm’s early development on Monday the low drifted southwest and intensified, coming within 300 nautical miles east of Sydney on Tuesday morning. Over the next 6 hours or so it’s forecast to drift slowly north, before turning east overnight and drifting slowly out towards NZ for the remainder of the week.

The Tasman low does its worst on Wednesday, sending in a bombing SE/SSE swell across the NSW coast, with plenty to more to follow into the end of the week. Source: Wave Tracker.

The Tasman low does its worst on Wednesday, sending in a bombing SE/SSE swell across the NSW coast, with plenty to more to follow into the end of the week. Source: Wave Tracker.

Recent satellite passes depict a broad and strong SSE/SE/ESE fetch wrapping around the storm’s south-western flank; mostly at speeds of 30 to 40 knots.  As shown above, the low is supported by a high pressure system to the southwest – and the negligible movement of both the Tasman low and cradling high will support continued development of the rotating wind-field over the next few days; maintaining broad and vigorous SSE/SE fetch across the low’s south-western sector throughout Tuesday and Wednesday.

This satellite pass depicts the slow-moving 30 to 40 kt SE fetch offshore; the source of Wednesday's impending peak in SE swell across the region. Source: NOAA.

This satellite pass depicts the slow-moving 30 to 40 kt SE fetch offshore; the source of Wednesday's impending peak in SE swell across the region. Source: NOAA.

Consecutive WW3 runs s reflect a very large, 20 odd foot deepwater SSE swell peaking across Sydney and surrounds on Wednesday, with a bit more size moving up the mid-northern NSW coast over the same time-frame, owing the more direct alignment of the strongest fetch-area  with Hunter and mid-northern NSW coasts.

The low's slow movement across the Tasman Sea on Wednesday and Thursday guarantees many days of large and powerful surf for the NSW coast. Source: BOM.

The low's slow movement across the Tasman Sea on Wednesday and Thursday guarantees many days of large and powerful surf for the NSW coast. Source: BOM.

As mentioned, the low’s extended forecast duration over the central-eastern Tasman Sea has the low still active inside our easterly swell window this weekend; initially slowly weakening as it approaches the NZ on Thursday and Friday, before hovering just above the North Island on Saturday and Sunday. The slow eastward movement of the low sees the fetch area spanning its southern flank eventually setting up over the south-eastern Tasman Sea, extending westward off NZ’s west coast.


These late-stage developments are really shaping up as the icing on the cake for this event; giving rise to a still powerful, but far more accessible pulse of SE and ESE groundswell, arriving across the region throughout Friday and slowly easing over weekend of Saturday 18th, with smaller ESE leftovers still likely running at a fun size on Monday 20. For now, there are good indications the entire Eastern Seaboard will benefit from lighter westerly quarter wind-regime; opening up several days of great conditions across the region. Stay tuned the detailed forecast for updates on your local region as the week progresses.



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