Forecaster Blog Updated: Deep Tasman Low to break the East Coast Swell-Drought
Issued Thursday, 11 January 2018
The models are now showing more cohesion surrounding the evolution of a deep Tasman low off the NSW coast this weekend – so it’s fair to say the first major swell-event for the NSW coast of 2018 is on track. Still, the complexity of the emerging low is making life difficult for this forecaster – particularly leading into the later stages of the low’s development and subsequent swell potential for the region mid to late next week.
To begin with, it looks pretty straightforward. A leading cold front sweeping up the southern half of the NSW coast on Saturday leads in an initial spike in S windswell on Sunday; probably a little undersized early before building steeply as southerly winds strengthen to 20 to 30 knots during the day. This presents fairly high chances for steep rising wave heights towards a stormy 3 to 6ft across southern exposures on Sunday afternoon, leaving sheltered southern corners as your best bet for much smaller, but cleaner waves in the 2 to 3ft range.
By Monday morning the windswell is likely to have evolved into a solid, mid-period SSE swell in the 3 to 4ft plus range, whipped up by a broader southerly fetch expanding across the southern Tasman Sea on Sunday. From there we may initially see wave heights easing a little during the day; following the continued eastward movement of the low across the Tasman during the day.
However, a new cold front sweeping south to north across Tasmania during the morning should reinvigorate a 30kt southerly fetch over the south-western Tasman; triggering renewed wave growth throughout the afternoon. Whether or not this translates into a late rise in SSE swell on Monday arvo isn’t so clear – but either way it should lead in a major boost in size by Tuesday morning; pushing surf up into the 4 to 6ft plus range across the southern exposures.
Beyond Tuesday the models are starting to show some loose agreement on the later stages of the low’s development over the Tasman Sea. Both GFS and EC runs are now indicating the low’s central pressure will fall steeply on Monday evening; giving rise to a severe gale force southerly quarter fetch across its western flank on Tuesday. However, both models are now positioning the low further east than earlier runs; indicating it will sit closer to New Zealand’s side of the southern Tasman; possibly stalling within close proximity of the South Island.
On the one hand, potential for a phenomenal spike in SSE swell into the 8 to 12ft plus range on Wednesday now looks unlikely. Instead, it’s looking like we’ll see a long-period SSE/ SE groundswell arriving out of the 150 to 160 degree band on Wednesday arvo and Thursday morning; potentially generating large surf in the 6 to 8ft range along the more exposed breaks before easing a foot or two during the afternoon. Thursday morning’s sizeable SSE groundswell looks like coinciding with early N to NW winds tending moderate NE during the day – and given this proves accurate we’ll undoubtedly be looking at an epic morning of surf state-wide.
For now, the extended lead time on these developments continues to lend a degree of uncertainty to the forecast. There’s still scope for changes to the low’s projected behaviour this weekend and that means some revision to size and timing of the swell is likely by Monday. Fingers crossed until then.
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