Forecaster Blog: Swell Inbound From All Angles Across The East Coast
Issued Tuesday, 12 January 2016
To date it’s fair to say the surf hasn’t been bad across the East Coast so far this summer. In fact, one might go as far as to say it’s been pretty good.
In the wake of early January’s sustained run of back to back E/SE and SSE swells conditions finally settled to more typical 1 to 2ft levels across the entire Eastern Seaboard by Tuesday – marking a brief hiatus in activity preceding the onset of multiple new swell trains from Wednesday onwards.
The first swell off the rank is a small pulse of ENE groundswell emanating from the long-enduring Tropical Cyclone Ula (TC Ula). The storm has been tracking slowly westward across the tropical Southwest Pacific since January 1; initially meandering through Western Samoa before dipping southwest below Fiji with some modest intensification during January 4 and 5. TC Ula subsequently weakened and drifted northwest over the course of last week before re-intensifying and curving to the southwest last weekend. The system achieved Category 4 strength on Sunday and on Monday morning appeared on the BOM’s latest MSLP chart as a 945 hPa tropical cyclone centred a couple of hundred nautical miles east of Noumea.
While TC Ula looks good from afar, associated swell potential is far from good. The storm’s diminutive size coupled with a weak subtropical ridge to the south precludes it as a major source of long groundswell. Although the strongest winds generated by TC Ula were clocked at 130 knots, these winds maintained a compact, clockwise orientation, with wind speeds rapidly falling to 34 knots within 100 nautical miles of its eye. The absence of any length of fetch coupled with its remote proximity to the East Coast heavily constrains wave potential. This is reflected in latest WW3 runs showing a few feet of deepwater groundswell inbound from 60 to 75 degrees over the next few days. So while in theory there should be sets in the 2 to 3ft range materialising at the height of the swell, the effects of wave dispersion and wave attenuation are likely to dilute this energy; producing inconsistent sets across exposed open breaks.
By Tuesday morning TC Ula had significantly weakened under increasing vertical wind shear and lower sea surface temperatures and over the next few days the system is projected to further deteriorate as it accelerates away to the south and southeast – drawing it swiftly out of range of our swell window by Wednesday morning. However, as the tail end of the TC Ula wave-train dwindles on Thursday it’s set to cross over with by a new round of long-range groundswell – this time originating from a remote corner of our swell window, far below New Zealand. The source is a deep, 942 hPa polar storm that retrograded southeast of the South Island last weekend, generating an area of strong southerly gales across the 60S to 70S latitudinal bands – a region roughly 2,300 to 2,500 nautical miles SSE of Sydney. Regardless of its remote origin, groundswells arising from this quadrant tend to exceed expectations, so the prospect of a well-organised (albeit inconsistent) 2 to 3ft SSE groundswell is expected to arrive across Sydney and the South Coast on Thursday before filling in further north on Friday.
However this manifests, Saturday then marks the onset of a more substantial S/ SSE swell event originating closer to home. The eastward progression of a large, complex Southern Ocean low beneath Tasmania later this week heralds the arrival of an extended run of robust wave energy; kicking off with a steep rise in SSE windswell across Sydney and the South Coast on Thursday evening. This follows on the heels of a vigorous cold front forecast to move up the NSW coast during the day and it’s now expected to push up windblown peaks of 4 to 5ft plus across southern exposures on Friday. This episode should push further north at slightly lower levels this weekend and there are good indications it will spill over into Sunday and Monday.
Latest GFS runs form a low on the front/ trough as it drifts out into the southern Tasman Sea over the weekend; contributing to a reinvigoration of a moderate to strong SSE fetch 20 to 30 knots. For the time being this points to a new pulse of SSE swell gracing the coast on Sunday and Monday – again producing surf ranging from 2 to 4ft across the region on both days. Keep in mind it’s still early days so revisions to the outlook for this weekend can be expected in Wednesday’s detailed updates.
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