East Coast Swell Alert: Tuesday January 27, 2015

27 Jan 2015 3 Share

Ben Macartney

Chief Surf Forecaster

The month of January isn’t exactly renowned for major East Coast Lows (ECLs) and associated, big swell events across the East Coast. Although the development of ECLs is far more common during the Autumn and Winter months, they can and do occur at anytime of year. A deep ECL is exactly what’s in store for the region into the last days of January 2015. Over the next few days the evolution of an intense low pressure system over the southern Tasman Sea holds a high probability of generating a large swell event for the NSW coast. It also looks like local winds will, by and large, be favourable through the height of the swell event’s lifecycle.

The evolution of a deep ECL over the Tasman Sea promises big clean surf for the NSW coast. Photo: Randy Sparrow.

The evolution of a deep ECL over the Tasman Sea promises big clean surf for the NSW coast. Photo: Randy Sparrow.

  • A low pressure system embedded on a surface trough over the central Tasman Sea rapidly deepens throughout Thursday, generating a gale force SSE fetch spanning the southwest Tasman Sea.
  • The East Coast Low (ECL) triggers a steep rise in SSE swell across the southern half of the NSW coast, commencing Thursday afternoon and peaking in the 6 to 10ft plus range across south facing breaks throughout Friday/ early Saturday before gradually easing throughout the remainder of the weekend
  • The peak of the swell is likely to coincide with light WSW to W/NW winds both on Friday and Saturday mornings.
  • The close proximity of the fetch to the southern NSW region reduces northern NSW and southern Queensland’s exposure to the swell source, translating into a moderate SSE swell peaking across exposed breaks in the 4 to 6ft range on Saturday.
  • Revisions to the timing and size of this episode remain a distinct possibility, contingent on the timing of the ECL’s development and subsequent movement over the southern Tasman Sea, so stay tuned to the detailed forecast over the next few days.

Overview
The early stages of a large SSE swell event were already underway over the Tasman Sea on Tuesday. An extensive SSE fetch spanning the central Tasman Sea is slowly strengthening in response to a near stationary high pressure system west of Tasmania forming a steep pressure gradient in conjunction with the surface trough extending north to south across central Tasman Sea. Both the high pressure system and trough are forecast to slowly intensify over the next few days - initially triggering a rising trend in short range SSE windswell that should become evident across the region throughout Wednesday. Although the strongest winds associated with the developing fetch are likely to be situated a little too far offshore to have a major impact on Sydney and the South Coast, mid and northern NSW locations see better exposure to the early stages of the swell-source – thereby generating an earlier push in SSE swell to solid 4 to 6ft levels on Thursday. However, it’s the latter stages of the system’s development that hold major SSE swell potential leading into Friday and the weekend. A low pressure centre embedded along the length of the surface trough is forecast to deepen marginally on Wednesday before moving closer to the southern NSW coast and rapidly intensifying into a fully blown East Coast Low on Thursday.

These impending developments follow the passage of an upper level low over the far southeast of the continent contributing to rapid intensification of the system on Thursday and Friday. Although early computer modelled projections were struggling to get a handle on the timing of the low’s development and it’s subsequent intensity and movement, there’s now fair agreement among several key model runs – all of which lend weight to a large SSE swell event unfolding on Friday and Saturday. The low is forecast to track slowly south to southwest as it rapidly intensifies over the southern Tasman Sea on Thursday, giving rise to a broadening southerly fetch across the low’s western quadrant. Although the key models begin to vary regarding the system’s subsequent movement and intensity on Friday and Saturday, there’s little doubt the system will support gale force southerly winds between Thursday night and Saturday morning; enduring long enough to, drive a rapid build up in SSE swell to deep-water peaks in the 20ft plus range over the southern Tasman Sea early on Friday.

A deep easterly trough spawns a surface low slowly developing over the Tasman Sea on Wednesday. Source: BOM.

A deep easterly trough spawns a surface low slowly developing over the Tasman Sea on Wednesday. Source: BOM.

The low rapidly intensifies over the southern Tasman on Thursday and Friday, setting up a gale force southerly fetch in conjunction with a strong high over the eastern Bight. Source: BOM.

The low rapidly intensifies over the southern Tasman on Thursday and Friday, setting up a gale force southerly fetch in conjunction with a strong high over the eastern Bight. Source: BOM.

A subsequent, large to heavy increase mid period SSE groundswell should fill in on Thursday night, producing large surf of varying heights depending on each location’s exposure. The more exposed south facing beaches are likely to see surf ranging from 6 to 8ft plus on Friday and deepwater reefs focussing this energy are likely see substantially larger surf in the 10 to 15ft range. Although Friday should see peak size for this event, Saturday morning should also see a continuation of still large surf in the 6 to 8ft early, but easing a foot or two afternoon. The rate at which the swell declines is contingent on the ECL’s movement and intensity throughout Friday and early Saturday and as discussed there’s still some divergence present among the key models. Latest GFS and also ACCESS runs both present a case for a long tail-end to this episode; indicating the low will stall over the southern Tasman Sea somewhere near 40S, 155E and slowly weaken throughout Friday and Saturday – thereby sustaining large surf across the region throughout the weekend – speculatively holding at 4 to 6ft on Sunday before easing a couple of notches by Monday. However, various other model runs (namely the EC) move the low further to the southwest over this time frame, placing it much closer to Tasmania. This would effectively squeeze the southerly fetch into a narrower band bounded by Tasmania’s East Coast and the low and also reorient the directional band to the SSW and hence out of direct alignment with the coast. This would constrain both the size and duration of the swell event, so some revision to these parameters would appear likely as the system evolves over the next few days.

The ECL also plays a key role in determining local winds. As the low moves southward the NSW coast should increasingly become exposed to a westerly airflow extending across the low’s upper western sector. This is reflected in latest wind forecasts that continue to show westerly winds across the coast both on Friday and Saturday morning’s, producing a couple of days of big, clean surf – predominantly effecting locations from Seal Rocks south.

This image depicts forecast surface wind for Friday morning. At the height of it's intensity the low supports a vast swathe of southerly gales, angled SSE across the southern Tasman Sea while reorienting SW across the NSW coast on Friday morning. Source: BOM

This image depicts forecast surface wind for Friday morning. At the height of it's intensity the low supports a vast swathe of southerly gales, angled SSE across the southern Tasman Sea while reorienting SW across the NSW coast on Friday morning. Source: BOM

Consecutive Wave Tracker model runs pick up seas and swell reaching peaks of 20ft plus over the southern Tasman Sea on Thursday night, setting up a big spike in SSE swell for the southern half of the NSW coast on Friday.

Consecutive Wave Tracker model runs pick up seas and swell reaching peaks of 20ft plus over the southern Tasman Sea on Thursday night, setting up a big spike in SSE swell for the southern half of the NSW coast on Friday.

Latest offshore virtual buoy readings for Sydney pick up a large run-up in mid period SSE groundswell, peaking on Friday before gradually abating over the weekend.

Latest offshore virtual buoy readings for Sydney pick up a large run-up in mid period SSE groundswell, peaking on Friday before gradually abating over the weekend.


Tags: East Coast Low , swell alert (create Alert from these tags)

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