East Coast Swell Alert: March 20, 2014
Swell Alert issued 5.30pm, Thursday 20 March 2014. By Chief Swell Forecaster Ben Macartney.
- Tropical Cyclone Mike, located 600 nautical miles southwest of Tahiti, is the source of a long range E groundswell inbound across the East Coast.
- A leading round of mid period energy arrives on Sunday, ahead of the bulk of swell peaking on Monday.
- Conditions will be best across Sydney and the South Coast under early light westerly winds.
- From Sydney south: The swell is anticipated to peak at five to six feet on Monday afternoon as a southerly change lends the focus to north facing breaks and sheltered southern corners.
- Southern Queensland and far northern NSW sees NE winds developing through Monday’s peak in swell, lending the focus to sheltered northern corners.
Another long interval easterly groundswell looms for the Eastern Seaboard early next week, but it’s a pretty safe bet to say it won’t live up to last weekend’s TC Lusi swell. The swell source is Tropical Cyclone Mike; a relatively weak, category one system that set up at the furthest eastern fringes of our swell window, out near the Cook Islands early this week; commencing its lifecycle as a tropical low before steadily intensifying and moving slowly south over the last few days. The system reached tropical cyclone-strength on Wednesday and then began to weaken as it accelerated on a SSE trajectory on Thursday. In most cases such a remote tropical storm wouldn’t be considered as a realistic swell producing candidate. However, in this case (not unlike TC Lusi), a large high pressure system supporting TC Mike from the south facilitated the development of an extensive, 30 to 45 knot easterly fetch spanning the cyclone’s southern flank. Further, the fetch has remained intact for several days and this has compounded the size of the resulting swell generated by the system.
Inconsistent E groundswell peaks on Monday
A long interval E groundswell spreading out off the source closes the distance on Australia’s east coast over the next few days before substantially filling in on Monday. However, it’s worth keeping in mind that TC Mike is located roughly 2,500 nautical miles east of Australia’s eastern seaboard; a distance long enough to heavily dilute the impact of the groundswell through wave dispersion and erosion. Wave dispersion refers to a steady increase in distance between each consecutive set of waves as the swell travels away from the source. This occurs because the sets are travelling at slightly different speeds. When the source is close to the coast the differing speeds are of little consequence, but over 2,500 miles the distance between each set grows incrementally – the end result being long waits between sets – possibly up to 20 minutes or more.
Wave erosion occurs as a result of the swell encountering surface tension, currents and opposing winds. Although groundswells predominantly travel below the sea surface and therefore retain most of their energy over vast distances, the distance covered by the swell will inevitably knock down the size of the swell to some degree. As a result, the larger projected set waves in the five to six foot plus range are likely to prove scarce through the height of this episode, so it’s worth keeping your expectations in check.
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