Axis of Evil
For the last two weeks the NSW coast has experienced a run of large and powerful swell. Stories and photos have abounded of standout days across various reefs and beaches on the East Coast.
The first week of swell was linked to a slow moving Tasman Low that encompassed the entire Southern Tasman Sea for almost a week, while the more recent east/south-east swell was associated with a much more dynamic weather system. That being, a small and concentrated low pressure cell sitting directly east of Jervis Bay.
While the system didn't quite classify as an East Coast Low it had very similar characteristics regarding torrential rain and large onshore storm swells stretching from Sydney south.
The most interesting, and also the best, characteristic regarding this low was its proximity to the coast resulted in a wide range of winds in a very short stretch of coastline. The following lesson is something to keep in mind the next time a similar sytem strikes.
For a moment try and imagine the form a low pressure system takes when it's near the coast: winds swing clockwise around its centre coming in from the Tasman Sea on its southern flank, while around the northern side of the low they're blowing off the land resulting in offshore winds and clean surf.
The point at which the winds swing nearly 180 degrees in direction is known as the axis of the low and is the most important location for surfers to take note of. North of this point the swell will be clean under offshore winds, while to the south poor surf can be expected with strong onshore winds. And the difference between offshore and onshore can be just a thirty minute drive away.
During Monday and Tuesday this week the axis of the low pressure cell sat just south of Sydney, directing onshore winds into the South Coast, while from Cronulla north, conditions were clean with six feet of swell. The weather models struggled to pick when this low would move north and spoil the surf in Sydney with computer forecast data indicating onshores would hit early Tuesday morning.
At Swellnet we noticed the trouble the models were having picking the movement and axis of the low and went against their forecasts, issuing instead a light variable W/NW wind for most of Tuesday in Sydney ahead of a late S/SE change.
The low indeed did hold south nearly all day creating great waves across most of the Sydney coast but by Wednesday the system finally moved north bringing fresh onshore winds with it. Places north of Sydney remained offshore till the axis passed them too (Forster remained offshore till midday Wednesday).
The crucial point is this: when such dynamic weather systems are on the coast, it's possible to escape the onshore winds by travelling north of the low's axis, opening up better weather and clean offshore surf. Local wind observation stations help a great deal in locating the axis and knowing when and where to head. // CRAIG BROKENSHA
Image 1 shows satellite observations of the low on Monday evening with the red-brown barbs indicating gale-force E/SE winds aimed into the South Coast. The axis of the low can be seen just south of Sydney, with winds immediately north clearly shown to be light Westerly and offshore. Image 2 is onshore Bondi. Image 3 is offshore Northern Beaches.