South Arm Surf Forecast by Ben Matson (issued Monday 12th October)

Best Days: Wed: small easing swell with offshore winds. Fri: very good new swell with offshore winds. Maybe hanging into early Sat but fading rapidly and with a developing S'ly change. 

Recap: Small clean surf today with light winds, freshening from the north throughout the day. 

This week (Oct 13 - 16)

*This week’s Forecaster Notes will be a little erratic as Craig’s on annual leave*

I gotta say, of all the times to pick up the reins of the South Arm forecasting bench, this arvo is the least desirable of the lot (having not looked at the Southern Ocean synoptics in quite a while). 

We’re right on the cusp of two interesting long period groundswells - the first of which is running late and thus presently undersized; the second of which is trailing close behind but was generated by an impressive cut-off low. 

Truth be told, whilst the parent systems that generated these approaching swells do look impressive, there are a couple of limiting factors. Today’s swell source was a very long way from the mainland (midway between Heard Island and West Oz longitudes), which means the enormous travel distance will shave off a lot of size and (more importantly) consistency. So set waves will be extremely infrequent.

The issue I have with tomorrow’s swell is that whilst the cut-off low responsible for the swell was very strong, and travelling in a manner conducive to a capture fetch, the actual fetch was relatively short and narrow, compared to a typical Southern Ocean front being driven by a core Polar Low. 

This somewhat weakens my confidence. In my experience, these kinds of swells tend to favour a small number of spots where the magic numbers (size/period/direction) are optimal, but many locations usually miss out. 

Regardless, the model data is holding steady so I’m going to remain with Craig’s estimate for 2-3ft surf building Tuesday. It’ll be bumpy anyway with moderate W/SW onshore winds in the wake of a front clipping the state tonight. Keep your expectations low.

Wave heights will then slowly ease from Wednesday onwards and winds will freshen from the N/NE, so it’ll be clean but becoming blustery throughout the day as the surf eases from an inconsistent 2-3ft to 1-2ft by lunchtime.

Thursday looks to be a bit of lay-day with minor leftover energy and early NW winds tending W’ly then S/SW in the wake of a vigorous front. This will be attached to an impressive polar low skirting the ice shelf over the coming days (see below), which will line up nicely within our swell window, providing a solid swell increase for Friday.

The good news is that winds will be back around to the N/NE by this time, so it’s well worth pencilling in a session during the day. Wave heights should build from 2-3ft to 3-4ft through the day, and there’ll be great options at many locations.

This weekend (Oct 17 - 18)

There’s quite a bit of divergence regarding the weekend outlook.

A weak coastal trough expected to be near South Australia on Friday is unclear as to how it’ll evolve. We’ll either see a near-stationary ridge of high pressure extending west from Tasmania dominating the weekend outlook with generally lightor, the trough will move slowly west, swinging the winds to the north ahead of a fresh S’ly change overnight that’ll maintain onshores throughout Sunday. 

However, either solution won’t provide any significant new swell for the state.

Saturday morning will see easing SW groundswell from Friday’s increase (early 2-3ft sets, smaller throughout the day), so let’s hope there’s a brief window of offshore conditions. 

Sunday looks to be small and choppy. I’ll take a closer look on Wednesday. 

Next week (Oct 19 onwards)

With the weekend outlook already divergent, there’s not a great deal of point in looking beyond into next week, as it’ll all change quite a bit over the coming days. 

See you Wednesday!