South Arm Surf Forecast by Ben Matson (issued Monday 11th September)

Features of the Forecast (tl;dr)

• Today's swell easing from Tuesday
• Smaller Wednesday, tiny by late afternoon into Thurs AM
• Chance for a small pulse Thurs PM, but probably wind affected
• Low chance for upcoming W'ly swells (Sun thru' most of next week) to benefit the South Arm

Recap

Small, generally wind affected surf over the weekend gave way to lovely clean lines of S/SW groundswell today, building into the 3-5ft range across open beaches by late afternoon, under light offshore winds. 

This week (Sep 12 - 15)

Today’s swell will peak overnight and then ease gradually through Tuesday. With winds out of the NW it’ll be a great morning for the South Arm though a shallow W/SW change is due later so aim for a morning paddle (it’ll have the most size too). Let’s peg size around the 3-4ft mark at dawn, easing to 2-3ft by the end of the day. Expect long breaks between the sets too.

By Wednesday we'll be back to small 1-2ft leftovers with freshening NW winds ahead of a late gusty W'ly change associated with a small but intense low passing just south of the state. Although the synoptic chart (below) looks impressive, this low really only develops right on the cusp of our swell window, and it also moves pretty fast to the east - so I don't think there's enough time for it to generate a decent swell increase. However, keep an eye out for a brief pulse through Thursday (probably the afternoon, maybe some 2ft+ sets at the swell magnets if we're super lucky). 

Otherwise, small residual energy will prevail for the rest of the week with variable winds Thursday tending NW and freshening on Friday.

This weekend (Sep 16 - 17)

The regional swell charts look active for the weekend but I fear the dominant westerly swell direction will have an ongoing negative impact across the South Arm.

The storm track all week is expected to ride quite north in latitude, generally above 50S but positioned quite some distance from the Tasmanian region. The best source of winds will be a broad W/NW infeed developing in the lee of a subtropical low near WA longitudes mid-week (see chart above). Prior stages of this low displayed very strong but off-axis winds, and were positioned inside Victoria’s long range swell window between Madagascar and Heard Island. Not great for size potential along southern Tasmania, at any rate.

So, with the automated surf model calling 1ft on Saturday and then 3ft on Sunday, I’m inclined to peg those figures back - especially Sunday - and highlight the likelihood for major inconsistency in set waves. Plus, westerly winds will be quite strong at times too, as the frontal conveyor belt slides past the state. If you have to pick a day, Sunday afternoon is your best option, but I don't think we'll see much action. Be prepared for a weekend of nil waves.

Next week (Sep 18 onwards)

In general, the Long Wave Trough moves slowly eastwards through the Southern Ocean, so these kinds of westerly swell regimes are often replaced with south-west patterns once the LWT passes Tasmanian longitudes.

However, in the current scenario there is a suggestion that the LWT may stall over WA longitudes later this week, delaying its eastward movement and prolonging activity in our western swell window. It’ll eventually shift, and in the interim, we do have plenty of surf potential as the storm track will be closer to the mainland (so, wave decay will be reduced, owing to a shorter travel distance), however it does mean that we may need to be a little more patient for the arrival of the next major swell event - it may not be until after next weekend.

More on this in Wednesday’s update.