Every so often a mystery swell appears at the coast, leaving even die-hard locals scratching their heads in puzzlement as to the swell's origin.

Sunday afternoon delivered one of those swells to the Surf Coast, and the waves were pumping; six feet plus and ruler edge with a fresh nor'wester – much bigger, stronger and cleaner than expected.

So where did this swell come from and why did we miss it?

Well, technically we didn't miss the swell, we just didn't think it'd make its way into Bass Strait (6ft+ surf was accurately forecast for South Australia from the same event).

The swell originated from a deep low pressure system that developed west of Bass Strait over the weekend. Throughout the latter part of last week the computer models varied considerably in their forecast of the exact position of the low.

When our Victorian forecast notes were compiled on Friday, the computer models finally came into agreement with each other, predicting that the low would stall west of about Portland. Such developments were expected to steer the bulk of southerly gales (on the western side of the low) outside of Torquay's swell window and up into South Australia.

However, the low ended up moving about 170 kilometers further to the east than Friday's predictions, extending southerly gales off Tasmania's West Coast, ideally aligned within Torquay's swell window. This generated a large southerly swell that arrived early afternoon, and provided excellent waves across the entire region.

And then to ice the cake, the low remained near-stationary throughout Sunday and early Monday, creating fresh north-westerly winds about the Surf Coast.

Take a look at the two images to see the difference in the surface wind forecast between Thursday and Saturday. This shows how just a small alteration in the low's position dramatically changed the outcome for Torquay surfers. It's rare to see such acute positioning of a low pressure system near Bass Strait, resulting in perfect, sizeable waves along the Surf Coast.

Here's hoping that Sunday's swell is a positive sign of things to come for the rest of autumn. //CRAIG BROKENSHA