So, what happened to yesterday's south swell in Sydney?

Following an extended period of tiny Tasman energy, our new swell was expected to arrive early Tuesday morning, peaking for a brief period with five-to-six feet waves at exposed south facing beaches before easing into the afternoon.

Unfortunately, the swell was smaller than expected, arrived later than forecast and eased more rapidly than predicted.

Actually, some locations did quite well on Tuesday. Despite the meagre offerings along the Northern Beaches, Cronulla picked up good four feet waves mid morning. So too did a number of other locations in the Hunter region, as per today's Wave Of The Day. And up on the North Coast, six foot sets were reported at Yamba and Ballina for the afternoon session.

However, the swell bypassed just about every major Sydney beach - even some of the more reliable swell magnets saw just a fraction of the size compared to those few locations where the swell actually delivered.

The problem with specific size forecasting for such weather systems is that small changes to computer model output can drastically alter potential wave size. We were consistent with our five-to-six feet call for this swell from the outset, originally predicting this size range the Monday prior (eight days out) even though the computer models were initially way over the top with estimates of ten feet plus surf, and more.

As the computer models slowly downgraded their forecasts over the coming week to be more inline with our predictions, it was thought that they'd finally stabilised their estimates over the weekend.

To a degree, this was true - however the actual developments within the low pressure system itself were knife-edge for Sydney's swell potential - south swells are notoriously fickle in their beach alignment, and this was quite evident on Tuesday with such a wide range of wave heights between Ulladulla and Seal Rocks.

In hindsight, I believe the responsible low pressure system developed too quickly to generate any notable size for Sydney, and its rapid track to the southeast - which was discussed on Monday's notes - exacerbated these effects. It seems that the primary fetch around the low was more south-west than south, with the resulting outer bands of swell energy briefly glancing the swell magnets enroute to New Caledonia and Fiji.

A secondary front behind the low resulted in a similar, renewed coverage of directional south swell today - exposed Sydney beaches are barely picking up two feet, while it's four feet in Newcastle. South facing beaches up on the North Coast are seeing five foot sets this morning, even though it's further away from the swell source than Sydney - this phenomena in itself requires a technical explanation that I won't go into here.

In the case of Sydney, there's unfortunately no real way of knowing the specific directional nature of incoming swells. This most recent event is certainly one of the more extreme examples.

The benefit of hindcasting these kinds of events is that our surf forecasting knowledge-bank is further built upon, meaning we'll be able to reference this event in the coming months and years when a similar low pressure system develops to our south. Hopefully next time it'll hang around a little longer and give us the bigger waves we all hope for! //BEN MATSON