If you've been reading all our weather articles on Swellnet you'll know we've got beef with cyclones.

Why...?

Well, here's the reason and its borne out by experience: the most classic East Coast swells come, not from isolated Tropical Cyclones, but the combination of a cyclone with a strong supporting high pressure ridge.

You see, a cyclone by itself is so small in scope that the strongest core winds only act over a relatively small area of ocean. This results in a highly focussed swell event that may perform excellently at breaks close to the cyclone but falls away quite rapidly any further away the swell has to travel.

If, however, the cyclone interacts with a broad high pressure ridge, a larger overall fetch of winds is established creating a much more significant and all-encompassing swell event.

One such pattern is currently setting up through the Western Pacific Ocean and Coral Sea and it's all due to a 'wave' of tropical activity known as the Madden Julien Oscillation (MJO). The MJO recently passed through northern Australia, setting off Tropical Cyclones Narelle and Mitchell in WA but it's since moved further east, and is now positioned in the Fiji-Samoa region. In saying this, North-eastern Australia is still under the effects of the recent MJO passing, with the monsoon trough (area of high rainfall and instability) remaining active across Northern Queensland.

 A couple of tropical lows have spawned off the Queensland coast owing to the monsoon trough, with one of them - Oswald - temporarily reaching cyclone strength (we'll come back to Ossie later).

Of greater significance is the effect the MJO is having through the Western Pacific, with this broad area of instability and low pressure starting to squeeze against a strong high pressure system positioned just east of New Zealand.

The high will be a stubborn bastard and remain anchored in the same position over the next week while being squeezed from the north as a small tropical low deepens south of Fiji. Pressure gradients will tighten resulting in a strengthening fetch of easterly winds extending from above New Zealand into the Tasman Sea, much like the shape a balloon would take if you squeezed it between your hands.

The benefit of such a setup is that the fetch of easterly winds will blow over the same area of ocean for an extended period of time setting up an active and ever increasing sea state. As winds strengthen on top of this active sea state, less work is needed to kick up wave heights further, resulting in the development of a strong and prolonged swell event. This is the beauty of trade-swell episodes.

By itself this setup is expected to produce a medium sized easterly trade-swell event for the whole East Coast that will build through the weekend and hold between 3-5ft from Monday to at least Wednesday. But a further deepening of the low on top of New Zealand early next week should generate a larger and stronger easterly groundswell event to 6ft to possibly 8ft for Thursday and Friday.

But if this all sounds too good to be true, well, unfortunately there's going to be a curveball.

Ex-Tropical Cyclone Oswald, who we quickly passed over earlier, will enter the fray and mess things up, particularly for QLD and Northern NSW surfers. Ossie is currently south of Cairns and forecast to track south-east and deepen off the Queensland coast during the weekend. This will produce a large and stormy swell with strong east to north-east winds over the weekend for South-eastern Queensland and Northern NSW that will then move down into Sydney on Monday and Tuesday as Ossie moves south.

A reprieve should be seen into Tuesday and Wednesday when Ossie moves away from the East Coast and winds swing more southerly, but where and when this will happen is all dependant on where the low actually tracks off the coast. At this stage the low is forecast to move offshore somewhere on the Southern NSW coast, with the Goldy and North Coast unfortunately seeing winds linger from the northern quadrant through much of the event creating average conditions. Keen surf chasers will have to observe this closely to score the best combination of swell size and local conditions.

 In summary, the whole Australian East Coast is in for a large and extended E'ly swell event with wave heights not likely to drop below 3-4ft until at least next weekend, all due to the active monsoon trough and recent passing of the MJO. We should see the MJO returning in another 30-60 days, spawning another burst of tropical activity for the start of autumn. Till then keep an eye on your local forecast to try and plan around the coming swell and winds. //CRAIG BROKENSHA