Indonesia is set to fire over the coming fortnight, with large back to back groundswells forecast to impact the islands.

While not uncommon for this time of year, what is different is the weather dynamics behind this run of swell.

A combination of a broad node of the Long Wave Trough stalling across the southern Indian Ocean and strong negative Southern Annular Mode (SAM/AAO) event will see a progression of vigorous polar and mid-latitude storms firing up through Indonesia's swell window.

The Southern Annular Mode (SAM/AAO) is an index showing how far north or south the polar westerlies (storm track) is from normal. When it goes positive the westerlies shift further south and away from us (smaller less consistent swells), but when negative they shift closer to the equator (larger more consistent swells).

The strong negative SAM event combined with a strong node of the Long Wave Trough in the southern Indian Ocean will result in the production of large consistent swells for the Indonesian region.

The Rip Curl Padang Cup has been put on high alert for a very long-range SW groundswell Thursday morning, but the swell proper isn't due until the afternoon and will likely only come in around 8ft+ across exposed breaks, not the size needed for the competition in my opinion (there's better to come). The storm that generated the swell actually provided building surf for the last day of the Corona J-Bay Open.

Of more significance are the storms firing up later under the influence of the Long Wave Trough. The first real juicy system is forecast to develop west-southwest of WA mid-week, projecting a good fetch of SW gales towards the islands before moving slowly east.

A large S/SW groundswell should result, building Sunday and peaking Monday in the 8ft range across the Bukit.

A better secondary polar front forming south-east of South Africa Thursday should project a much broader and longer-lived fetch of gales towards Indonesia from Friday through early next week. We'll actually see the final stages of this progression push up past the North West coast of Western Australia, resulting in an oversized consistent S/SW groundswell.

The swell is due Thursday and should come in at 10-12ft+ across exposed breaks across the Indonesian archipelago, tapering off slowly Friday.

Following this, an even stronger and more significant polar low forming in the Southern Ocean will generate another oversized long-period SW groundswell, but with the storm staying at polar latitudes we can expect a touch less size and consistency.

At this stage would fill in Saturday week, building to the 10-12ft range, but keep an eye on the the bi-weekly Indonesian Forecaster Notes for more info on the timing and size of this final large swell.

16 day Bali Forecast Graph
16 day Bali Forecast WAMs
16 day Mentawai forecast graph
16 day Mentawai forecast WAMs