Bali and Mentawais forecast​​​​​​ by Craig Brokensha (issued Tuesday 24th July)

Best Days: Experienced surfers only over the coming period, in protected spots in the Ments

This week and next (Jul 25 – Aug 3)

Bali: After easing through the weekend, a strong new long-period SW groundswell pushed in yesterday, generated by a tight mid-latitude low that formed south-east of Madagascar last week before tracking east.

This provided strong sets at swell magnets, but today, we're now seeing the fore-runners of the biggest swell of the season pushing in, producing large dangerous and building surf across most of Indonesia.

Through late last week and the weekend we saw a very significant polar storm developing in the Heard Island region and projecting slowly up into the south-east Indian Ocean in a 'captured fetch' motion, higher in latitude than normally seen.

This was due to a strong protruding node of the Long Wave Trough stalling just west of Western Australia, and the swell impacted their region Sunday evening and yesterday, producing some of the largest readings in years on the local wave buoys.

As a result we can expect the surf to come in very very large and powerful across our region later today, peaking tomorrow morning in the 12ft to likely 15ft range at exposed breaks, large even in super protected spots and only for experienced surfers.

The swell should start easing through the day, dropping from a still extra-large 10-12ft Thursday morning and more manageable 8ft range Friday morning.

Fresh and gusty E/SE trades will persist through tomorrow, swinging more SE in direction from Thursday and easing off from Sunday. Early each morning we should see more variable breezes, lasting longer on the days with weaker trades.

Into Saturday a new large consistent SW groundswell is due to fill in, produced by a mid-latitude front that's currently projecting east through the Indian Ocean.

While not especially strong this front will project a good fetch of W/SW gales relatively close to Indonesia, with the swell building through Saturday, peaking later and then easing Sunday.

It won't be as big as the models are forecasting, with the existing swell being incorrectly combined with the new energy, but we can expect exposed breaks to kick back to 8ft on the sets through the afternoon, dropping back from 6-8ft Sunday.

Of greater importance is a much larger and more powerful long-period SW groundswell due across the region Monday.

A few days ago the models were forecasting this swell to be similar in scope to the current building XXL groundswell, but it now looks to be just under.

Currently a vigorous polar low is forming west of Heard Island, with a fetch of severe-gale to near storm-force W/NW winds, creating an active open ocean sea state for a much broader and expansive fetch of storm-force W/SW winds to move over, projecting slightly north-east towards us through the middle of this week.

An extra-large long-period SW groundswell will be generated, with the fore-runners arriving Sunday and kicking large late in the day.

We'll see the groundswell fill in proper on Monday and peak through the day to what looks to be 10-12ft+. Following this it looks like we'll see the Indian Ocean settle down to normal levels, but more on this Thursday.

16 day Bali Forecast Graph
16 day East Java Forecast Graph
16 day Sumbawa Forecast Graph

Ments: Today we're seeing the XXL S/SW groundswell filling in across the region along with fresh SE-S/SE winds, favouring protected breaks.

The swell should peak tomorrow in the 12ft+ range across exposed breaks, easing through the afternoon and down further into Thursday and Friday.

Fresh SE winds will persist through the end of the week and most of the forecast period, limiting options with the coming swells.

The new S/SW groundswell for Saturday was generated a touch later in our swell window, and as a result we're looking at surf to 6-8ft, under the incorrectly combined model forecasts.

We then look ahead to the extra-large long-period S/SW groundswell for Sunday/Monday. The swell will arrive a touch earlier across the Ments and should kick to an easy 10ft by dark at magnets Sunday, peaking Monday morning to 10-12ft, easing back slowly through the afternoon and further into the middle of the week and bottoming out Friday.

We may see a new moderate to large S/SW groundswell for the weekend, but have a check back Thursday for more on this.

16 day Mentawai forecast graph
16 day Nias forecast graph
16 day South Sumatra forecast graph