Nias, Mentawai, South Sumatra forecast by Craig Brokensha (issued Tue 17th Jun)

Best Days: Every day over the coming period

This week and weekend (Jun 17 - 22)

The surf's been slow going the last few days but a new swell should start filling in today ahead of a peak through tomorrow. Of greater importance is the large S/SW groundswell due Thursday across the region, generated by a long-lived frontal progression that started it's life south of South Africa around Wednesday/Thursday last week and pushed through the Southern Indian Ocean while aiming bursts of severe-gale W/SW winds through our swell window.

There's been no real change to the expected size with the Ments due to peak around a large 8-10ft at exposed spots, with the possibility of the odd bigger sneaker, with similar waves around Nias to 8-10ft and larger bombs in Southern Sumatra.

A peak is due through the morning ahead of a late drop in size and more noticeable easing in size Friday and further into the weekend.

Winds are looking excellent and will be generally light and variable favouring exposed and more protected spots.

Next Monday onwards (Jun 23 onwards)

Next week is looking more than active as the westerly storm track through the Southern Indian Ocean remains over-active through the coming week with a couple of strong nodes of the Long Wave Trough traversing the basin.

With this a series of vigorous polar fronts will fire up towards us, the first through the coming days, generating a large pulse that should fill in Monday and peak to 6ft to occasionally 8ft at exposed breaks in the Ments.

The swell should back off a touch into Tuesday and further Wednesday with a low point in swell activity due through Wednesday afternoon.

Of greater importance is the formation of a very strong, long-period and powerful SW groundswell for Thursday/Friday next week.

This swell will be generated by a much stronger and broader frontal progression (right) firing up under South Africa, under the effects of the Long Wave Trough (shown below in the video as the red/purple stream lines pushing under South Africa), generating large pumping swell for J-Bay but also a very strong long-period SW groundswell for us.

At this stage we're looking at another swell in the 10ft range but we'll review this again on Thursday.

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