Nias, Mentawai, South Sumatra forecast by Craig Brokensha (issued Tue 12th Aug)

Best Days: Every day over the coming period

This week onwards (Aug 13 onwards)

A large and powerful S/SW groundswell provided excellent waves across the Ments yesterday. The swell is now on the way out but there were still large waves on offer this morning with light variable winds.

A further drop in size is expected through tomorrow from the 3-5ft range in the Ments before bottoming out on Thursday morning.

Into the afternoon though a new long-range and inconsistent S/SW groundswell is due across the region, generated by a broad but not especially strong polar front east of Heard Island.

An afternoon kick to 4-5ft is due Thursday while Friday morning should offer better 4-6ft sets, but there'll be long waits for the bigger ones. Nias is expected to be a touch smaller while more consistent sets to 6ft are due in South Sumatra.

Winds should be favourable around the Ments and Nias but South Sumatra looks average with fresh to strong N/NW tending W'ly winds on Thursday, tending more variable Friday as a weak surface depression moves through the area (right).

Into the weekend, Friday's swell should back off slightly, but a new long-range SW groundswell is due later in the afternoon, peaking Sunday afternoon. This swell and a secondary similar pulse on Tuesday will be generated by a flurry of broad but unconsolidated frontal activity north of Heard Island over the coming days.

Size wise we should see a peak in the 5-6ft range across the Ments for both swells, with the odd bigger bomb possible in South Sumatra.

Light variable winds should create good conditions across most regions, with weaker SE trades developing in South Sumatra from Monday.

Longer term the outlook is much better, with the westerly storm track expected to fire up around this time next week, possibly setting in motion a couple of larger SW groundswells for late next weekend and the following week (August 24/25th). We'll look at this in more detail on Thursday.

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