Nias, Mentawai, South Sumatra forecast by Craig Brokensha (issued Tue 22nd Jul)

Best Days: From Saturday onwards

This week (Jul 23 - 25)

A fun pulse of S/SW groundswell filled in yesterday but this has slowly eased off into today. Another slightly smaller pulse of inconsistent S/SW groundswell is due tomorrow and this should keep exposed spots topped up with 4-5ft sets before tailing off into the afternoon and further Thursday morning.

For the rest of the week there's nothing too significant on the cards, with an acute S'ly swell filling in Thursday afternoon and then backing off again Friday, with South Sumatra expected to offer the most size. Moderate to fresh E/SE trades are expected to ease around South Sumatra from Thursday morning onwards, while further north towards the Ments, weaker SE winds will become variable into the end of the week.

This weekend onwards (Jul 26 onwards)

The outlook from this weekend into early August is great!

This will be related to two separate amplifications of the Long Wave Trough moving west to east across the Southern Indian Ocean, bringing with it front after front and with strength.

Currently a vigorous frontal system south-east of South Africa is generating a fetch of severe-gale W/SW winds over an already active sea state and we'll see this storm deepen further this afternoon, with winds speeds expected to reach the storm-force range.

This should set in motion an inconsistent but strong SW groundswell that's expected to arrive Sunday and peak through the day to 8ft to occasionally 10ft across exposed breaks in the Ments. Before this though, the front generating the active sea state for this strong weather system to move over will produce an inconsistent but good SW groundswell for Saturday, reaching 6ft+ into the afternoon.

Sunday's large swell should ease into Monday but a new large S/SW groundswell is due to arrive Tuesday evening and peak Wednesday morning.

This will be produced on the tail of the first strong storm as it pushes towards WA, with a broad and elongated fetch of gale to severe-gale W/SW winds forecast to be generated on the edge of our southern swell window.

This swell will be a touch more consistent and should peak around 6-8ft in the Ments, with larger 10ft sets around South Sumatra. From here on a slow drop in swell is due into Thursday and further Friday.

Winds should be variable for the most part over the weekend around the Ments and Nias, while moderate E/SE trades are due around South Sumatra. Into the end of the week winds may become a little dicey around Northern Sumatra with freshening NW winds associated with a weak trough, but we'll review this Thursday.

Longer term another two vigorous polar lows are forecast to fire up through the Southern Indian Ocean, producing two separate pulses of S/SW groundswell likely for Saturday and Sunday the 2nd and 3rd of August respectively, but we'll go over this again on Thursday.

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