Nias, Mentawai, South Sumatra forecast by Craig Brokensha (issued Tue 6th May)

Best Days: Wednesday afternoon onwards

This week (May 6 - 9)

After a fun weekend of waves, we're on downwards trend in size, and we'll see wave heights bottom out this evening and tomorrow morning.

A bump in size is due tomorrow afternoon ahead of a peak Thursday as a long-range S/SW groundswell fills in, generated last week by a series of broad but not especially strong frontal activity in the Southern Ocean.

This should provide inconsistent 3-5ft sets at exposed Thursday before backing off a touch Friday. Light and variable winds are expected for the most part during the end of the week and further into next week.

This weekend onwards (May 10 onwards)

From this weekend onwards, we've got an active period ahead with a moderate to large S/SW groundswell Saturday afternoon expected to be backed up by a much larger and more powerful S/SW groundswell Tuesday.

The first increase due over the weekend is being generated by a mid-latitude front pushing fairly north into the Indian Ocean, aiming a fetch of strong to gale-force S/SW winds through our swell window.

The swell from this system should arrive Saturday and peak later in the day to 5-6ft+ across exposed spots before easing slowly on Sunday. There should be a fairly even spread of size, with Southern Sumatra coming in similar to the Ments, while Nias will be a touch smaller and in the 4-6ft range.

Of greater importance is the development of a vigorous polar frontal progression around the Heard Island region from today.

An initial broad and vigorous system should generate a fetch of severe-gale W/SW winds tonight and early tomorrow, setting in motion a large active sea state for a secondary stronger and more favourably aligned polar front to piggyback up and over.

This secondary system should generate an additional fetch of severe-gale SW winds towards WA, just on the periphery of our southern swell window generating a large S/SW groundswell.

The swell should arrive strongly later Monday and peak Tuesday in the 8ft+ range with 10ft bombs at swell magnets across the Ments, 8-10ft+ in Southern Sumatra and 6-8ft around Nias.

After Tuesday's peak the swell should drop through Wednesday, but another large S/SW groundswell is on the cards for later Thursday and more so Friday, but just under the size of Tuesday's swell. We'll review this again on Thursday so check back then for an update.

Mentawai Forecast
Nias Forecast
Southern Sumatra Forecast