Nias, Mentawai, South Sumatra forecast by Craig Brokensha (issued Tuesday April 15th)
Best Days: The whole period
This week (Apr 15 - 18)
The coming forecast period is quite exciting with a couple of medium sized + S/SW groundswells due over the coming days ahead of much larger and stronger groundswell over the weekend.
A moderate increase in long-range S/SW groundswell is due today, but a better increase is due tomorrow. This swell was generated last week in the Heard Island region by a vigorous polar low, with a peak in size to an inconsistent but strong 4-6ft due across the Ments, with larger 5-6ft+ surf in South Sumatra.
A reinforcing S/SW groundswell will help steady wave heights into Thursday, keeping similar size waves to Wednesday hitting exposed locations ahead of a more pronounced drop into Friday morning.
Winds in general are expected to be light to moderate from the north-western quadrant over the coming week, favouring south and east facing breaks.
This weekend onwards (Apr 19 onwards)
The first pulse in a series of large S/SW groundswells generated by an amplification of the Long Wave Trough across WA's South West is due to arrive later Friday afternoon, peak overnight and ease slightly into Saturday morning.
The Long Wave Trough steers and supercharges polar fronts up to where ever it is focussed and currently we're seeing a series of vigorous frontal systems falling under the effects of this upper atmospheric weather feature.
The storm track isn't perfect for maximum size across the Mentawai Islands, but we'll still see plenty of swell spreading up radially into us.
The first system was a tight but strong mid-latitude pushing in towards WA's South West yesterday and should kick up 4-6ft waves in the Ments later Friday and early Saturday, with larger 6ft+ surf around South Sumatra.
Of much greater importance is a very large and powerful long-range S/SW groundswell due on Sunday generated by a vigorous and broad polar front that's fired up west of Heard Island, and has already generated a fetch of gale to severe-gale W/SW winds.
This system will maintain its strength through today and tomorrow while pushing north-east towards WA before slipping into Australia's shadow on Thursday morning.
Size wise we're expecting to see large 6-8ft waves across exposed locations in the Ments Sunday afternoon, with larger 8-10ft surf in South Sumatra, while Nias should be a touch smaller and in the 6ft+ range (peaking Monay morning).
The swell should drop slowly through Monday, but a reinforcing S/SW groundswell is due Tuesday afternoon generated by a trailing front behind the huge system responsible for Sunday's swell. This should keep solid waves above 5ft hitting exposed spots before easing noticeably into the end of the week.
Winds on Sunday should tend more variable and remain so through most of the following week.
South Sumatra Forecast