Bali and Mentawais forecast​​​​​​ by Craig Brokensha (issued Thursday 19th July)

Best Days: Every day over the coming period for experienced surfers

This week and next (Jul 20 - 27)

Bali: A large new long-period and inconsistent S/SW groundswell filled in yesterday, the precursor to a much larger and stronger swell today that's providing oversized sets across exposed locations, and pumping waves for the Rip Curl Padang Cup.

This swell is due to start easing later today, but more so tomorrow, easing off from the 10ft range on the sets across exposed south swell magnets.

A reinforcing long-period S/SW groundswell will slow the easing trend into Saturday, produced by a severe low forming on the back of the progression linked to today's swell, generating a short-lived fetch of storm-force W/SW winds in our southern swell window.

This swell will be acutely more south with a touch less size getting into the protected Bukit reefs, but exposed south facing breaks look to still offer 8ft+ sets, easing through the day and smaller into Sunday.

Easing E/SE trades are still on the cards over the coming days, weakest through the weekend resulting in morning variable winds lasting longer. We'll then see the trades strengthen and pick back up through early next week and persisting through the rest of the period.

Our new pulse of long-period SW groundswell for Monday is still on track, with a mid-latitude low forming south-east of Madagascar through this week, generating a good fetch of gales, with this swell due to fill in Monday and provide 6ft sets through the day (most likely afternoon).

Of much more significance is the developments into Tuesday and Wednesday.

As touched on in previous forecasts and swell analysis articles, we're set to see another strong and protruding node of the Long Wave Trough forming in the Indian Ocean.

This node is expected to move very slowly east and effectively stall just west of Western Australia while reaching a peak in intensity, directing back to back storms more northward towards Indonesia, rather than eastward under Australia as was the case with the activity earlier this week.

With the node protruding further north than normal, we'll also see these storms push much further north in latitude, reaching a line drawn west from Ningaloo. This all points to a more consistent swell event for Indonesia, along with less size loss due to swell decay.

Looking at the structure of the frontal progression, and a mid-latitude storm has created an active sea state for a vigorous polar front to generate an expansive, slow moving fetch of severe-gale to storm-force winds over.

The fetch will remain near stationary while a secondary intensification will project storm-force winds north-east at a similar speed to the swell it's producing (known as a captured fetch) an over a very large open ocean sea state.

The biggest swell of the season, and even bigger than the June 2015 Muzza swell is expected across eastern Indonesia, building through Tuesday and likely reaching 10-12ft+ late in the day across exposed breaks, peaking Wednesday morning to 12ft to possibly 15ft.

Beyond this there's plenty more large to extra-large surf to come, but more on this Tuesday.

16 day Bali Forecast Graph
16 day East Java Forecast Graph
16 day Sumbawa Forecast Graph

Ments: We're currently seeing over-sized S/SW groundswell filling in on the coast and we'll see this swell peak later today, before easing back from 8-10ft at south magnets tomorrow morning, smaller into Saturday, bottoming out dawn Sunday.

Winds should ease off through tomorrow and become more variable, remaining so into Saturday before kicking in again from the SE-S/SE on Sunday.

The SW groundswell from the mid-latitude low is expected to arrive through Sunday afternoon across our region and kick back to 6ft on the sets, and holding a similar size through Monday.

The S/SW groundswell for Tuesday/Wednesday will be a bit more south than the Muzza swell seen in 2015 but still come in over-sized across our region, at this stage kicking to 10-12ft by dark on Tuesday and peaking Wednesday morning to 12ft+.

Winds look to ease off through Tuesday especially across the northern part of the region, and become more variable on Wednesday, but if not have a SE-E/SE tendency. More on this Tuesday.

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