Forecaster Blog: Major Indian Ocean Storm Activation

14 Mar 2017 1

Ben Macartney

Chief Surf Forecaster

COASTALWATCH | FORECASTER BLOG

Issued by Ben Macartney, Tuesday, 14 March, 2017

With the southern hemisphere winter starting to draw closer, our thoughts begin to turn to the warmer climes of Indonesia – or perhaps just to the prospect of a new steamer. As if to signal the change of seasons, the southern Indian Ocean is beginning to come alive with storm activity – and it’s fair to say latest model runs are looking decidedly more like mid-winter than early autumn.

A first big, stormy pulse of SW swell lends the focus to the sheltered bays on Wednesday. Photo: Dan Wyer.

A first big, stormy pulse of SW swell lends the focus to the sheltered bays on Wednesday. Photo: Dan Wyer.

Over the course of this week Western Australian coasts will feel the brunt of two large, back to back SW and WSW pulses. Unfortunately both of these events are set to coincide with strong onshore winds – although it’s now starting to look like Thursday morning will offer up a short-window of big, clean conditions.

Tuesday marks the onset of a first, steep rise in mid-period SW swell; generated by a deep polar low that’s currently positioned deep below Indonesia. The low is moving northeast and it’s set to continue on this trajectory into Tuesday; dragging a broad, 40kt SW fetch up towards the West Coast as it goes. As the low slips below WA during the day  broad front will slam the Southwest Coast; leading in near gale force WNW winds and  steep rising SW swell into the afternoon. The bulk of SW groundswell, however, is slated to arrive early on Wednesday, pushing surf-heights up to very large and stormy 10 to 15ft levels before it settles a little into the afternoon.

Latest Wave Tracker runs show a heavy SW pulse bearing down on the Southwest on Tuesday night.

Latest Wave Tracker runs show a heavy SW pulse bearing down on the Southwest on Tuesday night.

While the prospect of a big SW storm-swell isn’t exactly anything to write home about across the West Coast, the next storm system forecast to develop inside Western Australia’s swell window is. The source is an intense extratropical low currently positioned well south of Madagascar. As the storm tracks slowly eastward over the next few days it’s projected to support a severe gale to storm-force (40 to 60kts) W fetch throughout latitudes bounded by 50S and 60S. Initially, this is projected drive deepwater seas and swell to heights of 40 to 50ft southwest of the Kerguelen Islands on Tuesday. The system is then projected to briefly weaken before regaining strength as it moves south of the aforementioned islands on Wednesday and Thursday. This should rejuvenate seas and swell in the vicinity of 40 to 45ft; in turn spawning a large SW groundswell that’s set to spread out across the entire Indian Ocean later this week. 

This Wave Tracker image depicts peak wave period, showing the leading edge of the second pulse of long-period groundswell arriving at peak intervals in excess of 20 seconds on Saturday morning ahead of the bulk of swell filling in on Saturday evening and Sunday morning.

This Wave Tracker image depicts peak wave period, showing the leading edge of the second pulse of long-period groundswell arriving at peak intervals in excess of 20 seconds on Saturday morning ahead of the bulk of swell filling in on Saturday evening and Sunday morning.

Given the storm behaves as current model runs suggest, we could be looking at a rapid rise in large SW groundswell across the West Coast late Saturday, ahead of the bulk of swell holding peak size on Sunday morning; speculatively in the 8 to 12ft range. For Victoria, this points to leading forerunner swells arriving at 18 to 20 second intervals on Sunday, ahead of the bulk of groundswell arriving in the 4 to 6ft range on Monday.

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