thermalben

Strong surf ahead, but tricky winds

Western Australian Surf Forecast by Ben Matson (issued Monday 3rd May)

Features of the Forecast (tl;dr)

  • Easing swell Tues with great conditions for open beaches and small wave reefs around Margs
  • Another solid groundswell for Margs building Wed PM, peaking Thurs AM, only small on metro beaches
  • Tricky conditions Wed/Thurs with onshore breezes likely at some point, probably as the swell builds/peaks
  • Strong though easing swell in Margs on Fri, improving conditions
  • Large surf for Margs on Saturday, though potentially tricky winds; good surf across metro coasts
  • Another potentially large swell mid-next week

Recap

Strong though easing swells on Saturday maintained 5-6ft sets across the Margs region, with only smaller leftovers in Perth and Mandurah around 1-2ft. Sunday started off around a similar size but a new long period swell built throughout the day, reaching a peak late afternoon with solid 12ft+ sets across the reefs. Much smaller surf was reported throughout Mandurah (3ft+) and Perth (2-3ft), owing in part to a little more south in the swell direction than usual. Conditions were clean with offshore winds. Wave heights eased steadily today, down from 10ft in Margs, 2-3ft in Mandurah and 2ft in Perth, with similarly clean conditions. 

This week (May 4 - 7)

There’s a lot to unpack this week so let’s split up the forecast into two parameters - winds, and then swell.

First up, winds - which will dictate local conditions.

A broad trough off the west coast is pushing against an elongated region of high pressure to the south. This is tightening the pressure gradient across the lower SW region, resulting in fresh E/SE winds (Cape Leeuwin gusting 35kts this morning) though the trough will gradually weaken through Tuesday, allowing winds to slowly ease and veer more north-east, which is ideal for surface conditions across all coasts. 

However, the trough will then move slowly west on Wednesday morning, bringing a freshening northerly wind to locations north of Bunbury in the morning (slim chance for an early window of N/NE winds), tending W’ly in the afternoon. 

Margaret River should see light variable winds early Wednesday but the onshore westerly is due just before lunch and will slowly strengthen into the afternoon - nothing overly strong but probably around 15kts+.

Thursday looks a little tricky wind wise. A small cut-off low is expected to pinch off a frontal system embedded in a vigorous westerly flow at low latitudes, and it'll glance the lower SW region during Thursday. We may see an early window of light to moderate onshore winds through the morning, but it’ll freshen through the day and tend southerly, creating bumpy conditions at exposed spots. 

However, Perth and Mandurah won’t see anywhere near as much influence from this system so a more standard pattern of early light winds and sea breezes is likely here. 

A high pressure system ridging in behind Thursday’s front/cut off low combo will then maintain a synoptic southerly flow into Friday. However, early indications are that the lower SW region may be positioned close to the ridge’s axis, where we may see only moderate-strength winds at best. So, surf conditions could be workable. Emphasis on the 'could'.

Conversely, locations positioned much further north along the Mid West region (i.e. Geraldton) will see a stronger influence from the eastern flank of the high, resulting in fresh S/SE winds developing through Friday. This will probably extend south to about Perth and maybe Mandurah latitudes through the day, though early SE winds are likely here too. 

So, in summary - Tuesday will have the best conditions this week, with Wednesday and maybe Friday a little dicey. Thursday looks below average.

How does this tie in with the expected swell pulses?

Well, we’re already on the backside of Sunday’s impressive event, and Tuesday will continue the easing trend. Open beaches and reefs around the Margaret River region may see occasional 4-5ft+ sets early but expect a decrease into the 3ft+ range by late afternoon. There won’t be much size leftover across Perth and Mandurah coasts.

Wednesday morning will see a low point in size, ahead of a building trend through the day, courtesy of a long period groundswell that was generated by another powerful storm in the Southern Indian Ocean, roughly in about the same spot as the storm that generated Sunday’s swell. However, the fetch was a little smaller, and not quite as strong, so the resulting swell will be scaled back - reaching a peak sometime very late Wednesday or (more likely) early Thursday around 8ft in Margs. Unfortunately, this won’t be well timed, coinciding with the onshore breeze, so quality options will be limited to sheltered locations, which will be much smaller. 

The more favourable winds for Perth and Mandurah will tie in with a much smaller swell increase to 1-2ft and 2ft+ respectively (again, the slight S’ly component in the SW direction negatively influencing surf prospects here).

Friday looks reasonable with wave heights easing back to 5-6ft across the Margs region, and a chance for OK winds at times. Expect small surf and early clean conditions to finish the working week across metro coasts. 

This weekend (May 8 - 9)

Saturday is looking very solid. A powerful low pressure system developing south of Madagascar today will project through our swell window over the next few days, generating large swells with an impressive leading edge in the 18-19 second range that’ll arrive overnight Friday.

Although the initial core of this system was located quite some distance from the mainland - which would ordinarily result in very inconsistent set waves - a series of secondary fronts trailing behind (see below) will be working on the active sea state generated by the primary system, and they'll generate additional large, overlapping swells with slightly lower periods, that will nicely fill in the gaps and boost the perceived consistency in the surf zone. 

This energy is expected to reach a pretty comfortable 10ft+ by Saturday afternoon across the Margs region. The only fly in the ointment is a potential moderate westerly flow, associated with the passage of a couple of fronts immediately south of the mainland, that'll probably just the coast. Model guidance only suggests 10-12kts right now but that’s right on the threshold for workable conditions, and there’s every chance it could end up stronger than this. I’ll keep one eye on it and will update over the coming days.

The good news for Perth and Mandurah surfers is that the storm track is a little more favourable, so we should see more size (3ft Perth, 4ft Mandurah), and conditions will be better with light variable winds as this region won’t be under the influence of the weak fronts. 

Size will then ease steadily into Sunday, though still very large across the Margs region early morning (maybe 8ft+ at dawn, smaller later).

Next week (May 10 onwards)

Long term models show another strong swell later Wednesday/Thursday next week, as the Southern Indian Ocean remains quite active. 

More on this in Wednesday’s update.