Western Australian Surf Forecast by Ben Matson (issued Tuesday 11th May)

Features of the Forecast (tl;dr)

• Large surf in Margs on Thurs, with offshore winds
• Easing size Fri/Sat, clean with E'ly winds
• Inconsistent mix of new groundswells Sun, continuing offshore winds
• Plenty of swell next week, most size due around Thurs

Recap

Saturday’s saw large surf around the 10ft mark across the Margs region. Early offshore winds provided clean faces (on a lumpy but improving ocean) ahead of moderate onshore breezes that kicked in late morning. Sunday remained bumpy under the prevailing onshore, however surf size didn’t ease quite as much as expected - buoy data showing no discernible trend - and therefore Monday came in slightly above forecast expectations, with sets in the 5-6ft range, with clean conditions under a light offshore breeze. Metro beaches picked up some OK waves on Saturday with the new swell but there wasn’t much leftover on Sunday or Monday. Today we’re back to small leftovers with freshening northerly winds in the south, more E/NE tending N/NE across metro coasts. 

This week (May 12 - 14)

The broad scale forecast trend is relatively straightforward, as we have one significant swell event due Thursday, and then a steady easing trend for three days thanks to a blocking pattern. 

A cold front associated with the low that is generating Thursday’s swell will clip the coast on Wednesday morning, swinging winds to a fresh S/SE - it’s due into Margs between midnight and 2am. With easing size from today there won’t be many options on hand; a local windswell will build during the day but it’ll be low quality. 

Surf size will remain very small across the metro beaches, and apart from a brief window of light winds - the change is due into Mandurah between 6am - 8am, and Perth about half to one hour later - we’ll see gusty S/SE winds across the coast. 

The bulk of the groundswell is expected to push through very late Wednesday ahead of a peak in size on Thursday, and should max out around 10ft across the Margs region. Even better, the models are holding steady with their expectation for a small trough to pinch off the front in the Indian Ocean, forming a closed low about west of Geraldton through Thursday. This won’t generate any swell for us but it will quickly steer Thursday’s winds around to a fresh E/SE, then E’ly.

As for size along the metro beaches, we should see Perth pushing up into the 2-3ft range, with 3-4ft sets across exposed Mandurah spots, with similar winds out of the eastern quadrant.

The cut off low in the Indian Ocean will remain slow moving for a few days, and will maintain moderate E’ly winds across all coasts for the next couple of days as wave heights steadily ease - Friday morning should see occasional 6-8ft leftovers, down to 4-6ft by the afternoon. Expect 2ft surf in Perth and 3ft in Mandurah, easing throughout the day, equally clean with offshores.
 
This weekend (May 15 - 16)

A further easing trend on Saturday will be accompanied by regionally moderate E’ly winds, tending NE into Sunday as an approaching front captures the (by then, disintegrating) low, both moving towards the mainland.

Saturday’s size should still manage 4-5ft sets across the Margs region with very small leftovers at metro beaches. 

Sunday however has two new new long period groundswells on the way. The first has already been generated over the last few days by an impressive polar low well below South Africa, however the enormous travel distance will see significant wave decay by the time this energy reaches the mainland. Swell direction should be straight SW from this source. 

The second swell will arrive concurrently, having been generated by a rapidly deepening low pressure system near Heard Island on Thursday (actually, related to the first low, by way of a secondary front that’ll spin off the main system and then evolve into its own separate low). This intensifying low will be steered south-east underneath the blocking cut-off low (mentioned above, responsible for our 4-day E’ly pattern) but impressive core winds around the low should generate peak swell periods in the 18-19 range from both sources, though the second swell will have been generated closer to the mainland and so will contribute a little more size.

However! The storm track isn’t great - the low will track SE as it reaches peak strength - and the resulting direction will be more of a sideband S/SW at the coast, which won’t get into most breaks as efficiently as a SW swell.

So, the models are calling one groundswell event at 17-18 seconds from late Saturday onwards, though I think they’re probably combining the two swell trains. As such, I need to be a little cautious with size estimates, and probably aim for 5-6ft+ across the Margs region. The periods suggest more size, and this is certainly possible at those deepwater reefs well angled for southerly swells, but I'm just not 100% on the orientation of the main fetch of the second source (see below).

I don’t think there’ll be much size from these swell across the metro beaches, maybe 1-2ft sets in Perth and 2ft+ in Mandurah. Expect long breaks between waves at all coasts.

Next week (May 17 onwards)

The Southern Ocean conveyor belt is looking active for next week, though the storm track will remain active mainly west form Heard Island, which will keep a lid on size. As such, there are no major swell events on the cards - just a bunch of fluctuating days in the 4-5ft range throughout Margs, with one day that’ll likely push up into the 8ft range (currently pegged for Thursday). Local winds look to be favourable, generally light for most of the week apart from Monday which may see a northerly as the weakening remnants of the cut-off low push on to the coast.

More on that in Thursday’s update. See you then!