Effective from: Tue 21 May '13, 04:07PM

Forecast prepared by Ben Matson.
Next update Thurs 23/5 5pm.

Previous day's summary:

There's been lots of waves over the last few days, Monday in particular saw a large swell pulse through the Margs region in the afternoon, with early E/SE winds tending fresh southerly through the day as a cut off low tracked across the southern part of the state. This swell has eased steadily today but the morning saw 6-8ft sets at exposed spots with moderate offshore winds. The Perth metro beaches saw clean 2-3ft waves from this swell, whilst wave heights were up in the 6ft+ range in Geraldton.

Forecast period ahead:

Today's easing swell will bottom out on Wednesday and light onshore winds will maintain just-below average conditions in the Margs region (winds should be variable in Perth/Mandurah and offshore in Geraldton).

Thursday sees the arrival of a large, long period groundswell that's going to deliver some the biggest clean surf we've seen in quite some time. Wave heights are expected to ramp up quickly during the day reaching 10-12ft+ in the Margs region late afternoon. Winds should be variable for much of the day so conditions will be relatively clean. Wave heights should reach 3-5ft at exposed Perth beaches late afternoon however the peak may be slightly delayed in the far north (and may not arrive until early Friday). Nevertheless a steady building trend will also be seen here during the day.

Friday sees moderate offshore winds envelop the coast as a large high pressure system sets up camp across the western half of the continent. Very large waves will continue throughout the Margs region, probably 10-12ft+ early morning before easing to 8ft+ by late in the day. A similar trend will play out across the rest of the WA coast, so it's well worth booking in some flexi time as most locations should be delivering excellent surf.

The weekend will herald a more prominent decrease in size although Margs should still be somewhere in the 6ft+ range at first light, before easing to 4-6ft into the afternoon. Winds will maintain moderate strength from the E/NE so it's going to be hard to ignore the surf this day as well.

A trough of low pressure off the West Coast will move close to the mainland on Sunday, bringing about a period of light variable winds ahead of a possible moderate westerly flow in the afternoon (ahead of a reasonable front due late Monday). Surf size should muscle up with the arrival of mid-period energy from a reasonable low in the southern Indian Ocean, but compared to this week's efforts it won't be worth writing home about (maybe 4ft to occasionally 6ft Margs, with 2ft waves in Perth).

Monday will see freshening onshore winds and a moderate swell ahead of a more prominent increase in new long period energy on Tuesday, originating from a deep low tracking south of Madagascar over the next few days. Due to the large travel distance from the swell source to the mainland, the swell will be very inconsistent but we should see some solid sets at exposed beaches at times.

Looking further ahead and a continual progression of cut off lows in the southern Indian Ocean should maintain strong swell activity through the second half of next week and the weekend. I'll update this in more detail on Thursday.