From computer models.

Damn you Bill Gates.

Nah they get it from an official source
The wa bureau of meterology
This guy is the head forecaster!

Not whinging, just curiosity, the bureau of meteorology boating forecasts for the last few days have consistently predicted lighter winds than Seabreeze. Although I have just now noticed that the explanatory guide to the graphs states:

"The Middle of the arrow is where you should read the wind speed from the scale at the sides. Picture the arrow as being like a compass needle, where it rotates around a central point of balance."

Well I always thought it was the top. That probably explains a large part of my confusion

Bails said..
Is your "Perth" report based on Rottnest or something?

not sure about seabreeze, but pretty sure the BOM system works something like this

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Following Windxtasy's ideas the same is true for the forecasts for Albany. It seems that most of the the forecasts may be reasonably accurate for the airport where the weather station is but the harbour can be quite different. The 2 spots are 10 km apart at significantly different elevations and are affected by very different geographical features.
Recently I sailed in an eastely on the harbour, I left the harbour when it was dead calm and at the airport (5 mins drive away) it was about 18kts south westerly. No forecast is going to be able to accurately predict this for both locations.
The closest forecast for the harbour is Willyweather but as always is a prediction of wind which will not always (often?) be right.

It can be a bit hit and miss and is only meant as a guide.

Weatherzone.com.au isn't too bad. not much detail but reasonably accurate 9am 3pm