Because the forecast data corresponds to data not actually at the beach but further out to sea, under most circumstances the Swell Height forecasted for a location will actually be smaller when it hits your local break. Among the main factors that effect the relative size of nearshore waves are the swell direction and period. A higher swell period means the average wave height will tend towards a higher height as the swell has more energy. Therefore, the Swell Height should be seen as an upper bound, which is nearer to being reached on a higher period swell.
It is important to consider that the direction swell is heading in determines how much energy may or may not be dissipated as it hits your beach. If the swell has to wrap a wide angle to reach your beach because of the swell direction then you can expect the swell height at the beach to be smaller than if it hits it directly head on. The higher the period of a swell, the more successfully it can wrap around headlands. Usually the more a swell has to wrap, the more organised the sets will be.
Swell Period is key. You may be able to tell from the previous explanations of the importance of Swell Period to a wave’s characteristics as it reaches the beach. Swell period is literally the time between successive peaks of a swell. A high period swell has the following characteristics over a low period swell: wider more spaced out waves; a higher energy swell with this the more likely a decent wave will get through to your beach as it overcomes local currents and wind; a faster wave; more wrapping potential. A high period swell can be critical at certain spots for making them work. For example, a high period swell is required to make certain beaches like Avon Beach on the South Coast work as a wave riding location. This is because the wave has to often wrap around a point to overcome strong local tide conditions and retain energy after travelling over shallow shelving shelf.