Yup Mfitz is right. Once the land warms the air rises and the cold air off the ocean rushes in to fill the space. Once the sun starts to set the land cools and allows the wind to blow back off shore. Even on days when the weather system itself is bringing the onshore wind if the front moves off the wind can't switch offshore until the land cools back down. That's why when a local storm rolls up mid summer it often goes offshore until the sun comes back out. Mid winter can have offshore all day because both the ocean and the land are cold but once warm spring days move in look for a switch to SE winds by 11am.