Special Weather Statements will still be issued for weather events that are more than 6 hours away. Meteorologists use pressure gradients in many ways, but primarily for determining wind speed and direction. Without going into too much detail or technicalities, in the upper atmosphere, air generally flows clockwise around areas of high pressure and counter-clockwise around areas of low pressure. However, at the surface, due to friction and other smaller scale effects, air flows more from high pressure to low pressure. In Southern California, pressure gradients are critical in forecasting the weather.
Onshore flow refers to air flow from the water onto the shore.
In other words, west to east. A majority of the time, the pressure gradients in Southern California are onshore. Well, that would take a little while to explain in detail. However, to put it simply, climatologically speaking, there is an area of high pressure in the eastern Pacific and low pressure in the desert southwest. The low pressure in the desert intensifies during the day because when the air heats up, it rises, causing the air pressure at the surface to lower.