Surf Dog Championship 2013, Coronado Bay Resort, California, United States

Swell is generated when wind blows consistently over a large area open water, called the wind's fetch. The size a swell is determined by the strength the wind and the length its fetch and duration. Because this, surf tends to be larger and more prevalent on coastlines exposed to large expanses ocean traversed by intense low pressure systems.
Local wind conditions affect wave quality, since the surface a wave can become choppy in blustery conditions. Ideal conditions include a light to moderate "fshore" wind, because it blows into the front the wave, making it a "barrel" or "tube" wave.
The most important influence on wave shape is the topography the seabed directly behind and immediately beneath the breaking wave. The contours the reef or bar front becomes stretched by diffraction. Each break is different, since the underwater topography one place is unlike any other. At beach breaks, sandbanks change shape from week to week. Surf forecasting is aided by advances in information technology. Mathematical modeling graphically depicts the size and direction swells around the globe.
Swell regularity varies across the globe and throughout the year. During winter, heavy swells are generated in the mid-latitudes, when the north and south polar fronts shift toward the Equator. The predominantly westerly winds generate swells that advance eastward, so waves tend to be largest on west coasts during winter months. However, an endless train mid-latitude cyclones cause the isobars to become undulated, redirecting swells at regular intervals toward the tropics.