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

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.
East coasts also receive heavy winter swells when low-pressure cells form in the sub-tropics, where slow moving highs inhibits their movement. These lows produce a shorter fetch than polar fronts, however they can still generate heavy swells, since their slower movement increases the duration a particular wind direction. The variables fetch and duration both influence how long wind acts over a wave as it travels, since a wave reaching the end a fetch behaves as if the wind died.
During summer, heavy swells are generated when cyclones form in the tropics. Tropical cyclones form over warm seas, so their occurrence is influenced by El Niño & La Niña cycles. Their movements are unpredictable. They can move westward as in 1979, when Tropical Cyclone Kerry wandered for three weeks across the Coral Sea and into Queensland before dissipating.