A term used by Surfline surf forecasters to identify swells with swell periods over 16 seconds between successive waves. These swells are able to wrap into many protected areas because the swell energy extends much deeper below the ocean surface and interacts much more with the ocean floor. They are also able to grow much more during the transition from deep water to shallow water, contrary to short period swells under 15 seconds, which wrap and grow very little during the transition from deep to shallow water. See also groundswell.