NOAA Moving Swell Model

If you ever have a chance to fly over a solid swell, you can
watch wave groups travel, and waves moving within the group. If
the groups are well-separated, and the swell clean, these groups
become sets of waves at the beach. The number of waves per set is
based on how "fully developed" the seas become. If the wind is
consistent in direction and velocity for a long time, over a
large area, the wind will create lots of single wave travelling
at the same speed. As they synchronize with one another, the wave
groups will increase in the number of waves per group. With
particularly well developed seas, wave groups with 10+ waves can
result. More often, there are fewer waves per group, and the
storm has variable winds from hour to hour and mile to mile which
prevent the seas from reaching a larger wave group size.

NOAA Moving Swell Model