high waves during the dog days of summer
Refers to large waves that often appear along the Pacific coast of Japan during the late summer 土用 (doyō) period, typically associated with distant typhoons.
土用波が立つと、海水浴は危険になる。
When the doyō waves rise, swimming becomes dangerous.
This year's doyō waves are higher than usual.
Compound of 土用 (doyō, 'dog days of summer') and 波 (nami, 'wave'). The term describes a well-known seasonal phenomenon.