The cold wave of January 1977 produced the only known trace of snow in the greater Miami area of Florida ever reported. It occurred following the passage of a strong cold front, in combination with a high-pressure area situated over the Mississippi River Valley. As a result, cold air moved far to the south across Florida, causing both snow flurries and record-low temperatures. Most notably, the weather system brought snow flurries (seen in the air, but not on the ground) as far south as Homestead on January 19. No snow had ever been reported in southeastern Florida before or since.

Source: Wikipedia