How large do the largest snowflakes grow to?

Dear Tom,
How large do the largest snowflakes grow to? I heard on the radio that snowflakes 16 inches in diameter were reported in the western states several years ago. Is that even possible?
— T. Standlin, Dubuque, Iowa
Dear T.,
A snowflake that large stretches the bounds of credibility. Guinness World Records lists a snowflake 15 inches in diameter and 8 inches thick as measured at Fort Keogh, Montana, in 1887, as the largest.
Large snowflakes consist of “packets” of many smaller snow crystals loosely clinging together. As they fall through the air, individual snow crystals brush past each other, matting together to form larger snowflakes whose fall speeds increase, thereby accelerating the gathering process. Routine measurements of snowflake sizes are not made but meteorologists believe two-inch snowflakes are credible.