It's pronounced "GEEN". The folks who are confused aren't used to seeing "Jean" Spelled "Jeanne".
I interviewed with a Jeanne once and fortunately, I called her "GEEN" instead of "GEENEE" - I heard that she would immediately end the interview with anyone making that faux pas.
My mother's sister-in-law Jeanne pronounced her name ZHAHN - Tante Jeanne to me.
If the NHC wanted a GEEN, why not Jean?
-------------------- From Brevard's Barrier Island
28°08'56"N; 80°35'11"W