Whoever told you September in Belize is "chilly" must have been hallucinating, thinking they were in Belize when they were really somewhere up north. It's been known to happen!
I have not been there in September but the general consensus is that is probably the worst month to go, due to (1) hot; (2) days with no breeze at all; (3) unfriendly flying critters; (4) rain rain rain; and (5) possible hurricanes. October is supposedly not such a hot time either.
I have been to Belize over Thanksgiving and had great weather. Still technically the rainy season but tapering off quite a bit...usually (tropical weather is NEVER predictable). I'd say the second half of November is a pretty good bet for at least some nice sunny days...SO if your choice is between September and November, definitely go for November!
Susan Guberman-Garcia, Attorney at Law. Phone: 510-792-2639
Fax/Voicemail:: 510-405-2016 Email: firstname.lastname@example.org