Jim:
I live in Austin TX and we normally have several days above 100 degrees in summer, however, we've already had about 20 this year and it's anyone's guess where it will end. Our humidity isn't as bad as Houston but we get a lot of those hottest days that include high humidity because the air is coming off the Gulf of Mexico.
I keep my personal (demonstration) MFT's here in the garage, which is subject to normal ambient air, and they are fine. I've had a KAPEX sitting on the original MFT/3 KAPEX since it came out and it is neat and tight. I also take the pair around and they are pretty darn tough. I could see a freak accident or a mistake ruining them but not normal use. Occasionally, people on the FOG talk about leaving their MFT out in the rain... I wouldn't do that very long and expect the top to last.
I grew up in Houston and it's different than Dallas, Austin, El Paso, or anywhere else in Texas that has many people. Houston is soaked...
When I lived in Houston, I left a bristle dart board on a rain dry, screened-in porch. It had gnarly short growths on it after a couple of really rainy months and we used it occasionally. I can't say exactly how an MFT would react in Houston during one of those spells. In my 25-plus years there, it was not uncommon to have a few months in-a-row of over 95-percent humidity and temp's above 90. I know a couple of people that have more current perspectives though.
Tom
EDIT:
Jim: I think you're probably a lot drier than Austin. You should be good...