Deb,IMHO I think biscuits would be adequate for this application. Suggest you use an exterior glue, maybe even one of the polyurethane glues. Though the cleanup can be a pain, they will fill any minor voids in the joint and help prevent future water intrusion.
Method I use for wide panels is to not plane the boards down to finish thickness at first. I then plane these mini panels down to the final thickness. You can now glue these mini panels together using whatever method you wish.
What We Hear Most & Agree With Is This …. Biscuits joints serve best as a quick and easy way to keep glue-up parts in alignment, and that they add appreciable pull-apart to strength joints that would be otherwise too weak to stand on their own – like butt joints and miter joints. We’re guessing that most woodworkers will find more use for a biscuit joiner, with its ability to invisibly align and secure joints so quickly. Doweling also makes for a stronger joint, and if you own a drill, it’s less of an initial investment: You just need a doweling jig and a few drill bits.
biscuit joiner vs tongue and groove Related Question:
Are biscuit joiners worth it?
Biscuits joints serve best as a quick and easy way to keep glue-up parts in alignment, and that they add appreciable pull-apart to strength joints that would be otherwise too weak to stand on their own – like butt joints and miter joints.
What are the disadvantages of a biscuit joint?
Biscuit joints do not stand up well against other joinery methods. We won’t spend any time comparing with mortise and tenon joints because all other joinery methods pale in comparison when measuring strength. But, as an example, the use of dowels as opposed to biscuits is a step up in strength.