Can't keep it out. Best advice I'd give would be to provide a drain hole. Something like we have for signs. See 600.9(D) for size for signs. Probably wouldn't need to be that large diameter for boxes but the concept is the same. If one installs the MC cable with a drip loop this would help. Also look at 225.22 and Exception. Interesting.
The only MC I have installed in wet locations is what we call 'parking deck cable' which is jacketed MC. There area WP connectors for it.
As for standard MC I think George is onto it.
348.12 Uses Not Permitted. FMC shall not be used in the following: (1) In wet locations unless the conductors are approved for the specific conditions and the installation is such that liquid is not likely to enter raceways or enclosures to which the conduit is connected
It goes without saying MC is not FMC but I think we could apply the same logic.
Bob Badger Construction & Maintenance Electrician Massachusetts
I've seen it strung from a house to a garage and I thought for sure this was a violation when I first saw it, then I checked its listing, I was wrong. It's raintight and I can see a few scenarios where this would be very convenient.
Interesting timing on this thread....I just finished wiring an exhaust fan for a dishwasher in a commercial kitchen. It was the type with the 1/2 inch tube for the wire to drop through and was roof mounted. The duct guy promptly told me that I needed to install sealtite and a pitch box. He said he has never seen MC used in his career I peeled back the aluminum armor and exposed the THHN-THWN conductors to him.