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.