There's a type of MC cable that has a PVC jacket on top of the armor, Southwire calls it Parking Lot/Deck Cable.
Special connectors must be used with it, as well. The connectors have a gland nut, much like the gland nut on a cord grip.
From 2011 NEC, 330.10(A) :
(11) In wet locations where any of the following conditions are met:
a. The metallic covering is impervious to moisture.
b. A moisture-impervious jacket is provided under
the metal covering.
c. The insulated conductors under the metallic covering
are listed for use in wet locations, and a corrosion resistant jacket is provided over the metallic sheath.
Normal MC does not fit the bill