We were there for Hurricane Charley...they closed the parks the afternoon of the day the storm was predicted to hit. Then the parks reopened on time the next day. AK opened late due to much more tree damage.
The parks remain open during rainstorms/thunderstorms tho. I have to say that our trip in August '03 did away with my dd's fear of thunder!! They will close down the outside attractions if there is any threat whatsoever of lightening. but all the inside stuff is open.
M:S is pretty cool. Very enclosed, small area tho. The take-off is pretty intense...you are really forced back into the seat...a terrific face-lift tho!!!