I have and store my MES 30 outside with a good cover on it year round. Out side storage has not come with out some well one problem. I have a very good vinyl cover that is water proof. when I used the the Master built controller i would put a bowl over it filled with paper towels just in case it leaked. I did not. During the rainy days the heater element will soak up a little moisture from the air. This September and early October it has been more wet that dry on the East coast. I has soaked up Just enough to trip my GFI receptacle. I used a volt ohm meeter to determine I did not have a short and I just plug it into a non GFI receptacle. After it heats up to over 200 degrees I can pick up my extension cord and use the GFI.
I could close all the air off but then it would get a case of mold.
My buddy DaRicksta lives in Seattle which also gets a lot of rain. He stores his on a hand truck in his garage and does not have any problems. Jted