Best case scenario is nothing bad happens at all and your exterior outlet is a gfci like it SHOULD be, so it simply turns itself off as needed.
Worst case scenario is it isn't gfci protected, water gets inside of it, it shorts out, and either simply trips the breaker (or a gfci upstream from it) or starts an electrical fire.
Good luck not burning your house down! If it was me, id Google "gfci outlet" so you know if you have one or not. If you dont have one, change the outlet to one (super duper easy if your not an idiot.. and even then), and get a cover on it. Your talking about unscrewing and rescrewing a few screws.. no big deal. If yoyr in the US, do black to gold screw, white to silver screw, ground to green screw.. done deal