The important part of the forecast here is the inches of precipitation listed, not the percentage! For example, if it says 80% chance of rain, that just means that 80% of the forecast area will get *some* precipitation. A lot of times, especially in the summer, the percentage will be very high, but the accumulation will only be 0.02 inches of rain or something tiny like that - the afternoon storm rolls through for about 15-30 minutes and then is gone again as if it never happened, so just hide out in a gift shop until it clears out.
Every now and then, there are days where there are truly storms all day and several inches of rainfall, when you might want to find alternative plans other than the parks, but those days are much less common than the quick, narrow storms that will sweep through in less than an hour.
So just be sure that you're checking the predicted inches rather than the percentages before you get too concerned!