Plan your trip in the dry season. Pay attention to the forecast. Listen to the rangers weather advice when you get your permit. Read the posted forecast info at the ranger station. Unlike 10% at home, a 10% chance of rain in Zion means it will rain, somewhere, on 10% of the park.
That is actually how weather reports are supposed to be interpreted. 20% chance of rain in your area means 100% chance of rain affecting 20% of your area.