Don't panic. There's almost always a chance of rain there. It rarely lasts long. Buy yourself some cheepo ponchos from the $1 store before you leave and keep them with you while in the parks. Only a handful of rides will close during rain, but they are things you wouldn't want to ride in the rain anyway.
Don't let the forecast change your plans. Just adjust your touring plan as needed while in the park. Hopefully you have your touring plans already set up. Just make sure to know how to use it on the Lines app so you can change or re-optimize your plan if/when needed.