If you think that the day you will be in the parks is just too risky to try because of the rain, you can always try and rebook your Theme Park Reservation and change the date of your ticket. You will be able to do both from your My Disney Experience account online or your My Disney Experience app.

Disneyland generally does not allow you to reschedule or refund tickets specifically because of rain; the parks operate on a "rain or shine" policy. Most attractions at both Disneyland Park and Disney California Adventure are indoors or under cover, and the "show" continues unless there is extreme weather like lightning or high winds. However, because Disneyland tickets now require a Theme Park Reservation, you have the flexibility to "Modify" your reservation to a different date through the Disneyland app or website as long as there is availability for the new date and you do so before the park opens on your original day. If you simply don't show up because of the rain, the ticket itself doesn't lose its value (it remains valid until its expiration date), but you will lose your reservation and may be subject to "no-show" penalties if you are an Annual Passholder.