California weather is inherently fickle.
That said, prepare for the worst, hope for the best.
Personally, I love
Disneyland in the rain, and did a torrential downpour solo adventure on our last trip while the rest of the family rested in the hotel:
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Most stuff runs in the rain. Alice, Teacups, RSR, and Luigis close for just about any precipitation. The Treehouse will also likely close (I know Tarzan's always closed in the rain, I assume it will be the same for Adventureland Treehouse).
Most everything else will run unless there are thunderstorms (rare) or the downpour is really strong (also rare)
You'll want something weatherproof on your feet.
And you'll want something weatherproof on your bum. Seats on many attractions will be wet, so you'll want to bring at the least something to sit on, or weatherproof pants to wear.
If your hotel is close, a mid day change may be in order. If your hotel is not close, bring clothes for a mid day change and store them in a locker.
Allow yourself a slower pace and take some time to watch the ducks frolic. Be aware that restaurants will often close their outdoor seating when it is wet, so consider this when checking in for dining reservations as your wait may be a little longer. Alternatively, if you are doing counter/quick service, consider eating at times on either side of the lunch/dinner rush to help with the seating. As with any day, it works well to have one person order/pick up the mobile order and another person try to snag seating.
Remember also the hack that if you do get caught unexpectedly and buy the disneyland parkas, if it gets torn on an attraction or otherwise during the course of your day they will typically replace it free of charge wherever they are sold.
And take the opportunity to check out may of the great indoor attractions that are often overlooked for a nice respite from the rain (Great moments with Mr. Lincoln, the Main St. Cinema, The animation building in DCA, the Red Car Trolley, etc.)
A rainy day at the park beats a sunny day almost anywhere else. Have a great trip, rain or shine!