Hello Cheri,
Welcome to planDisney! It's good to see that you are planning ahead for a sporadic spring shower or rainy day. You don't want anything to dampen your fun during your magical vacation!
Umbrellas are allowed inside the parks and can come in handy, although I don't always love maneuvering them through some of the more crowded areas. It may be easier to bring a couple of inexpensive ponchos in your park bag. They take up less space, so you'll be more likely to have one with you even on days that you aren't expecting it to rain. Plus, if you really don't want to get wet on an attraction like
Splash Mountain or
Kali River Rapids, you can throw one on before you ride.
Bonus: if you happen to be at Magic Kingdom in damp day, the Rainy Day Character Cavalcade will put a smile on your face and have you singing in the rain!
Hope this helps, have a magical vacation!
Diane