Plenty of times. Be prepared and have good rain gear. Consider wearing wind pants and NOT jeans, take forever to dry, tend to get and stay damp. I don't like umbrellas since they occupy your hands and have gotten a lot of use out of my cheap long poncho- avoid the short ones if possible.
You can almost always expect rain in Alaska, hopefully it will not ruin an entire day, but after all it is a rain forest. Be sure to pack some rain-gear along with those hats and gloves, but most of all ENJOY! Its absolutely beautiful, rain and all, have fun.