When we were there in October 2004, it rained a lot. Our first full day in the park it rained from about 10am to 8pm. Was quite a bummer. Since your trip is an adult trip, it might not be so bad. But unfortunately, our kids got miserable in the rain, so we had to head back to the resort. That was our AK day. We tried to do a make-up day on our last day at WDW for AK, but it rained again. And not just rain.....DOWNPOURS!
It definitely happens, unfortunately there are no guarantees. Be prepared that it might happen (maybe have an alternate weather plan) and just go with the flow.