Can you open and close the crack by applying pressure beside or behind the crack, or by bending and twisting the door?
If the crack opens and closes, and the wood adjacent to the crack is sound you may be able to work a water resistant glue into the crack by opening and closing it, squeezing the glue as deep as possible into the crack. Also using strips of paper or pins, push the glue in as deep as possible. Get as much glue as possible into the crack. Clamp tight to squeeze the glue in. Unclamp and repeat. I've used this method a few times. If the glue isn't too thick it can be worked quite deep into the wood. Messy but could save the door for a few more years possibly longer.
LdB