Certainty vs Possibility

If you say something is not going to happen, are you saying that it is impossible (0% chance of happening)? Is there a direct correlation between certainty and possibility? (If you're certain something is not going to happen, you think it is impossible?) Example:"Im going to propose to my wife.""She's not going to say yes." Does this mean the person thinks it is impossible that the person says yes? Is he saying that it is impossible? If he does, isn't he wrong as it is possible (she does have the ability to say yes).