As far as I know doors don't have a weather rating - they do have a fire rating and sometimes have a rating for insulation.
Since you have a varnish I am assuming you have a wood door. How well it stands up to the weather depends on how well you small porch shelters it, the outer coating, the type of wood, and if it is solid or not.
If it is a solid wood door you would have to really neglect it for it to get torn up by the elements. But that being said - test it out. Spray it with the hose. Does the water soak or run off? If it soaks in a little you might want to add some poly. If not don't worry about it. You can also run hose over porch to see how much water would get to it.
Really this is something that shouldn't be a major concern. It should be part of your bi-yearly house checkup. You should be able to notice water damage way before your door isn't repairable.
And to answer your question a wood door isn't water-proof in the purest sense. You would need to get a fiberglass door for that. I would rather have wood rather than steel which in your situation - taking off screen door - could lead to rust on the bottom of steel door.