So b_sta and aimpoint, let's look at this from a cloud viewpoint. It is a moonless night (you can't see the cloud until all the lights below you disappear, then you're in it...oops...too late). If there is any cloud forecast below your lsalt on the arfor, then you cannot depart. During the day you have the option to see the cloud and avoid, but not always possible at night.
If we apply the same logic to visibility, (you don't know you're in the crap until you're in the crap), thence you cannot legally depart if there is any chance vis will be below 5km. TAFs cannot remove or allow selective application of the restriction as they only apply to a 5nm area around the issued location. What about everywhere in between?
A favourable TAF will only remove an alternate requirement at the destination, not make the entire flight legal.