If it is a portal, the impassable pass being clear is kind of like opening the gates of your fortress, especially if there was a magical Ancient Widow side to the snows that keeps nastiness on the other side.
High Pass is usually clear for at least part of summer. The deal with the Ice Witches was to open it up earlier so there was a long enough window for the Expedition to be able to leave and return through it in the same year.