Hi all,
Thanks very much for the replies.
The notes I have says in a TAF, FM is used to show the beginning of a self-contained part in the forecast. All conditions before this group are superseded. So I took it as the weather will change to replace the previous weather complete from this 1800 so the worst case is 3000m visi at 1800. If the weather will change before 1800 (to give 9999) at 1800, I'm sure they would stated BCMG 1719 or something like this?