I suspect that you're confusing visibility with cloud base.
BKN110 is telling us our cloud base is 1,100 feet. 6000 is telling us our visibility is 6km. Two very different things.
First things first, BECMG indicates a permanent change lasting 2 hours or more is forecasted to occur within the time period specified ... BECMG 2521/2522 simply means a permanent change in conditions between 2100 and 2200, our new cloud base is gonna be 1,100', the exact time of this change is unknown, just that it's between 2100 and 2200.
Now that's the cloud base (it also happens to be the ceiling considering its BKN) ... the visibility is something else as mentioned.
A TEMPO is a change in the weather, but it's not a permanent change. It's a change lasting a minimum of 30 mins but no greater than an hour. So basically TEMPO means there's a change in the weather for at least a half an hour to 1 hour during the time period specified.The key is realising that outside of the TEMPO time specified, the conditions are to be the same as they were previous to the TEMPO time period, they revert back (or never change if they weren't included in the list of things to change beside the TEMPO notice).
As you correctly alluded to, the 6000M visibility at the start applies unless we're told its changed by a new visibility being mentioned. The only time that occurs is during the TEMPO notice of a change to 800M, however as above, this is only a change lasting 30-60 mins at some point in time within the specified time period. Outside of that, the 6000M from the start prevails. It would only change if a new visibility was offered after a BECMG notice.
Last edited by Officer Kite; 4th Feb 2018 at 07:19.