If there is no TAF, then the area QNH prevails.
Given that the area QNH is guaranteed to be within +/- 5 Mb, and at 30' per Mb, you then add 150' to the minima, unless your chart has a correction factor built in. Check for PANS-OPS.
If the area QNH is going to vary by more than5 Mb, then a speci would be issued.
QED?
