I would be inclined to use the maximum wind speed which has been recorded historically for fifty (50) years at the location of the plant. For a measure of comfort, I would then use an additional ten (10) percent.
This would represent the worst case scenario in your model and drive the design to handle for maximum loads.
Just my two (2) cents worth.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada