Originally Posted by
slough
If you have a destination and two alternates, one of those alternates being good enough weather to be listed as the only alternate:
Why not just list your destination and the good weather alternate and forget about the marginal weather alternate? That is what is confusing me.
If 3585 is used, the destination weather isn't only low enough to require an alternate, it's also too low to dispatch a flight to under normal circumstances (even with an alternate). You can't dispatch a flight under 121 to a destination unless the weather is forecast to at least meet approach minimums (e.x. 1/2 mile for a typical ILS). If the main body of the TAF calls for 1/4 mile visibility, you couldn't dispatch a flight to that airport even with a suitable alternate. But if that 1/4 mile visibility is only forecast in a conditional part of the TAF (such as TEMPO, BECMG, or PROB) then you can apply 3585 and dispatch the flight provided the soft and hard alternates are added.