Originally Posted by txlnnbl

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.