If i understand your question correctly then this is what i know...
The forecast weather at your destination must be at or above the airfields published landing minimas for the serviceble navaids at ETA(+/-) 1Hr. If the weather conditions are below this, you can still continue to another destination via your original destination and be recleared to your original destination over head or en route.
The designated alternates have to have the following forecast weather requirements for you to dispatch the Flt.
*If the Approach aid avail is CAT II/III :- Weather required CAT I.
*If the Approach aid avail is CAT I :- Weather required Non precision minima.
(and so on......)
Also as a note you can use the VIS/RVR conversions for planning purposes.
ie. at night you can assume RVR will be reported vis times 1.5-2!
Would give more info but have no time right now, sorry. hope this helps...
