The Leonine prayers can be omitted if the celebrant is to perform some other function per my pre-54 Missal, i.e. Procession as was done yesterday for the May Crowning, etc. As far as omitting them for a weather blessing, that has always seemed wrong to me. Leaving Low Mass with no prayers whatsoever left me stunned on more than one occasion, but I'm hoping this specific SSPX chapel has stopped that practice as I've not seen it in awhile.
With the Rogation Days being this week, I don't see anything wrong in omitting the Low Mass Prayers in favor of the Rogation Day prayers but that's my opinion.