RVR is always an observation, never a forecast. Therefore, it wouldn't be in a TAF.
Under EASA when vis drops below 800m, RVR is required. Since you CANNOT have the RVR, because it's not a forecast, but an observation, I would consider the wx below minima. It is permissible to dispatch in such a case, provided two alternates have been selected and fuel for the furthest is carried. Again, this is under EASA, other jurisdictions may have different rules.