Beech 1900
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness; Communication Breakdown
Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Human Factors; Weather
I received takeoff clearance for Runway XX at ZZZ. Shortly thereafter; the tower advised the Runway XY RVR had dropped to 2;100 after I had released brakes and was truing up the power settings. After the exchange with the tower what flashed through my mind was akin to 'That's rollout; not controlling; and they didn't drop the prevailing value' - and by then airspeed was alive and I put it out of my mind.It was only as I was climbing on course that I digested further - this RVR report made the ILS XY - my only viable option for returning to ZZZ - below minima. This was the first time I have departed using OpSpec C057. I had gone over the approaches & criteria about an hour before and hadn't imagined/'briefed' for this particular scenario - which I could have anticipated; as the forecast had been amended repeatedly. In retrospect; although I believe my departure was legal and this is not an 'event' - barely - I did have enough time to abort safely; just not enough time to think it through. If I had received this information while taxiing or holding short I think I would have decided against taking the runway. I was not 'gung-ho' and thinking of completing the 'mission.'Contributing factor may be [lack of staffing] in the tower at this hour - he got my release; issued takeoff clearance; and then proceeded to issue a new METAR.I reviewed the sequence of events repeatedly. Aborting the takeoff could have been an over-reaction. In future I will include this type of scenario in my departure brief/self-brief.
Beech 1900 Captain reported not recognizing during takeoff roll that the decreasing runway visibility would not permit a return to the field if necessary.