IAD controller described Runway 19C incursion event during heavy precipitation; suggesting limited visibility as the likely cause of the event; no formal event recorded.

Narrative

At approximately XA00; during a period of heavy to extreme precipitation; I was working Local Control position #3 (West Local). At this time; I observed an ASDE-X target (with an associated data tag) moving on a high-speed taxiway (Taxiway Y7) toward Runway 19C. I remarked aloud; 'Wow; the rain must be causing target displacement....' I then looked out the window and observed aircraft lights on the high-speed taxiway (Taxiway Y6) exiting Runway 19C. At that moment; I deduced that an aircraft had; in fact; encroached upon Runway 19C. I pointed at the ASDE-X display and exclaimed; 'That guy just went on the runway!' The Ground Controller then stated 'Good catch....' At the time; the visibility was limited due to heavy to extreme precipitation at the airport. Fortunately; due to this heavy precipitation; there were no arrivals or departures at the time of the incident. I do not know what the taxi instructions were for the aircraft involved; so I do not know what the pilots were told to do as how it influenced their taxi route. I do believe that the limited visibility was a factor in the air crew turning onto the high-speed and encroaching upon Runway 19C. However; there are 'wigwag' lights at the hold short lines for all of the runways at IAD. As far as I know; the aircrew did not inform Ground Control that they had crossed the hold lines and encroached upon Runway 19C. At the time of this report; as far as I know; there was/is no formal investigation (tapes listened to; etc.) for this incident.