Narrative:

I was working the final sector. Weather conditions were VFR; but beginning to deteriorate. Simultaneous monitor checks had been completed and monitors were standing by for the start of simultaneous ILS's. Visual approaches where in use for downwind aircraft; ILS approaches in use for straight in aircraft. A crj; on the downwind reported the airport in sight and was cleared for a visual approach to runway 34L. Another air carrier was on the ILS to runway 34R. As the crj turned base to final the pilot reported he had lost sight of the airport. I cleared the crj for an ILS approach and immediately switch him to the tower frequency for ILS monitor. At this point the other air carrier was approximately 800 feet below and two miles in front of the crj on the ILS final for runway 34R. Separation was lost at the moment the crj lost sight of the airport. I'm unsure if initiating a go around would have been more or less safe than the action I took. In retrospect; I should have been more aware of the rapidly deteriorating weather; and planned on ILS approaches for all aircraft. I recommend a better awareness of rapidly changing weather conditions; in this case; a cold front was arriving into the area from the northwest. Until its arrival; weather conditions had been VFR with all aircraft accepting visual approaches without problem. Better communications to and from the tower to provide updated weather information; would help prevent a similar situation.

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Original NASA ASRS Text

Title: S56 controller experienced a loss of separation during the transition from visual approach procedures to IFR simultaneous approaches when an aircraft on a visual approach lost sight of the airport and was issued an ILS clearance.

Narrative: I was working the Final sector. Weather conditions were VFR; but beginning to deteriorate. Simultaneous monitor checks had been completed and monitors were standing by for the start of Simultaneous ILS's. Visual approaches where in use for downwind aircraft; ILS approaches in use for straight in aircraft. A CRJ; on the downwind reported the airport in sight and was cleared for a visual approach to Runway 34L. Another Air Carrier was on the ILS to Runway 34R. As the CRJ turned base to final the pilot reported he had lost sight of the airport. I cleared the CRJ for an ILS approach and immediately switch him to the tower frequency for ILS monitor. At this point the other Air Carrier was approximately 800 feet below and two miles in front of the CRJ on the ILS final for Runway 34R. Separation was lost at the moment the CRJ lost sight of the airport. I'm unsure if initiating a go around would have been more or less safe than the action I took. In retrospect; I should have been more aware of the rapidly deteriorating weather; and planned on ILS approaches for all aircraft. I recommend a better awareness of rapidly changing weather conditions; in this case; a cold front was arriving into the area from the northwest. Until its arrival; weather conditions had been VFR with all aircraft accepting visual approaches without problem. Better communications to and from the Tower to provide updated weather information; would help prevent a similar situation.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.