Narrative:

I had been on local for about an hour. During that time; weather had changed rapidly from good VFR to low IFR as a low overcast marine layer moved in quickly. It was still reported clear to the east of the field; and I could still see the runway 21 final from the tower. Within the few minutes leading up to this event; ground control/FD and the controller in charge had generated a special observation that showed the airport IFR and passed it to concerned parties. Aircraft Y exited lax airspace and requested a transition northbound; in VFR conditions; via the I-405 freeway; which intersects the runway 21 centerline at about a 3/4 mi final. That aircraft was at 1300 ft. Around the same time; I received the hand off on an arrival; aircraft X. Scratch pad coordination at the time of the hand off showed this aircraft assigned 'V21' (visual approach); but that would not have been appropriate with the newest weather observation. This coordination notation was later removed; indicating that the aircraft would arrive on the VOR-a approach. I used the stars ptl (predicted track line) feature (vector lines) to determine that aircraft Y would get across the runway 21 centerline in time; and I advised aircraft Y that they would be crossing my final in advance of an arriving jet.aircraft X subsequently checked on late; high; and with faster ground speed than I would normally anticipate at that stage. Step down altitude at wurud is 1540 ft (a 3 mi final); but I recall aircraft X crossed that point above 3000. I advised aircraft X of the traffic (now 1/2 mi south of the centerline at 1300 and heading northbound) and issued landing clearance. Just afterward; I observed aircraft Y passing the extended centerline; as aircraft X was on a 2 mi final; also noting that they had arrested their descent at 1900 ft. I advised aircraft X that their traffic was clear of final. On a 1 mile final; aircraft X reported missed approach; still at 1900 ft and passing 1 mile south of the diverging traffic at 1300 ft. I issued missed approach instructions in accordance with the sct LOA and performed required coordination. I switched aircraft X to sct malibu sector. Before going over; the pilot informed me he had received an RA with the traffic.aircraft X returned later and landed via the RNAV GPS 21 approach. I asked the pilot for some more information about the RA situation. I had guessed; and the pilot confirmed; that they were unexpectedly 'dumped' on the VOR-a approach. The pilots had briefed and were expecting the RNAV GPS 21 approach. Per local procedures; smo ATIS should advertise both the VOR-a and RNAV GPS 21 approaches under these conditions. Sct burbank area may issue either approach. I did not record or listen to the ATIS at the time because that is not a local control responsibility.I would not have approved aircraft Y's I-405 transition with an aircraft arriving runway 21 on [a] day where weather was not a consideration. However; this was a complex decision in that aircraft Y had a need to maintain VFR east of the cloud layer. My judgment was correct that this aircraft would pass the centerline in advance of the arriving jet. Plus; I realized that aircraft X would be out of position somehow; but I admit I did not know what the pilot's actions would be. Aircraft X's higher-than-anticipated ground speed and trajectory ended up reducing the separation. Ideally; aircraft X would have been permitted to fly a stable approach and would have passed well beneath and behind the traffic; but the actual situation generated a TCAS RA because the device projected the aircraft descending on top of the traffic.I think the important lessons here are:1. Knowingly anticipating that an aircraft will have an unpredictable/unusual flight path should be a case for more positive separation. I could have asked aircraft Y to fly eastbound until aircraft X was in sight as weather would not have been a factor to the east. My plan a (with aircraft Y expeditiously crossing final) was very conditional; I realize.2. Sct's handling of the flight; aircraft X was never in a position to land and a missed approach/go-around was likely regardless of the crossing traffic. This underscores the importance of adequate vectoring to allow an aircraft to fly a stable approach. Also at issue was possible mis-coordination or miscommunication regarding the approach to be flown by the aircraft X crew. The weather changed very rapidly; but I believe our tower team made every attempt to provide the latest information to the arrival controller.the RNAV GPS 21 approach has been commissioned for [a short period] at the time of writing. It's hard for me to say what happened on that end; but I suppose the pilots may have briefed the RNAV approach as it would be preferred in general (lpv minimums); but the arrival controller may not have had the comfort and familiarity with that approach to issue it on very short notice.

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Original NASA ASRS Text

Title: SMO Tower Controller and inbound jet pilot reported an airborne conflict between a jet arrival and a helicopter transitioning the area when weather deteriorated.

Narrative: I had been on Local for about an hour. During that time; weather had changed rapidly from good VFR to low IFR as a low overcast marine layer moved in quickly. It was still reported clear to the east of the field; and I could still see the RWY 21 final from the tower. Within the few minutes leading up to this event; GC/FD and the CIC had generated a special observation that showed the airport IFR and passed it to concerned parties. Aircraft Y exited LAX airspace and requested a transition northbound; in VFR conditions; via the I-405 freeway; which intersects the RWY 21 centerline at about a 3/4 mi final. That aircraft was at 1300 ft. Around the same time; I received the hand off on an arrival; Aircraft X. Scratch pad coordination at the time of the hand off showed this aircraft assigned 'V21' (visual approach); but that would not have been appropriate with the newest weather observation. This coordination notation was later removed; indicating that the aircraft would arrive on the VOR-A approach. I used the STARS PTL (Predicted Track Line) feature (vector lines) to determine that Aircraft Y would get across the RWY 21 centerline in time; and I advised Aircraft Y that they would be crossing my final in advance of an arriving jet.Aircraft X subsequently checked on late; high; and with faster ground speed than I would normally anticipate at that stage. Step down altitude at WURUD is 1540 ft (a 3 mi final); but I recall Aircraft X crossed that point above 3000. I advised Aircraft X of the traffic (now 1/2 mi south of the centerline at 1300 and heading northbound) and issued landing clearance. Just afterward; I observed Aircraft Y passing the extended centerline; as Aircraft X was on a 2 mi final; also noting that they had arrested their descent at 1900 ft. I advised Aircraft X that their traffic was clear of final. On a 1 mile final; Aircraft X reported missed approach; still at 1900 ft and passing 1 mile south of the diverging traffic at 1300 ft. I issued missed approach instructions in accordance with the SCT LOA and performed required coordination. I switched Aircraft X to SCT Malibu sector. Before going over; the pilot informed me he had received an RA with the traffic.Aircraft X returned later and landed via the RNAV GPS 21 approach. I asked the pilot for some more information about the RA situation. I had guessed; and the pilot confirmed; that they were unexpectedly 'dumped' on the VOR-A approach. The pilots had briefed and were expecting the RNAV GPS 21 approach. Per local procedures; SMO ATIS should advertise both the VOR-A and RNAV GPS 21 approaches under these conditions. SCT Burbank area may issue either approach. I did not record or listen to the ATIS at the time because that is not a LC responsibility.I would not have approved Aircraft Y's I-405 transition with an aircraft arriving RWY 21 on [a] day where weather was not a consideration. However; this was a complex decision in that Aircraft Y had a need to maintain VFR east of the cloud layer. My judgment was correct that this aircraft would pass the centerline in advance of the arriving jet. Plus; I realized that Aircraft X would be out of position somehow; but I admit I did not know what the pilot's actions would be. Aircraft X's higher-than-anticipated ground speed and trajectory ended up reducing the separation. Ideally; Aircraft X would have been permitted to fly a stable approach and would have passed well beneath and behind the traffic; but the actual situation generated a TCAS RA because the device projected the aircraft descending on top of the traffic.I think the important lessons here are:1. Knowingly anticipating that an aircraft will have an unpredictable/unusual flight path should be a case for more positive separation. I could have asked Aircraft Y to fly eastbound until Aircraft X was in sight as weather would not have been a factor to the east. My Plan A (with Aircraft Y expeditiously crossing final) was very conditional; I realize.2. SCT's handling of the flight; Aircraft X was never in a position to land and a missed approach/go-around was likely regardless of the crossing traffic. This underscores the importance of adequate vectoring to allow an aircraft to fly a stable approach. Also at issue was possible mis-coordination or miscommunication regarding the approach to be flown by the Aircraft X crew. The weather changed very rapidly; but I believe our tower team made every attempt to provide the latest information to the arrival controller.The RNAV GPS 21 approach has been commissioned for [a short period] at the time of writing. It's hard for me to say what happened on that end; but I suppose the pilots may have briefed the RNAV approach as it would be preferred in general (LPV minimums); but the arrival controller may not have had the comfort and familiarity with that approach to issue it on very short notice.

Data retrieved from NASA's ASRS site 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.