Narrative:

I was working as the pm. During the descent through 8;500 ft on approach into ZZZ; I was advised that runway xxr was being closed for storm passage and we'd be using runway xxl. As I was preparing the FMS for runway xxl; I looked up and noticed that we were flying directly to a large storm cell. I glanced over at the pilots mfd and noticed his weather radar was set at 100 miles; in spite of ZZZ being literally 15-20 miles away. I turned it down to 12.5 miles and then I asked approach if they could give us vectors around the rapidly approaching cell that was now at about 11:30 from our position. He'd replied he was trying to get a hold of us to fly south; avoiding the cell on the east side of the cell; but we'd not responded. That was when I started to suspect the radio was beginning to have issues. I immediately asked for a right 270 to avoid the cell and proceeded to sync the heading bug and placed the flight guidance in 'heading mode' while turning the heading bug to a north heading to avoid the cell. At this point; approach advised us that we'd approach the field from the north and get vectors for runway xxl. The captain then went into his FMS and began to finish programming the box for runway xxl. I then noticed the cell over the airport and reran the performance numbers for runway xxl and yyr (the opposite direction); anticipating wind gusts and direction change. At this point; the lav smoke warning alarm came on and I heard the captain call for the QRH for that alarm. Being this close to the ground; I immediately called the fas and asked them to check both lavs for indication of smoke or fire and then ran the QRH; which said to consider diverting to nearby airfield. Shortly after my radio page went blank on my side.at this point; approach advised of runway change to runway yyr and that another aircraft in front of us had reported windshear loss of 20 kts; and then gave us vectors to set up for the runway yyr approach. I then set up the box for runway yyr; briefed the approach and advised them that we would need fire trucks to roll for us and how many souls on board. The fas called me back and informed there was not smoke in the aft lav; but fog. I advised approach what the flight attendant's reported and still wanted the truck to be there; just in case. I then noticed we were still flying along at 250 kts. I put the flight guidance in manual speeds and set the plane to slow down to 131kts; and on speed; lowered the gear to slow the airplane down as fast as possible; and set flaps at their appropriate speeds to configure the airplane. We were given vectors to the final approach course and clearance for the approach. I'd set the appropriate altitude for the glide slope intercept altitude and watched the storm set over the east half of the airfield. As we approached the field; trying to stabilize the approach; I'd briefly considered taking the controls from him until he'd finally established the proper descent and approach. At no time did the captain give any commands to do any of these things; other than to reference the QRH for the lav smoke warning; and I apologize for having exceeded my capacity as first officer; but; felt the actions taken to be necessary to ensure the successful termination of this flight.

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Original NASA ASRS Text

Title: EMB-175 First Officer reported they received a lavatory smoke warning while on approach. They were vectored around weather with a late runway change.

Narrative: I was working as the PM. During the descent through 8;500 ft on approach into ZZZ; I was advised that runway XXR was being closed for storm passage and we'd be using Runway XXL. As I was preparing the FMS for Runway XXL; I looked up and noticed that we were flying directly to a large storm cell. I glanced over at the pilots MFD and noticed his weather radar was set at 100 miles; in spite of ZZZ being literally 15-20 miles away. I turned it down to 12.5 miles and then I asked Approach if they could give us vectors around the rapidly approaching cell that was now at about 11:30 from our position. He'd replied he was trying to get a hold of us to fly south; avoiding the cell on the east side of the cell; but we'd not responded. That was when I started to suspect the radio was beginning to have issues. I immediately asked for a Right 270 to avoid the cell and proceeded to sync the heading bug and placed the flight guidance in 'HEADING MODE' while turning the heading bug to a north heading to avoid the cell. At this point; Approach advised us that we'd approach the field from the north and get vectors for Runway XXL. The Captain then went into his FMS and began to finish programming the box for Runway XXL. I then noticed the cell over the airport and reran the performance numbers for Runway XXL and YYR (the opposite direction); anticipating wind gusts and direction change. At this point; the LAV SMOKE WARNING alarm came on and I heard the Captain call for the QRH for that alarm. Being this close to the ground; I immediately called the FAs and asked them to check both lavs for indication of smoke or fire and then ran the QRH; which said to consider diverting to nearby airfield. Shortly after my radio page went blank on my side.At this point; Approach advised of runway change to Runway YYR and that another aircraft in front of us had reported windshear loss of 20 kts; and then gave us vectors to set up for the Runway YYR approach. I then set up the box for Runway YYR; briefed the approach and advised them that we would need Fire Trucks to roll for us and how many souls on board. The FAs called me back and informed there was not smoke in the AFT LAV; but fog. I advised APPROACH what the FA's reported and still wanted the truck to be there; just in case. I then noticed we were still flying along at 250 kts. I put the flight guidance in MANUAL SPEEDS and set the plane to slow down to 131kts; and on speed; lowered the gear to slow the airplane down as fast as possible; and set flaps at their appropriate speeds to configure the airplane. We were given vectors to the Final Approach Course and clearance for the approach. I'd set the appropriate altitude for the Glide Slope Intercept Altitude and watched the storm set over the east half of the airfield. As we approached the field; trying to stabilize the approach; I'd briefly considered taking the controls from him until he'd finally established the proper descent and approach. At no time did the Captain give any commands to do any of these things; other than to reference the QRH for the LAV SMOKE WARNING; and I apologize for having exceeded my capacity as FO; but; felt the actions taken to be necessary to ensure the successful termination of this flight.

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.