Narrative:

We were descending via the [arrival] into ZZZ. After zzzzz we requested deviations around cell build ups on the arrival. Approach approved and said 'direct ZZZZZ1 when able and descend to 6000 feet.' I used cws (control wheel steering) to avoid the cells. We reset the FMC to direct ZZZZZ1 and were descending to 6000 feet. I was high due to still avoiding some lower buildups but turned toward ZZZZZ1 and was descending to be at ZZZZZ1 by 6000 feet. Everything seemed fine until I noticed that it didn't look like the aircraft was going to level at 6000 feet. I was trying to figure out 'what is it doing now' when I realized that I had not gone from cws back to cmd on the autopilot. I reacted - a little too late - and the aircraft descended below 6000 feet by about 400 feet. Approach queried as to our altitude and we responded that we were aware and correcting. The correct altitude was re-established and cmd was re-engaged; the rest of the approach and landing was completed without incident.this was a bonehead mistake. I don't use cws much; and in an attempt to keep the ride smooth for the passengers opted to use this mode instead of heading select. I should have told the pm (pilot monitoring) to help me remember to reengage cmd since I don't use cws much; this is a busy part of the flight and I was a little tired - in the yellow. This would have helped maintain the shared mental model and prevent this rookie mistake.

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Original NASA ASRS Text

Title: B737 First Officer reported an altitude overshoot due to autopilot usage while evading weather.

Narrative: We were descending via the [Arrival] into ZZZ. After ZZZZZ we requested deviations around cell build ups on the arrival. Approach approved and said 'Direct ZZZZZ1 when able and descend to 6000 feet.' I used CWS (Control Wheel Steering) to avoid the cells. We reset the FMC to direct ZZZZZ1 and were descending to 6000 feet. I was high due to still avoiding some lower buildups but turned toward ZZZZZ1 and was descending to be at ZZZZZ1 by 6000 feet. Everything seemed fine until I noticed that it didn't look like the aircraft was going to level at 6000 feet. I was trying to figure out 'what is it doing now' when I realized that I had not gone from CWS back to CMD on the autopilot. I reacted - a little too late - and the aircraft descended below 6000 feet by about 400 feet. Approach queried as to our altitude and we responded that we were aware and correcting. The correct altitude was re-established and CMD was re-engaged; the rest of the approach and landing was completed without incident.This was a bonehead mistake. I don't use CWS much; and in an attempt to keep the ride smooth for the passengers opted to use this mode instead of Heading Select. I should have told the PM (Pilot Monitoring) to help me remember to reengage CMD since I don't use CWS much; this is a busy part of the flight and I was a little tired - in the Yellow. This would have helped maintain the shared mental model and prevent this rookie mistake.

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.