I can relate a story from my own experience with my daughter's car. Turns out it had two bad ignition coils that caused the engine to misfire when it was raining. Did she stop when the check engine light came on? Nope!
So the unburned fuel from the misfiring cylinders ends up in the catalytic converter. These things are designed to handle mostly burned ...
TL;DR: bad coil pack.
Pop hood and remove engine cover
Start car, note rough idling
Starting from the left of the engine block:
Remove the female ignition coil connector plug from coil pack
If the idling noise changes, replace ignition coil connector
If the idling noise DOES NOT change, that's the bad cylinder (*)
Remove 5/8" ...