Check engine light came on during normal driving in bad weather (snow storm).

Need a pro scanner to view and *compare* live data for the pre and post cat O2 sensors.. Probably need hands-on help by someone with that equipment. That code usually indicates the post (REAR) O2sensor sees a problem with the exhaust oxygen content and 'most likely' is caused by a depleted catalyst.

P0420 refers to the front catalytic converter in the exhaust manifold. It will most likely need to be replaced. But, I would first erase the code from the computer and see it comes back.