Based on the NAM index, you might think strat-trop coupling shuts off ~23 Jan

BUT take a look at the longitude/height plot:

Strat high pressure barotropically tied to high pressure along US West coast (-120 deg)

Strat low baroclincally tied to low near the dateline pic.twitter.com/2PcGqSRUjC

— Hannah Attard (@HannahAttard) January 11, 2021