Don't be too surprised about NW lounges! No arrivals services of any sort as far as I know. I think the only one with a shower is main one at DTW? Might try the CO and DL lounges. The NW, CO and DL lounges (if any for the latter 2 and yu can access them having travelled internationaly at ST+) are in D concourse and a quick airside bus ride away from somewhere from the C concourse that AS uses.

Was in PDX Satrday night but no time to look.