Ubiquitous initialism in the area, generally living here a few years i find the climate and culture to be relatively homogenous from far northern california through seattle (and presumably vancouver BC). Very different (for still American) from the other areas Ive lived or frequent (Northeast, south, parts of Cali). Mild, wet climate; passive, introverted, polite culturally (very generally speaking).
I think acronyms (or initialisms) are a form of community enforcement. If using jargon to reinforce that people in the group understand.
Or maybe that’s my best interpretation. As it’s also just faster to type PNW than Pacific Northwest and someone who lives there must need to type it quite a bit.
But either way, HN is a community and many people value efficiency and, more specifically, code systems for efficient communication.
I have never heard of PNW and I live in the US. Also, this person is from Croatia.
Edit: I guess that doesn't disqualify this person from picking up US lingo, but thinking about it further, someone who didn't grow up in the US might be more likely to use regional abbreviations in general contexts.
At first I thought it might be some part of Papua New Guinea