I spent some time this morning looking at how the diverse WFO's are presenting these graphics now. In short, now that the NWS has apparently unleashed them to do whatever they please with this experimental service they've gone berserk.
The graphics URL's are all over the place. Apparently some WFO's have abandoned the service: Topeka, Wichita, Hastings, Raleigh-Durham, and Portland. Whether this is temporary or permanent remains to be seen.
Some WFO's continue to use the old graphics URL's but with the new schema. They load just fine with the current wxnwswinter script, and are Billings, Glasgow, Memphis, Amarillo, Salt Lake City, and Milwaukee.
Some WFO's have adopted unique, apparently temporary, URL addresses on their Winter Weather Page: Omaha, Des Moines, Baltimore/Washington, Memphis, St. Louis, Fargo, Aberdeen, and Missoula. They actually have the graphics under the script's URL, just as a JPG rather than a PNG that the script looks for.
I will retain my scratch sheet of these findings and recheck them again in a few days before I set about modifying the script. Thanks for your patience.