NWS in outstate areas doesn't seem to use their radios much and most of the traffic I have heard is between the NWS and County Dispatchers. I have NWS Duluth logged as radio 794701 and my last log was on Group 36591 which is the NWS Duluth TG.
I have heard NWS Grand Forks on NW Call 48516 and S-Tac 1. I would bet that NE Call would be your best bet.
The NWS talkgroups, by standard, are rarely in field radios. Without affiliations the traffic is often only on console sites. The current standard does allow patching to a STAC for those console sites without the NWS talkgroup, but again this is not often affiliated in the field thus not going out on the radio sites. The new NWS standard will drop the STAC patch for simplification. I believe the new standard goes into effect in 2018.