You're right that much of the western U.S., including Montana, are in a warmer-than-average pattern right now and in the short-term. I don't see a clear indicator of when this general pattern will break, but I will say that it's common for such patterns to last a week or two (and I realize much of the west has been warm for a while, but it's not the same pattern), so I wouldn't be surprised if we see a change sometime in December. I also will note that the La Nina pattern tends to be strongest in mid-to-late winter even though the tropical sea surface temperatures peak a bit earlier.