I wonder how long they have had this idea of a judgement message?
I don't know how long they have been promoting this idea but I may know where they get the idea of a warning message. I think they are relating the Armageddon event with the Flood. Peter called Noah, "A preacher of righteousness" and the JW's assumed that meant a warning message to the pre-flood worldly people.
It's strange that the book of Genesis makes no mention of a warning or preaching campaign by Noah prior to the Flood. I don't know where Peter got his information unless it was by divine inspiration. I still find it strange that Noah's preaching activity is not mentioned in the OT. I think the large boat construction was the only witness given.
I find it hard to believe that if Noah really preached he would have been able to save more than his immediate family and their wives. What about grandparents, cousins, sisters of the wives, the wives parents and so forth. Something doesn't add up.