I agree that the baselines are rapidly changing for certain regions and climatic variables ... very good observation.  As you pointed out this is not a "one size fits all" solution and really works for the tropical Pacific b/c of unique atmospheric dynamics specific to the region.  I know this is a discussion that many user communities are having: NIDIS-- drought assessment and also Probable Maximum Precipitation (some links provided).  This will continue to be a hot topic!  [ pun intended ;-)  ]