In my (limited) experience, the barfies don't necessarily have to accompany core cooling. Every time I experienced them I wasn't shivering, let alone past shivering. In those cases, my hands got cold from being pressed against the ice, constricted by leashes, and plunged into snow. Another interesting sidenote: once my hands get cold and warm up, they then tend to stay warm and not get cold. Is this due to blood vessels expanding as blood is forced through them when they're cold? But overall, I don't think that the barfies are an indicator of hypothermia onset. Of course, it can be, but not necessarily.