I get that warmer air can hold more water content, but I don't think that necessarily translates to more 6"+ snows, because you also can lose some with bad ratios, more front end/back end rain, etc. IMO the increasing 6" number is more likely associated with storm track... fewer dry fast moving active N stream storms producing light snow, more offshore systems, for whatever reason... possibly more blocking, bigger ridges out west, warmer oceans? Maybe somebody has some real research that looks at both sides of the warmer=snowier equation (water content v accum loss) and tried to correct for track differences, not sure.