Bookworm - but you don't have an airframe temperature gauge and the metal skin of your aircraft in a 100 kt wind (cruise) is likely to be lower than the OAT if you have melted snow evaporating from it.
Well there's a mechanism, at least. I'm not convinced that the latent heat of evaporation is an issue -- otherwise we'd see the same cooling effect in rain, and I don't think that's observed. There's also the latent heat of fusion of the snow as it melts that can take some heat away from the airframe.
Add to that the variations in temp that will naturally occur in and around the base of convective cloud and you cannot in any way be sure that just because you see snow there is no risk of icing.
And I would agree with that. But the point is that the circumstances have to be quite special for SN or RASN to pose a significant icing risk. The mere presence of RASN doesn't have to set off alarm bells like FZRA should.