Just watched a video, from 7 years ago, where Ryan Maue explains how the Euro snow maps are done on Wxbell. He did not broach GFS. He said Euro output includes sleet AND Frz and he has an algorithm that attempts to back out the liquid equivalent. So this may explain why Wxbell maps are so different from TT and Pivotal re: each uses their own method. Here is TT for the 12Z GFS and its definitely lower in some areas despite including sleet. Important to know for model PBP. TT also uses the "SLR " which is explained by Ryan also below.
Today's 12Z GFS on TT