TBH, the tires are actually designed that way. half way their tread depth, they change to different compound, aimed at people that wears them in one season, so the final part of the tires (40%) are for less snowy conditions, close to the springtime.
http://www.tirerack.com/tires/tires....l=Blizzak+WS60
"The Blizzak WS60 features a dual-compound tread with Bridgestone's silica-enhanced Tube Multicell compound
layered on top of a standard winter compound.... The pores and tubes are uniformly distributed throughout the compound and are constantly being exposed around the tread as the tire wears.
When the Blizzak WS60 is
50-percent worn, a tread depth indicator molded into the tread design lets the driver know that only 10 percent of the remaining tread is the Tube Multicell compound."