The rear tire just spins out on you - no traction. I think you could ride these conditions with 5" tires and bigger knobs on the tires. Surly's Lou is a rear specific designed tire for these kind of conditions. I know a lot of people that love the Bud and Lou combo for snow riding.

Funny enough, I'm going to end up with more tires for my fatbike than the rest of my bikes combined