Without winter tires....DON'T even try it.
Yesterday I witnessed how a braking 997 Carrera S with summer tires slipped on a snow covered surface for at least one car length. And this car was farely doing 5(!) kph.
We all held our breath because it was a new car delivered and we were anxious that he (the driver) hits the window of the building.
Without winter tires....FORGET it.
My 997 Carrera S with 19'' winter tires (235s front and huge 295s rear) performs VERY well on snow, I'm actually surprised HOW well it performs.