from what you describe (1 week or so of snow and ice, and the rest cold) i would go with a performance snow.
two of the top contenders are the bridgestone lm60 and the dunlop wintersport 3D.
the lm60 will be biased to snow performance (its the same tread pattern as the ws60, just without the tube multicell)
the 3d would be biased toward dry and wet road performance.
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Past: 2001 330i ZSP/ZPP/Step
Past: 2004 S60R 6MT Sport
Past: 2007 335i 6MT ZSP/ZCW/322/655
Past: 2010 335xi ZMP/ZCW/ZPP/PPK/PE
Present: 2013 335i 6MT ZMM/ZDH/ZTP/3AG/494/508
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