WS70 are not run flat. Your options for winter run flat tires are pretty limited, especially in the size you need (245 50R18 probably). Tirerack.com shows three options:

Michelin Pilot Alpin PA2 ZP
Pirelli Winter Carving Edge Run Flat
Pirelli Winter Sottozero Serie II Run Flat

None of those have reviews on Tirerack (almost no one buys winter run flats), but I would guess the Pirelli Winter Carving Edge will give the best snow/ice traction, seeing as the other two are performance winter tires (which give up some snow/ice traction for better dry traction and handling).

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For August 14th ED: 2013 335i M Sport 6MT - Estoril Blue - Black Leatherette - Dynamic Handling Package - Heated Seats