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All nonsnow creatures get -X/-X until end of turn, where X is the number of snow permanents you control.
Dead of Winter affects only creatures that aren’t snow creatures at the time it resolves. Creatures that enter the battlefield later in the turn won’t get -X/-X.
The value of X is determined only as Dead of Winter begins to resolve. It won’t change later in the turn if the number of snow permanents you control changes.
