Dead of Winter

• WWizards of the Coast2019-06-14
  Official Ruling
  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.
• WWizards of the Coast2019-06-14
  Official Ruling
  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.

Rulings via Scryfall