Bernie Sanders trails Hillary by a very significant number of delegates.
Therefore, Bernie Sanders must win the state with the most delegates tonight. That state is Arizona. He can't just win there by 2 or 3%. He needs to win convincingly.
Besides that, he must win Idaho and Utah by a large margin. You don't catch up by scoring small victories.
I expect Sanders supporters and the media to act as if winning two states out of three represents a huge victory for Sanders, regardless of the margin of victory. Because 2 is a bigger number than 1.
Another prediction. Remember how Michigan was said to be more important than Mississippi because Michigan was the "biggest prize"?
Tonight it will not matter that Arizona is the biggest prize, UNLESS Bernie wins AZ. If Hillary wins Arizona, the media and Bernie supporters will prefer the"number of states" approach.
The media will pick whatever narrative boosts their favorite candidate, Bernie Sanders.
One final prediction: If old people prefer Hillary and young people prefer Bernie tonight, the media will write articles about young people only (OMG MILLENNIALS!) because who cares about those old farts.