Not really a WB issue IMO. Your camera's meter is designed to review the reflectivity of a middle-grey object (Neutral, or 18% grey). What happened here was that the camera saw the snow, assumed, as it's designed to, that it was middle grey and exposed appropriately. As has been mentioned, in situations like this, you can't [normally] use automatic modes successfully.

There are a couple of ways to deal with a situation like this. The best is to use incident metering; that is: using an incident meter which reads the light at the scene, rather than that which is reflected, but unless you have a separate light-meter, not really an option, OR the simple and easy method is to use the 'Sunny 16' rule. That is: You exposure for a bright, sunny day should be equivalent to f16 with the shutter speed the same as your ISO. In this case: F16 @ 1/200 second, or f11 @ 1/400, or f8 @ 1/800.

Like JJ said, tripod and release aren't really required here, even at 300mm (but don't forget to make sure that your shutter speed is at LEAST equal to FL if not higher when hand-holding long glass) and not using them will give you much greater freedom of composition.