Not easily. Basically, every pixel can only interpolate between two material textures (here: snow and ground). To do more would require a significantly more complex scaler texture (plus shader magic to texture *3* materials per output pixel) or subdividing landscape pixels so we can use a different interpolation method per direction (which wouldn't look very consistent either).

Best "easy" idea I could come up with would be changing the selection of that "other" material to sometimes prefer sky. However, that would mean that the earth edge would move (and get more pixely) when the snow falls on it. That seemed less desirable to me.