If you want natural snow at Christmas in Spain, the choice comes down to two resorts in Catalonia: Baqueira-Beret and Port-Ainé share the same region and height band, 2,100 m,, and they came out snowy in 25 of 30 Christmases across the winters 1984/85–2013/14, right on the threshold of reliability. The third, Boí Taüll, with its mid-mountain at 2,400 m, managed only 19 of 30. The rest of the Pyrenees get by on snowmaking over the holidays.
Sierra Nevada's altitude is no help
Sierra Nevada has the highest mid-mountain in the country, 2,700 m, yet natural snow at Christmas appeared in just 5 of 30 winters, the same as the median resort in Spain. With snowmaking that becomes 27 of 30. Formigal (5 of 30) and La Molina (3 of 30) are much the same story: with the snow guns running, 14 of Spain's 20 resorts come out snow-sure. For comparison, in France 30 of 125 are snow-sure on natural snow. These figures describe the climate of past winters, not a forecast (methodology). The bottom line: for Christmas in Spain, head for Baqueira-Beret; the other resorts, only if you are happy skiing pistes with snowmaking.