In reply to JamesRoddie:
I'm planning a similar trip but in November. I've never been before so I was going to ask the same question. I have found a bit of info....
This website;
http://www.tiempo.com/ram/2301/situaciones-meteorolgicas-para-el-recuerdo is in spanish but they say that between the second half of November through to March you could have snow anywhere in Spain from 800 - 1000m.
This site reports on a trip made last year to Ubiña on the 21st 22nd Dec:
http://guiasdelpicu.blogspot.co.uk/2013_12_01_archive.html . They said that it was snowing at1400m but still thin on the ground.
This site shows historical weather from last year in the Picos at 1546m:
https://www.meteoblue.com/en/spain/weather-picos-de-europa/archive .
It will depend on the weather but I would image that the snow line could be between 1000 - 1500m. Hope this helps a little.
