anyone know what this is??

I've got it too. Its a reflective texture, and it looks kind of bad in game, unless its being hit by shadows. /shrugs

It could be anything at all, it might be an indication they experimented with snow way back in the development of tf2 or that they have tried it out since, or that they are going to.
or it could be nothing to do with snow and tf2.
from just one texture you can't conclude anything, so don't get your hopes up for snow in tf2.

Perhaps they were going to include snow in Lumberyard, and they just kept it for future use in the alpine theme?