That sounds like a question that needs some practical hypothesis testing, to me. Do you currently have any snow?
(smallclanger made an impressive snow figure on a stone wall belonging to Christ Church while we were waiting in vain for a bus the Saturday before Christmas. The snow was useless for actual snowmen, being too powdery and light, but it clung to the wall beautifully.)
I'm thinking rather than friction, though that will play a part, the part-melt forms a seal against air, so essentially air pressure holds it in place: it's pushed into the wall by air pressure, but never away from it (and it's not heavy enough for gravity to take over)