You need to apply soiling on a monthly basis. It is not unusual to have 50% snow losses for peak winter months if you are in a snowy location and if your tilt is low.
There are a few studies available online that can help you estimate snow losses but it is very location and system dependent.
We operate projects in Canada and Japan and it’s always been a challenge to model snow losses. Thankfully winter months in snowy location are not high production to start with.
Check this from NREL https://www.nrel.gov/docs/fy17osti/68705.pdf
This from DNV https://solarprofessional.com/modeling- ... o-snowfall