What happens when the modules are covered in snow?

The solar modules need sunlight to produce electricity so a ‘sticky’ snowfall that blocks the sun will certainly reduce the power output until it melts or is removed. Once exposed to bright sun, the modules heat up and will shed any snow cover. A good system design will maximize power output with partial snow coverage. Also you may have some other benefits: Albedo effect (reflection coefficient) increase radiation and less temperature → more efficiency.