The Land of Barovia and its Peoples: An outsider’s perspective Volume IV: The Seasons

A Colorless World  The Changes between the seasons are minute
Autumn 
Autumn in Barovia is just a bit nippier than usual. The various pine trees don't shed their needles or anything, obviously. The more normal trees change colors, but mostly to a few different shades of muddy brown. The brilliant reds and oranges we associate with Fall don't come in Barovia. 
Winter 
It rarely snows during Barovian winters. Instead, it frosts. The trees and underbrush (what little there actually is) all dies and hardens with the cold and shallow springs and streams freeze over. Though the temperatures drop to frigid, the cold seems to petrify the landscape instead of transforming it into a white wonderland. 
The higher mountains are exempt to this, of course. It snows year round in such areas and winter is especially brutal. 
Spring/Summer 
There's actually little difference between the two warmer seasons. Once the ice of winter breaks, the valley becomes temperate. 
Luckily, Barovian winters are typically short, autumn and winter taking up no more than four or so months of the year. The rest of the year is the temperate climate.