Nuclear winter does not imply that there should be snow. That is not what 'nuclear winter' means lol
There is a very logical explanation within the lore for this. The nuclear war basically resulted in a global climate shift, resulting in essentially a year round summer. You're not going to see snow in a Fallout game because the canon explicitly states that this doesn't happen anymore.
From the Wiki -
"The nuclear exchange that characterized the Great War lasted for only a brief two hours, but was unbelievably destructive and reshaped the climate of the world even as it caused the fall of most of human civilization everywhere across the globe. More energy was released in the first moments of the Great War than all of the previous human conflicts in the history of the world combined. Entire mountain ranges were created as the ground buckled and moved under the strain of the cataclysmic pressure produced by numerous, concentrated atomic explosions. Rivers and oceans around the world were contaminated with the resulting radioactive fallout released by the relatively low-yield nuclear weapons used by all sides, and the climate changed horrifically. All the regions of Earth suffered from a single, permanent season once the initial dust blasted into the atmosphere by the nuclear explosions had settled – a scorching, radioactive desert summer."
http://fallout.wikia.com/wiki/Great_War