Supposedly the engine paints the world with snow dynamically but I doubt it allows much more than defining areas in the editor which snow, and hence have snowy terrain. Maybe a mod can pull this off, as I think it would be cool to see riverwood pelted with snow at one point, and see the throat of the world clear with green grass growing but until the creation kit comes out it's impossible to tell if something like that can be done.
I believe the dynamic part is in the EDITOR, not in the actual game... Painting snow on high elevations, as a default texture.
Seasons are possible... but at a heavy cost.
You can't just paint everything white. You have to add more polygons to create snow build-up on roofs, roads, rocks, trees, land... You need to strip the leaves off trees that shed... You need to dry-out grasses to dead, then turn them green again. (That can be done by just removing the color and colorizing it yellow/tan.) You need to change rain-weather to snow, and more-snow. You have to replace most of the road textures, and create muddy ones for spring and fall thaw-times. Then you have to make multi-level filled trees, as the leaves come back, day by day, until full... (Unless you want winter dead trees to just POP back to life with full branches of leaves again.)
It is a lot of overhead, for unrealized reality. We would all just be sick of snow... Like most northerners are... Oh, look, more snow, and mud, and dead trees... NEVERMORE, cries the raven! (That was a request for warmer weather.)
LOL. (I think it would be neat, and could be fun... but then I have to upgrade my kilt to leggings, or grow hair on my legs... thicker hair... bear hair... (As opposed to bare rabbit.)