Here is a render I did of the atlantis seed in 1.9.
It has the very edge of a HUGE snow biome in it.
Absolutely enormous. If a 1K x 1K (and maybe even more) map were centered on that biome the whole map would probably be snow.
I also FINALLY managed to come across a (tiny) mushroom biome. Sadly, no mooshrooms spawned.
Along the way, however, I encountered some weather:
Seed: dreamworld (which is actually very nice - when you spawn, you have a swamp and desert to the left, a snow biome immediately to the right, mountains to the north, and forest to the south)
Snow!
In between biomes: rain/snow border
Coming across the mushroom island!
Another view
Spores seem to come up from the mycelium and rise into the air. It isn't too visible in a still image, but it's those tiny black dots on the grass.
A tiny sibling island about 40 blocks away
I tried lighting the area, expanding it slightly and bonemealing red mushrooms in an attempt to get some mooshrooms spawned, but to no avail.
What an obscenely rare biome. I'd love to see how many of them even show up on a rendering of a seed.