Add a darker version of the fog from the original map and add some snowflakes outside with a wind howling effect so its like an evening blizzard.

Bonus: Easter-Egg ideas

Mount Everest: If you zoom in then you can see a frozen corpse at the top of the tallest mountain.

Snow Monster: Rarely outside you can hear a roar echoing throughout the mountains, like a Yeti or something.

Monster Meal: You can find strange footprints leading to a small nest of human bones.

Something Inside: While in the underground facility you can occasionally hear something in the cielling.

Monster News: In one of the buildings you can find a newspaper talking about some Bigfoot rip-off spotted in Boreas.