D&D 5E Campaign set during winter

[Eltab]

River travel is disrupted because ice floes or total freeze-over messes up boats.

Find information on exploration for the Northwest Passage.
An early adventure could be taking enough dogsleds to evacuate a stranded ship from where it became frozen in, and is beginning to crush. Some important person wants the cargo, too, even if the crew has to wait for another trip.

Polar bears &c moving south into inhabited lands because they can. Nobody ever knew there were so many mice / lemmings about until a Swarm of them ran through town and into the river. Owls, hares molted and the new grew in all-white.
Further south, elephants spontaneously mutate into mammoths and migrate north. And a bunch of tigers (or a pride of lions) grow sabre-toothed.

Neanderthal subrace of humans.

 

[Tonguez]

Winter means death, ghouls feasting on frozen corpses, ice wraiths seeking to drain any source of heat/life, starving animals desperately attacking settlements and animals that usually live further away (Yeti, White dragons) move into the former warm areas.

Snow is also really heavy, so buildings collapse, avalanche becomes are real danger,lake ice things, rivers have floes,

melting snows lead to floods, turning streets into bogs

 

[jasper]

From my 1E or 2E snow snakes. Choose your 5E snake make white. 90% undetectable in 2E. Call it stealth +12. Evil Snow Men 1d8 snow balls 2x per hit dice. Snow balls do 1d4 damage. Unless it a slush ball then does 1d6.
Jack Frost. Cr 12 I don't remember his stats.
Anything burrowing would get advantage to stealth due to snow cover. Frost Giants would move around more. I would do Evil Saint Nick but Futurama has done better than I did.

 

[aco175]

There may also be affects on health for people in towns that may be snowed in. Magic may take care of some of this, but typically there was limited fruits and vegetables in old-timey days that caused problems and eventually led to other diseases getting in. There are stories about the sled dog race having roots in delivering medicine to a snowed in town.

 

[WayOfTheFourElements]

An esoteric organization that imitates the exploits of the Polar Bear Club.