The last game I ran also had a cold/snowstorm-filled apocalypse, but it was an urban setting, and the PCs were largely stationary -- I think with a travel-focused game, you're going to want to focus on how the Threats act in that context, more than a generalized 'what is winter like?' approach.
Front: Winter
Express: despair
Dark future/agenda: cover the entire world?
Description & cast: ash snow, storms, freezing cold ?
Stakes questions:
- do summer still exist in the south?
- can people survive the winter?
This seems like a good start. Since the Front expresses Despair, I think you might also want to consider ways in which the Winter might isolate people from each other -- both geographically/physically, through actual storms and snow, and socially, through the hardship and tough choices it forces on people. If I were running with this I would probably add a question like 'will people band together in the face of winter, or be forced further apart?'
This also makes the dark agenda feel a bit more concrete -- winter will cover the world not just because it will be cold everywhere, but because it will have forced everyone apart, set man against man, and left what few good-hearted people remain shivering to a slow, inevitable death in a cave somewhere.
Threat 1: The road south
Kind: landscape/mirage
Impulse: entice & betray people
Cast: people migrating south, ?
Custom move: ?
Countdown: this is where I am hesitating. For now I have the idea that more and more people will migrate south. But should I focus on the actual "road" or the trip?
Have you read
The Grapes of Wrath? Forced, mass-migrations in response to environmental calamity create a lot of opportunities for people to exploit each other. All these people moving south are vulnerable -- they don't know the land they're travelling through, they have all their possessions strapped to their back, etc. I think focusing on the trip and the non-PCs making the trip with or around them is a great idea.
So like a really basic chain of escalation-through-human-violence, with events like: somebody steals someone else's stuff; violence erupts over where to go next; a local guide of questionable morals offers to lead migrants through a seemingly impassable landscape; a windfall of food turns out to be poisonous; desperate migrants kill and pillage local farmers; a con man swindles a family out of all of their food; a wealthy migrant dies on the trail, leaving her supplies up for grabs. Lots of things that the PCs will have opportunities to mitigate or avoid, but could also happen on their own, emerging naturally out of the fiction -- that way you get a really dynamic countdown clock, which advances at the pace of the story.
Threat 2: The freezing cold
Kind: affliction/condition
Impulse: expose people to danger
Cast: ?
Custom move: something about freezing?
Countdown: I am aslo hesitating here. The temperature could become colder and colder. Or maybe a super ash-snow storm could happen. Maybe something weird could lurk in the cold. Or maybe I should focus on the effects on the general population?
Again I would definitely focus on the effects on the general population -- you want things that can happen 'on their own' in the fiction, as much as things you can make happen yourself if the PCs give you the opportunity.
In the game I mentioned above, the snow and cold were deeply intertwined with the psychic maelstrom -- the overall front was about isolation and breaking down communication ("static") -- so I generally just had a rule where if the PCs missed an Open Your Brain roll, I would trigger a snowstorm. This wasn't on a countdown clock, though -- if there's ash storms all the time, they probably don't need to be distinct events. On the other hand, if the threat of the Storm to End All Storms is something you want, that's probably worth making 'Ash Storms' its own Threat type.
I think deciding whether or not the cold is supposed to be Weird or not is going to have a huge effect on what sort of things you want to include on the countdown. Assuming a mostly natural threat, I'd look up the symptoms of hypothermia and frostbite and consider what sort of horrible situations that could create for people. A child is accidentally left exposed, or someone who was travelling south loses a leg and may have to be abandoned. Think about the effect on resources, too: in our cold apocalypse, anything that could be burned pretty much had been burned (yes that included most of the gasoline), so books were super-rare and most people were illiterate. Clothing also becomes very important. Pack animals are a source of heat, etc.
Superstitious beliefs are good, too, especially since you have a Hocus; maybe a death cult develops around people who get lost in the snow, or someone decides that the cold can be kept at bay through human sacrifice. Maybe people have hypothermia-induced visions.