You've had a reply from @Citizen Mane. I fully agree with it.So the GM's not even allowed to narrate what the PCs see while travelling to town unless it directly affects a player-authored concern that already exists?
I would add: my friends and I will never run out of things to imagine, things to say to one another about the fiction we are creating together.
But we can make choices about what fiction we create, according to what principles.
The principle that is adopted by Burning Wheel is: the GM should say things about the fiction that put pressure on the player-authored Belief, Instincts etc of their PCs. The GM's goal is to thereby incite the players to declare actions for their PCs that will express, reflect on, perhaps contradict, those Beliefs. The slogan for the game is "Fight for what you believe".
If I, as GM, choose to narrate the weather, it will be because I see it as somehow contributing to my purposes in accordance with the preceding principle.
In addition: even when I am GMing Torchbearer, a game which invites the GM to take a greater hand than BW does in contributing to what is at stake, I will take try and take the result of my random weather and travel rolls and incorporate them into the fiction in ways that apply pressure to the players' goals, or speak to the dramatic needs they have chosen for their PCs.
Eg one time when the PCs were travelling through (randomly rolled) heavy rain, and (through random roll) found themselves harassed by bandits, I had those bandits be the same NPCs they were trying to find: so the opening framing of their meeting with those NPCs had the PCs finally, after trudging through the rain being harassed by enemies they couldn't quite see, coming to the bottom of a steep rise and seeing the NPCs, above them, calling out to them to turn back.
And I've already posted upthread how, when the PCs were returning to Megloss's house to try and bind the shadow spirit, I rolled up rain, incorporated that into my narration as a type of foreboding, and then - when the players were utterly defeated in their conflict with the spirit - incorporated the weather into my narration of the consequences, with a lightning bolt splitting Megloss's house in two.
In other words, when GMing Torchbearer, and incorporating the random elements that its rules call on me to generate, I always try to minimis mere colour as much as I can.