How large is the settlement? massive city, small port town, obscure village / hamlet in the midst of nature on the edge of the frontier?
Are any parts of the settlement abandoned or nearly so? Perhaps it is a small town founded in the ruins of a larger city from eras past?
Does the settlement have a wall around it? thick 50 ft stone wall? a small wooden fence or somewhat rotten palisade? a 4 foot hump in the earth?
You suggested winter as the season and some precipitation (snow or rain) is involved. Is that still the plan?
Ideas that come to mind:
The party finds a child lost due to the blizzard. As the streets do not have names and the child is not certain how many turns he took or where, leading him home could take some time. Night is approaching, and the blizzard conditions make the way difficult for those not used to the large town / city.
The town, built upon the ruins of a much larger city, has several areas that have few if any people in them. Some roads are pocked with holes leading into abandoned sewer systems. In other places buildings have collapsed or on the verge of such. Most roads leading into such areas are blocked off by simple walls (maybe even just a wooden fence and a sign). During a blizzard (or rain storm), the party are walking along when suddenly the road beneath them collapses down about 10 feet. Due to the rain, the current in the half filled sewer tunnel washes them away, into regions not yet explored in the town.
Combining the two, the party is passing a blocked off road during a sleet / rain situation when one of them (have them all make listen checks, don't set a DC - whomever rolls highest hear it) hears the cry of a child beyond the wall / fence. Exploring, they find that the road has collapsed shortly beyond the wall / fence and that a child is now holding onto the edge of a brick to keep from being swept away. The slick and icy conditions make climbing down or up difficult, and the current is slowly but noticeably (after a minute or so there) rising. Finally, just as it seems that success is immanent, the child loses his grip and is swept away in the ~4 to 5 feet deep water, down the sewer tunnel into parts unknown. If the party is good aligned they will probably choose to go after him, trying to save him before he drowns.
If they follow, have them go down a tunnel with no real branching but several turns for about, say 50-70 feet, after which they find the child upon a platform a scant inch or two higher than the (still rising) water, next to the entrance to a larger tunnel that (if explored) has several other tunnels connecting to it. The platform has a few steps up next to it, leading to a raised walkway along larger tunnel. Being a few feet higher than the water in the larger tunnel, this walkway seems the best option for finding a way out of the abandoned sewer system, especially with the water still rising (albeit slowly), and the current running against them if they go back the way they came.
If they do choose to go back, it should be difficult due to the depth (~shoulder to neck deep for medium humanoids) and the current, but it should not be impossible, especially if any of the party had a 100 ft rope (or tied together two 50 ft ropes) and tied it to something before entering the sewer. So the party escapes but has a later option for more exploration in the town.
If they choose to go forward, they have a long path with little light except from the rare but occasional hole in the sewer roof from above, usually from the smaller branching tunnels connecting to the one they are in. Eventually they find a stairway or ladder leading out, but they will have to break through the rusted shut door / lock. Or perhaps they find a former den of long ago thieves - or come upon a den of thieves recently set up in this out of the way place. Either way, within an hour or two (in game) they find a way out. As they have a child with them to return, the point is to introduce a new area to explore while also forging a new contact (or possibly two, if the modern thieves den idea is used) in the child's parents (who may be important or at least knowledgeable and perhaps well known in the town).
Does this idea work for you? Even if the town is not filling and fixing empty former houses in a ruin of a city, it is possible it was built upon a city buried in a landslide ages past, and rumors or local sages may know this if the player's question the existence of the formerly unknown sewer system - one far too large for or to have been built by the town they are in.
), so the "road" encounter should not involve combat too much, and I'm thinking about winter weather. One thought was setting up shelter and/or a fire against a late-season blizzard. What kind of challenge would you give your players? 
