So, as soon as our heroes enter Irrisen through the Hoarwood portal, it again states that the snowfall causes them to take movement penalties.
Once they leave the forest and begin their trek to the Pale Tower, this seems to remain the case (requiring that they get some snowshoes).
However, is this intended to remain this way throughout the entire adventure in Irrisen, including the trip to/through Whitethrone? Does the 8 day travel estimate given assume that the party is using dogsleds to travel now rather than on foot?
Not sure entirely how the snowfall builds up in Irrisen. So, does it only become difficult terrain after a fresh snowfall? Or is it fated that our adventurers (and their adversaries) will never be able to take a 5-foot step outdoors?
Whitethrone does seem maintained in some areas to the point where there wouldn't be difficult terrain, at least from what I've read so far.
Isn't it snow that doesn't melt because it is so cold. I have ruled that it is not constant snowfall in Irrisen (it becomes restrictive on certain PCs with constant -4 to ranged )
I would assume no restrictions in whitethrone unless stated specifically. But generally no 5 foot steps outside
Not sure about the journey. But I'm sure you can work it out by looking at the overland speeds (don't have them to hand)
Snow will sublimate over time. However, there are also occasional snowstorms which result in renewed snow cover. So the movement penalties remain in effect but the snow probably isn't tens of feet deep ;) and when you feel like it (dramatic purposes or the like) you can have another snowstorm hit.