Stormy Weather

Harried by bandits for the first hundred miles upstream, the party sees relief with the river turning to run from the north. The trade off is a more difficult river to navigate. The walls have closed in on either side of the river, pinching it to a narrow channel running fast and deep. Starting north, the trees thin as the ground becomes rockier and less fertile. The steep mountainsides tower over the water. It is still possible to travel upstream by boat, but this is as far as any larger river boats can travel. The currents and waterfalls of the upper stream require travel by smaller craft that can be portaged around the worst areas. You are slightly surprised to notice a sizable dock jutting out of the eastern bank with a good sized, low wooden structure built next to it, but a successful History DC16 check let you understand this is the turning point for bigger boats, and a spot for offloading into small boats.
There is no sign of smoke or movement to indicate anyone is at the dock. Pulling in, there are no boats present, and it doesn't look like any have been in some time. Given the onset of winter, this isn't very surprising. It does look like there might be some supplies around.
If the party decides to rummage for supplies, they can find enough dry rations for three days journey. There are also 3 lengths of rope, 10 feet of chain, a block and tackle, 10 pitons, and 2 hooded lanterns with 5 flasks of oil. Perception DC20 will let someone find a wooden box tucked behind a loose board in the wall. The box is 12 inches long, 6 inches wide and 6 inches deep. Inside it there is a fishing net and a spyglass. Arcana DC15 will perceive a magical aura to the box [the box is a folding boat, with 3 command words - one to open the boat, one to open a larger ship, and one to close it back into the box].
While the party is looking through the cabin for supplies, the weather begins to deteriorate. The sky is rapidly blackening and the wind begins to howl. There is a small lodging attached to the main building where they could hold up for the storm if they wish to. The two rooms share a fireplace, although the woodpile next to the building is almost exhausted. There are about two hours before the storm opens in full force. There is time to scavenge wood from further up the hillside.
During the height of the storm, a DC12 Perception roll will notice the chattering cackle of voices on the wind. Looking outside, the players will notice 5 imp-like creatures about 100 feet upriver and inland from the cabin. They are white, and appear to be made of the snow they are cavorting in. They seem very excited, and are actively enjoying the wildness of the storm. They are ice mephits.
Roll contesting perception checks to see whether they notice the PCs. Once they do, they will swoop forward to attack.