Depending on how much snow you get you need to build it differently. If you get a lot of snow then just make a big mound of dirt resembling the height with and length of the jump but you don't really need too much transition because that is best to add in with snow. If you don't get much snow then just build a jump out of dirt exactly how you want it to be in the winter and then when it snows just pack the snow on it. Also pack the dirt super tight and water the jumps to get the dirt to set right if you don't and there's pockets of air in the dirt then the dirt will collapse when the ground freezes
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