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Upslope does not require mountain ranges. Places just to the east of the Rocky Mountains, such as Denver, receive most of their precipitation from upslope snow. This is not caused by the Rocky Mountains, except to the extent that the Rocky Mountains prevent the Pacific moisture from falling on Denver.
No, the upslope is caused by the Great Plains themselves. Denver, as you may know, is a mile (5280 ft) above sea level. Western Kansas and Nebraska are closer to 3000 ft above sea level. When the wind blows from the east, as it does when a storm system is developing south of Denver, the air will ascend 2000 ft while traveling from far eastern Colorado to the foot of the Rockies. This elevation gain is sufficient in most cases to cause clouds and precipitation, possibly including heavy snow. So air doesn't have to be going over a mountain; enough elevation change by any mechanism will do.
Suppose air has gone up and over one mountain range, and then it encounters another of similar size. What will happen?
The actual amount of precipitation that falls on the second mountain depends on whether there's another source of vertical motion at the same time. Usually, if the air is moist enough for upslope to work, there's also synoptic-scale vertical motion going on, helping to lift the air. The stronger the synoptic-scale vertical motion, the more likely it is that some moisture will survive to fall on the second mountain.
Numerical forecast models usually don't have enough horizontal resolution to accurately simulate the flow over mountains. The precipitation that they do predict is a combination of the large- scale precipitation and the upslope flow produced by the overly smooth mountains in the model. To forecast precipitation amounts in topographically complex areas, you need to decide whether the details of the topography will cause the actual precipitation to be greater or less than the model forecast, and by how much. This comes from reasoning and experience. One useful technique is to decide how significant or strong the storm system is, compared to others that strike the area. Then, use the climatological precipitation totals for your forecast site to decide how much precipitation such a storm is likely to produce. The storm better be an exceptional monster for it to produce more than the monthly mean precipitation at a given location, for example.
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