Answer: It is challenging at your elevation and conditions. I've listed a few possibilities. If you have a good nursery in the vicinity, talk with the owner or a knowledgeable employee. They should be up-to-date on specific local conditions. I hope this information helps!
Smaller trees, up to 20 feet
Acer ginnala (Amur maple). Lots of color with yellow flowers, red seeds, red foliage in fall. Maples like deep watering, but since this is a smaller tree it wouldn't require as much. Takes the cold.
Malus sp. (Flowering crabapple.) There are numerous varieties--check with your nursery. Crabapples take rocky soil and moderate water.
Quercus phellos (Willow oak). You mentioned wanting a willow. This oak actually has long, narrow leaves like a willow, but takes far less water. Grows 50-90 feet.
Fagus sylvatia (European beech). Takes moderate to light water. Numerous varieties. 50-90 ft.
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