Suppose somehow you could delete the entire Rocky Mountain range down to sea level. No more tornadoes? Milder weather (no more cold air fronts from high altitudes or from way up north)? Would just the change itself make the weather crazy for a while?
It would rain less on the coast.
Oh! And let’s say there aren’t any humans, anywhere in the world. So no hot, dirty cities, no pollution of any kind, no cattle, no dogs or cats, no dams or levees. And no people, breathing, farting, smoking, burning, polluting. Just wind and rain and snow. More deer and wolves and bears. Lots more trees, wetlands.
Do you mean just the Rockies (the easternmost range), or all the western mountains (the Sierras, the Coast Range, etc)? Because if it was just prairies all the way to the west coast, there’d be no mountains to cast rain shadows, and the middle of the continent wotd be significantly wetter. At the cost of the west coast being drier, presumably.