As others have pointed out, the middle of Nevada (route 50/Western Express) is at higher elevations than the south (e.g. Las Vegas). As a result, temperatures are also somewhat cooler. As a desert climate there can be a high day/night swing in all of Nevada. Hence, April/May can have some cold desert nights and the passes can have snow.

The western parts of Kansas are still dry and there is a climate gradient where average rainfalls/humidity increases and eventually get into areas where (unless already cleared) a lot more trees. I find the cycling in drier parts, to be pleasant.

Adventure cycling recommends a mid-May to October timeframe for the Western Express and I would agree.