Amigo -

Well, if you want to be warm in late April you almost have to be in the desert.
Unless, of course, you tour in the Deep South during its azalea season.

A) If you're willing to risk chilly nights you could tour northern New Mexico.
Still desert, but lots of Puebloan culture and Route 66 kitsch, too.
Late April is still cool, but fairly dry in northern NM.

B) If you're willing to do a longer flight, the South has some good options - albeit rainy ones.
Have you considered the Natchez Trace? Maybe fly into New Orleans and out of Nashville?
Another option is to fly out to the Atlantic and trace some major Civil Rights locations - Albany, Selma.
The small towns will be resplendent in spring flowers.