Phoenix and Las Vegas are already major metropolitan areas. With enough money invested in desalination (expensive, but cheaper than migrating a whole country) it's doubtful that any part of the US will be rendered uninhabitable.

> With enough money invested in desalination

Cheaper to nullify meteorological services and browbeat media into silence and simply tell Americans everything is fine.

That would be good for Canada - better weather, more laborers and capital and more reasons to complain about their friendly neighbors to the south.