Easiest snowbird route from Seattle to Phoenix in January

It's all weather dependent, no matter which route you choose. If you prefer to avoid the Interstates as much as possible, take I82 and I84 to Twin Falls, then US93 through Nevada to Vegas, then Kingman and Phoenix. I've run it many times. Good road surfaces, biggest climb is Cabbage Hill at Pendleton.

But Nevada is very remote, and don't run it if snow is forecast.

Otherwise, Salt Lake, then Page, Flagstaff and Phoenix. Flag is the weather mess on that route.