This is great stuff, but the decision will likely be made based on where Amazon execs in Seattle can get a break from Seattle weather. That ain't Detroit.
Also, a city doesn't need good weather all year to have better weather than Seattle.
All that said, I personally doubt weather (and execs' preferences about it) will factor directly into the decision.
I can certainly see someone preferring Phoenix or Las Vegas, say, to Seattle in terms of weather but I wouldn't personally.
(I do agree that weather is perhaps one of the least relevant factors to this decision.)