I thought it would be helpful to list western US metropolitan areas with populations over 1MM from Wikipedia:
Los Angeles–Long Beach–Santa Ana - 12MM
San Francisco–Oakland–Fremont - 4MM
Phoenix–Mesa–Scottsdale - 4MM
Riverside–San Bernardino–Ontario - 4MM
Seattle–Tacoma–Bellevue - 3MM
San Diego–Carlsbad–San Marcos - 3MM
<Denver fits here> - 2MM
Portland–Vancouver–Beaverton - 2MM
Sacramento–Arden-Arcade–Roseville - 2MM
San Jose–Sunnyvale–Santa Clara - 2MM
Las Vegas–Paradise - 2MM
Salt Lake City - 1MM

Los Anageles and San Francisco would be my uninformed choices. The airports themselves would seem to be problematic and but I wonder if there are any solutions involving a big infrastructure investment from the municipalities?