Any suggestions for traveling the lowest elevation crossing of the Northern Rocky Mountains from Washington state to Montana?
You’ll want to travel Lookout Pass (Elevation ~4,710ft) via Interstate 90. It’s a paved highway crossing of the Rockies and likely the lowest major pass route you’ll encounter on I-90 heading east out of WA.
If for any reason that doesn’t work out (always check current conditions) Marias Pass (Elevation ~5,213ft) via US-2 in northern Montana is my next recommendation.