Yesterday I booked my a June 13th flight to Missoula--first class, of course.
Toying with the shoulder season. In 2016 it was 32 degrees when I woke up on the morning of the first day of riding. A couple of days before I arrived it snowed 5" on a pass I would climb on Day 3. Fortunately, it melted quickly. I did get a little sleet during the climb.
Here is most of the route:
https://ridewithgps.com/routes/31562518?beta=false
Should end up being around 665 miles when all is said and done. Quite manageable in 13 days with one rest day. A good deal of dirt though (some quite hilly/mountainous), which will make things harder.