I am scheduled to start a loop from Missoula on June 15th. I did much of this route at the same time of year in 2014. This year I follow the TransAm to Ennis but with a detour on into the Pioneer Mountains after Big Hole Pass east of Jackson, MT. In 2014 a woman I met on the first day of my trip told me that there has been several inches of slushy snow on Lost Trail Pass a day or two before. Crossing the Pioneer Mountains I had a bit of wet snow mixed with some with sleet and rain. I bring this up only to illustrate what Jama wrote.
Some unsolicited advice fort that part of the route:
If you have suitable tires and don't mind some gravel, take the Old Darby Rd. Alternative between Hamilton and Darby. It's not terribly rough and the views are excellent.
If you are planning to stay at the private campground in Sula, make sure you shop at the nice store in Darby. The campground store has a very limited selection of stuff, although you might find Ramen. However, the store closed at 5 p.m. when I was there. I arrived around 5:03 and the place was dark and locked up tight. You can still camp there and pay in the morning, but you won't be able to get anything to eat.
The spring-fed pool at Jackson Hot Springs in Jackson, MT is nice, but the place is pricey for camping. The only grocery source around is in Wisdom, the town before Jackson. IIRC, the store closes around 5 or 6 p.m. The mosquitoes in Wisdom will eat you alive unless you have repellant. There were some in Jackson, too, but they were not as bad.
Try to time things so that you stay in Twin Bridges, MT. The Bike Camp there is terrific and free (donations greatly appreciated), and the town has a very good grocery store considering the town's size.
There is a camping at the fishing access site just outside the center of Ennis. I haven't been there in 16 years, but it was nice back then. It used to be free, but now it's $12 if you don't have a MT fishing license.
