Roo,
Since you're a new kid on the block, you might want to add
Weatherbase to your favorites. It won't tell you what the weather will be like when you get there, but it will tell you what the weather was like, by month, over a number of years. In Yellowstone's case, 107 years. Its amazing, at least to me, how altitude affects the weather, especially temperatures and whether the precipitation is rain or snow. BTW, it defaults to Centigrade, but you can click on Fahrenheit, it will calculate for you.
FWIW, coastal Virginia is at zero feet altitude, and Yellowstone is at 8,000 feet. BIG difference.