Early September is ideal. The weather is fine (cool nights, warm days), the crowds are smaller and the foliage is beautiful.

We were there in mid June last year and it did not feel crowded at all.

How cold can it be and you will still wear shorts? You can never rely on the weather in Yellowstone! We've seen it snow a couple of inches in Mid July! We've also been twice in early June and once in September.

We were there the first week of June and we could really see the difference in the crowds by the second week! Nothing like the summer crowds thou!

If I had to guess, I'd say that fall might be your better chance at warmer weather...but don't hold me to it!

Utahtea

Utahtea, I vividly remembering an early September day when we wore gloves and warm jackets for a 7 a.m. float trip in the Teton and then wore shorts that same afternoon when it was considerably warmer. And, oh yes, when we flew out of Jackson on September 10th or 11th, it was snowing! But, for the rest of the 10-day trip, the weather was wonderful.
We've also been to the area in the summer, and we'll take September any time!

We went in early June last year; we all packed shorts but didn't wear them until we were on our way home. It was still cold, especially in Yellowstone where some of the lakes were still partially frozen and there was still snow on the ground. It was lovely, though, and we had a great time.

Lee Ann

we were there in mid July this year but it had only just got to shorts weather the week before.

I went this year in early June. The temperature dipped below freezing at night, and I don't think it made it above 50 degrees during the day.

Last year I went in early September. I think I never got to wear shorts, and I remember having a fleece on most of the time. I drove through a snowstorm on my way out of the South Entrance. The next day I was hiking in Arches NP in 90-degree heat.

I think what we are telling you Davemeister is that you will have to decide between shorts and crowds because you probably won't be able get both.

Utahtea

Utahtea, I think you mean that one can have shorts and crowds, or neither, but not one without the other.

Dave, I changed my profile picture to the weather forecast sign in the Moose Visitor Center at GTNP. On June 12th
(the week after the Pickles were there)

There are positives to both June and September. June has baby animals and longer daylight hours, September has animals in rut and fall color. Both have fewer people than mid summer, take your pick and go and enjoy a beautiful park.

I don't think it is definitely a choice between "crowds and shorts" and "no crowds and no shorts." Rather, I think we veterans of the area are saying that there's no guarantee of a certain type of weather, temperature-wise. As I previously mentioned, we definitely had no crowds and wore shorts most of the time during our early September visit. Meanwhile, others are saying that their weather wasn't was conducive to wearing shorts.

Looking at our pictures for the first two weeks of June of 2006, it was mostly short sleeves shirts, jeans and light jacket weather. Looking at the first two week in Sept of 2008 it was mostly long sleeves, jeans and heavier jacket weather. If my DH has on a jacket....you know it's cold!

Utahtea

utahtea, you prove my point! (PS: My September photos show us mosting in shorts!)

We were there last week. First morning at Yellowstone it was 25 degrees at 6:30 am, next morning it was 27 degrees. High for the days was 68-70. Grand Tetons was slightly warmer. It didn't seem very crowded but we did have some waits at restaurants and we couldn't get a dinner reservation at Jackson Lake Lodge Mural Room the first night we were there. The restaurant situation seemed to be mostly a factor of staffing since there were empty tables. Perhaps the college kids had returned to school.

Sorry for the typo. That obviously should read "mostly in shorts."

Wow, thanks for the feedback. So, in a nutshell: all bets are off when it comes to the weather! I'll pack light on the shorts.

Part II: Finances. Any real difference in the trip costs of June vs. September?

The only significant difference would be hotel rates. Check a couple of hotels you're interested in to see if there's a significant difference.

But, do take some shorts! While I seem in the distinct minority of posters in that I had warmer weather in September than the others, don't lose sight of the fact that it's a possibility that you could experience the same.
As I wrote earlier, we also did experience weather cold enough for us to wear gloves on an early morning float trip, but then experienced weather warm enough on the same day to wear shorts.
Keep your weather options open when it comes to packing clothes.

I was there the first week of August last year and wore shorts and t-shirt every day.

Crowds?

The only time it really felt crowded was the Jenny Lake hike on a Sunday. Obviously, if I had it to do over I would do that hike on Monday.

Also, the parking lot for the Mt Washburn hike filled up but the trail was very spread out.

Even the Old faithful deck was reasonable. A few crowds in some places such as Grand Geyser. But nothing bad.

I would do that again.

Okay. I have an opinion now. I just looked at a friend's pictures of the Tetons in the fall. Big difference from my pics in June. I'd definitely go earlier for more snow on the peaks.

But, Starrs, then you'll miss the beautiful foliage!

Oh, darn.
Let me think about this....

No matter when you go it's going to be a great experience!!!!

Utahtea

My vote: September. Aspens and tamaracks will be changing colors. And after growing up and living in the area most of my life, in general, you'll find better weather in September than June (especially early June). You'll have cool mornings but you're less likely to have rain. However, we ran into snow in Glacier over Labor Day.

But basically, overall, with all the weather caveats, etc.: better weather in September than June. September is my favorite month in MT.