We have never been out West. What is the best month to go weather vs crowds? This has always been a dream of ours and it is time to put it into action.
The "best" time to visit National Parks out west is any time you can go! Seriously, it's hard to pick a month that is the "best" generally.
There might be specific months that are best for one park, but not necessarily all. For example, my favorite time of the year at Yosemite is May, because the kids are still in school and crowds are light, the snow is melting, and the waterfalls are gushing. But if I go too early in May, some of the higher roads and trails may be closed or inaccessible.
A few parks have very short seasons. One of our most beautiful parks is Glacier NP on the Montana/Canada border, but it's only open to visitors during the peak of summer. The rest of the year it's snowed in.
You really have to factor in altitude and weather out West. We have had snowfall at Yellowstone in late July. At many of the parks in the Rockies, the
floor of the park may be at 7,500 feet or so. That makes for a much shorter summer, colder temperatures, etc.
Crowds are another issue in many parks. Yellowstone, Yosemite, and the South Rim of the Grand Canyon during the summer seem more crowded than midtown Manhattan at lunchtime. Traffic can be horrible at those parks, but may be very light at the North Rim, and down the road from Yellowstone at Grand Teton NP. OTOH, if you get 1/4 mile up many trails you will be completely alone.
Two other factors to consider in planning a western park trip. The first is, there are a LOT of parks out west. Not just National Parks, but National Monuments, Nat'l Recreation Areas, National everything. There is really way too much to try to even scratch the surface on one trip.
The other factor is driving distances. Living in the East, we don't really appreciate the distances sometimes, but you really do have to watch the distances. Just because Colorado and Wyoming are neighboring states doesn't mean you can drive from Mesa Verde NP to Yellowstone in one day.
Another consideration is
how you want to see a park. Many people go to the Grand Canyon, drive up to the South Rim, take the obligatory "standing-at-the-rim-of-the-Grand-Canyon" pictures, hit the gift shop, get their National Parks Passport stamped at the Visitor Center...and drive on. And they
loved the Grand Canyon! They'll talk about it for years! Those people, in fact, are the typical visitor to Grand Canyon, not the exception.
Other people stay a week, hiking, maybe taking the overnight mule ride to the bottom and back, sitting on the porch at El Tovar watching the sunset, etc. They also love Grand Canyon...but they love it in a different way. There's no right or wrong way to enjoy a park, but the manner in which you approach parks will affect how much time you need to allocate.
I'd recommend you use the Internet to research parks you might be interested in. Do NOT use a search engine to get information specific to a park. You will get old, cached information which will be terribly inaccurate. Go to
www.nps.gov and use the search feature (either by name or state) to locate your park.
Each park has its own webpage, and the general convention is www. nps.gov/first four letters of a one-word park name, or first two letters of each word in a two-word park name. For example /yell for Yellowstone, /yose for Yosemite, /ever for Everglades, but /grca for Grand Canyon.
You will find the websites a bit clunky to navigate at first -- you sometimes have to drill down through a lot of layers to find the info you want. But you'll figure it out. All of the parks use the same template, so once you can navigate one, you'll be fine with the others. Some of the websites are quite good. For example, at Glacier you can even take "e-hikes" online! I just looked at them for the first time today, and they are pretty darn cool!