Our original plan was to spend May visiting the Grand Canyon, Painted Desert, and Utah parks; June at Yellowstone; July at the Grand Tetons; August at Glacier-Waterton; September at the Washington national parks; October in California. However, the fall colors are spectacular in Yellowstone and the Grand Teton, so we decided to return there in September instead of continuing west to California and Oregon.
While planning the itinerary, we drove over the California mountain passes every other weekend, spending two night at each site. We discovered that changing locations every two days wasn't long enough to settle in, photograph in different lights, and left us exhausted. So, we replaced two night at each of the Utah parks with a week at the north rim of the Grand Canyon.
Meanwhile, I e-mailed the park rangers.
Grand Teton replied, "Camping in May is definitely feasible, although the weather is generally pretty varied during that time of year. Make sure that you are prepared for cold weather and potential rain or snow as you make camping preparations. It will hopefully be nice, but with mountain weather you never know! Below is a link to some average temperatures and precipitation measures for each month. http://www.nps.gov/grte/planyourvisit/weather.htm. As far as the time frame for seeing cubs and calves, it depends on the year, but I would say that you have a good chance of seeing baby animals beginning in late April and throughout the month of May."
Yellowstone replied, "I suggest you came later in May or early June. While we can still have snow and temporary road closures there will be more roads and services open and available. Bears are frequently sighted later in May, especially in the northern part of the park, and bison calves are readily visible. Much of the bison calving season is in May. Elk calves may begin to appear around late May as well, though they aren't seen as much as bison until they get older. Bears are sighted with less frequency beginning the middle of June as they start moving away from roads and into higher elevations as the snow disappears. Even if you came the first couple weeks in June you should still have some opportunities to see them, though sightings become less frequent. Even in May there are no guarantees but there are lots of sightings in May. Coming later in May or early June would also give you the opportunity to better enjoy the park. Even then we can have cool, wet weather, including snow. But early through mid May many roads are still closed and in some areas there is typically still snow cover."
It seemed that spending the month of May at the Grand Canyon may result in missing the bear cubs and, if we saw them, we would miss some of their youth.
Next, I made a chart of park temperatures so that we could easily compare park to park and month to month. During the day, the temperature variance was about ten degrees. 70 degrees (Grand Canyon) is more pleasant than 61 in the Grand Tetons. And, the 61 is just the average high. Snowstorms can occur in all months at Yellowstone and the Grand Teton.
But, we have experience camping in snow and really want to photograph the youth, so we dropped the Grand Canyon.
Next, I began researching the animals so that we would know when and where to find them. Guess what? The bison rut is late July and August. Oh no! Miss that? Good-bye Glacier.
Together is the best place to be and it will be in Wyoming.