This album collects views of Theodore Roosevelt National Park in North Dakota in May 2010. They should give you an idea what to expect this time of year, which is a good time to visit.
May brings some wildflowers and some thunderstorms. Snowstorms are possible but it was in the 70s when we were there, making it much more pleasant than later in the summer.
There's some non-potable water that feeds the general green-up. The Little Missouri River was running at six feet and unfordable.
Wildlife was easily spotted in the South Unit, especially in the Paddock Creek drainage. We saw bison calves and wild foals but no elk calves.