What Whee said is spot on.
I don't know why, but Memorial Day weekends produce rain about 90 percent of the time in Idaho! By mid-June the weather is generally good, and it can be before then too, but it is more variable so the odds of hitting a good weather window aren't as good. Given this is an El Ninio year, I'd definitely be thinking mid-June or later if you're traveling any distance to get there. Its not unheard have to get snow on the 4th of July so be prepared.
I live in Nevada, 2-hours from Johnson Creek by plane. If I'm planning an Idaho trip, I always have an alternate if the weather isn't looking good. SE Utah, Oregon Cost, Montana, etc.
Also, Hells Canyon is a good alternative if the weather is low or cold. You can generally get in and out of there. It is also warmer than the other Idaho destinations since it's about 1,200 feet in elevation. Big Bar or Dug Bar are worth looking at. The desert strips are often overlooked, but can certainly provide solitude, if that is what your after (Hollow Top, Bear Trap, Owyhee Reservoir, etc). They can be more accessible if the mountains are socked in. Lots of options in Idaho.
Keep in mind too, that northern Idaho is generally wetter than southern Idaho.