That's normal weather at high altitude, at least in the Rockies of Colorado and Wyoming, where such thunderstorms often occur daily, usually in mid to late afternoon. Plan your trip so you're down off high, exposed places (where lightning is a considerable risk) by early afternoon. Such planning may necessitate starting on the trail at dawn.

Also, put on that rain gear before you get soaked! Mine goes on at the first drops, even if I have to take it off 15 minutes later.