RE:CRJ GROUNDED BY MOTHER NATURERJ's can fly at 50 degrees they just require a longer runway and reduced weight depending on how full. The reason is at that temperature their wings are not large enough to produce lift from the ground level air thus require more runway to garner more speed for air and wings to produce lift. Larger aircraft (more powerfull do not have such issues)
Even at 50 degrees they can still take off but in AA's case safety was a prudent choice.