The weather this week for the Inner Grand Canyon (Phantom Ranch) is as follows:

There is a 50% chance of showers & thunderstorms tonight (Monday) between 11:00 p.m. and 11:00 a.m. (Tuesday).

After that the clouds will pass and a high pressure area will set up over the southwest. Temperatures will rise steadily throughout the week, starting in the high 70's and increasing quickly.
 
The Inner Canyon may hit the century mark (100 degrees) on Sunday for the first time this year. Make sure you have plenty of water/sport drinks and salty snacks to help keep you going, and be sure you plan to be off the trail between 10 a.m. and 4 p.m. (during the most extreme "triple digit" heat of the day).

Hike Smart & Have Fun!
Brian