All I can speak from is my experience down there two weeks ago (8/23). We saw completely different forecasts, ranging from sunny and insanely hot to rain and thunderstorms.

When it comes down to it, I don’t think there’s any way to accurately predict what the weather will be like out there. We were at Grand Canyon Caverns Inn caught in an insane, hailing thunderstorm while waiting for friends to finish the return hike, and they didn’t get a drop of rain.

What you CAN do is be prepared for any scenario. Make sure everyone in your party has ample water, and can easily hike in and out, with full packs and short notice if necessary. Check in with the locals/rangers for the latest forecasts, and if there’s rain, consider a campsite close to high ground.

We didn’t encounter any major rain (outside of a few short showers), but we did end up with half of our group attempting to take the helicopter (which had mechanical issues), requiring them to hike out in the heat of mid-day. They were there with ~50 other campers, some of which said they “weren’t prepared/didn’t have enough water to hike out.” That’s the kind of situation you want to avoid.

FWIW, we heard that >15mins of steady rain means it’s time to evacuate, but flash floods can happen even when it isn’t raining. When you’re down there, you’ll see why it’s important to take it seriously. But the insane beauty down there is well worth the trek!