It's common advice in dry regions: stay out of the canyons when it might rain! Even if it's blue skies and warm weather, a bit of rain can quickly come and ruin that.
This video was taken about 1 year ago in the transition from summer to fall (which we're going through again now). There guys are out canyoneering, clearly very experienced, but don't head the warnings about potential rain. As such, within about 20 minutes of the rain starting, they are nearly washed out. The four think they've made it out, but end up having to hide in a ledge while a waterfall of sorts forms over them. They finally make it out, and see just what they could have encountered!
Remember: check the weather before heading into the canyons! Locals will tell tourists the same, and people often feel like they're being teased, but it's a real danger.