Average Temperatures and Weather
April: Days 60–80 F/Nights 40–55 F;
May:Days 80–100 F/Nights 60–70 F;
June/July/August: Days 100–120 F/Nights 70–90 F.
Note: July/August is typically the monsoon season in
Arizona which usually means a cloud buildup in the
morning and short heavy rain in the afternoon.
September/October: Days 70–90 F/Nights 50–60 F.
THESE ARE NOT ABSOLUTES! It can rain in any month, or conversely, be arid and cloudless. Clouds and/or rain can cause the temperature in the Canyon's desert environment to plummet—even during the hot summer months.
You can check these sites for weather forecasts prior to your trip:
Marble Canyon (Full and Upper trip put-in)
Phantom Ranch (Lower trip hike-in)
The weather on your trip may even vary between hot and cold several times a day. Layering options are required as unexpected temperature extremes can occur at any time of year and body temperatures change with the activity (hiking in the middle of the day vs. boating early in the a.m. etc.)
It is possible, even probable, that the weather you encounter will be consistently sunny and warm/hot, but you will need to be prepared for the Canyon's many different moods.
There is a good possibility that spring and fall rafters will experience weather conditions that lower body temperatures. During these months, if the sky is cloudy, it will result in very cold conditions while you are rafting, and “dry” gear and layering options will be required on the water.
The river water temperature is ICE COLD at ALL TIMES (50–55 degrees F) – a big factor in body temperature as even the smallest waves will give you a soaking.