We live an hour northeast of San Francisco (Napa), and have done trips to Arizona in April. Things can get a little chilly at night in northern Arizona depending on your elevation. Canyon de Chelly often drops into the 30s at night in April and could easily go lower (it can also get really warm!). Lower humidity can mean wider variations in temperature. Overall, we generally *love* going to Arizona in April. Bring some warm clothes, use your furnace (or a heater/campfire if you don't have one), and have a great trip!