Having lived in Flagstaff for a time, the weather will be clear unless precipitation is predicted. In that case it will snow...even with temperatures in the 40s-50s the snow usually remains on the ground for a time.
Regarding the traffic...it will be wall-to-wall. It almost ALWAYS is nowadays, even on weekdays in the "off-season."
Sedona used to be beautiful, tranquil, colorful, peaceful, and awe-inspiring.
Now it's thousands of the same person crowding the streets, the shops, the cars, the atmosphere, and the air.
If you think there are alot of people at WalMart on Black Friday then multiply that by 10,000 and you get an idea of modern-day Sedona.
There are SOOOOOO many more amazing things to see in that area...without ALL THOSE people.