Being newly retired teachers, this will be our first chance to travel in the fall. We will be leaving on Sept 23. The trip includes the Black Hills, Yellowstone, South Eastern Utah, and Colorado. We have spent much of the summer planning this trip but our biggest question now becomes "How do we pack?" There are four of us in an SUV so we are limited to one suitcase and a carry-on each. How cold can we expect in Yellowstone in late September?