You burn faster at high altitudes. A complete change of clothes. I cannot overstate the fact that you will get dirty on Northstar's trails. Warm clothes for the morning and evening. At Northstar, getting dirty is guaranteed due to the powdery dirt there.
Rain is possible, and weather conditions change fast in the mountain environment.
It will be chilly in the morning, warm or hot during the day, and chilly again as the sun goes down, so dress accordingly. Make sure you bring a complete change of clothes both days, including shoes and especially socks. This will be required for all registered riders. Required for the races, recommended for all other riding. Required for the races, recommended for all other riding.