Upcountry covers a large range of elevation, and elevation is the primary determining factor in temperature. A rule of thumb is about 3°F temperature drop per 1000′ of elevation from sea level.
Average low/high in three common Upcountry destinations:
Makawao at 1600′ elevation in winter: 60°/75°F
Makawao at 1600′ elevation in summer: 62°/80°F
Kula at 3100′ elevation in winter: 55°/70°F
Kula at 3100′ elevation in summer: 57°/75°F
Haleakala Summit at 10,000′ elevation in winter: 37°/52°F
Haleakala Summit at 10,000′ elevation in summer: 41°/58°F
Much of the Upcountry area where visitors might typically find themselves is on the leeward side of Maui, and is mostly dry and sunny, with clouds moving in to the Ulupalakua and Kula areas later in the day most days. The more you move toward the Hana direction, the wetter it gets; in other words, Makawao is wetter than Kula.