December in Mount Isa is the hottest month of the year. In December, the weather in Mount Isa is typically hot. Average daily high temperature in December is about 33 degrees above zero. On average, Mount Isa has 10 cloudy, 9 partly cloudy, 7 rainy, and 5 sunny days in December. The rains are usually heavy. December as well as January is considered the most uncomfortable time to visit Mount Isa.