December in Hermiston is the coldest month of the year. In December, the weather in Hermiston is typically cold. Average daily high temperature in December is about 3 degrees above zero. On average, Hermiston has 13 cloudy, 9 rainy, 5 partly cloudy, and 4 sunny days in December. The rains are usually light. December as well as January and March is considered the most uncomfortable time to visit Hermiston.