The highest average temperature in Yakima is July at 74.9 degrees. The coldest average temperature in Yakima is December at 26.4 degrees. The most monthly precipitation in Yakima occurs in December with 1.5 inches.The Yakima weather information is based on the average of the previous 3-7 years of data.he Yakima, WA air quality index is a median value which considers the most hazardous air pollutants. The Yakima, WA air quality index is 10.6% greater than the Washington average and 19.5% less than the national average.