The highest monthly average temperature in Walla Walla for July is 78 degrees. The lowest monthly average temperature in Walla Walla for December is 32 degrees. The most monthly precipitation in Walla Walla occurs in November with 2.5 inches. The air quality index in Walla Walla is 312% worse than the national average. The pollution index in Walla Walla is 86% better than the national average.