1.2. The highest average temperature in St. Louis is July at 77.8 degrees. The coldest average temperature in St. Louis is February at 33.2 degrees. The most monthly precipitation in St. Louis occurs in September with 3.2 inches.The St. Louis weather information is based on the average of the previous 3-7 years of data..2. The highest average temperature in St. Louis is July at 77.8 degrees. The coldest average temperature in St. Louis is February at 33.2 degrees. The most monthly precipitation in St. Louis occurs in September with 3.2 inches.