The climate of St. Louis varies between a humid continental climate and a humid sub-tropical climate type and is subject to cold Arctic winds as well as warm, humid tropical air from the Gulf of Mexico.
Summer (June-August): The city experiences warm summers, with peak temperatures reaching 38 degree Celsius. The hot weather peaks in July and continues till August.
Winter (November to March): This is the coldest time of the year when snow and fierce winds can dip down the temperature to -21 degrees Celsius and this time of the year is off season in Saint Louis.
Spring (April to May): A short but glorious time which is also a good season to enjoy the flowers blooming in various Botanical gardens in Saint Louis.
Thus, the best time to visit St. Louis is between April – May and September – October. These months provide the most pleasant weather conditions and are filled with festivals and events.