Planning for the weather is an important factor for making your trip to Sioux City a success, especially when you're splurging. The hottest months are usually July and August, with an average temperature of 22°C, while the coldest months are January and February, with an average of -3°C. The snowiest months in Sioux City are January, October, February and November, with each month seeing an average of 23 cm of snowfall.