Planning for the weather is a good way to make your trip to Ottumwa relaxing and enjoyable, especially when you’re splurging. The hottest months are usually July and August with an average temp of 72°F, while the coldest months are January and February with an average of 30°F. The snowiest months in Ottumwa are January, November, December, and February, with each month seeing an average of 8 inches of snowfall.