If you’re hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in Onalaska: The hottest months are usually July and August, with an average temperature of 20°C, while the coldest months are January and February, with an average of -5°C. The snowiest months in Onalaska are January, March, November and December, with each month seeing an average of 14 cm of snowfall.