If you're hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Monrovia: The hottest months are usually January and April with an average temp of 81°F, while the coldest months are August and September with an average of 78°F. The rainiest months in Monrovia are September, June, August, and July, with each month seeing an average of 24 inches of rainfall.