Planning for the weather is a good way to make your trip to Monrovia relaxing and enjoyable, especially when you're splurging. 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.