You'll likely spend much of your trip indoors, but if you're planning to venture beyond the casino, taking a look at local weather data can help you. 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.