You might spend a lot of your vacation inside, but if you want to see more of the area, taking a look at local weather data can help you. The hottest months are usually February and January with an average temp of 57°F, while the coldest months are July and August with an average of 45°F. The rainiest months in Castro are August, June, July, and May, with each month seeing an average of 10 inches of rainfall.