Planning for the weather is a good way to make your trip to Opatija relaxing and enjoyable, especially when you’re splurging. The hottest months are usually August and July with an average temp of 69°F, while the coldest months are January and February with an average of 39°F. The rainiest months in Opatija are November, December, September, and October, with each month seeing an average of 7 inches of rainfall.