It could be a bunch of different things, but more than likely the doors weight has changed a bit due to the humidity and the downward force adjustment needs to be tweeked. It'll be located on the back of your opener and you can slightly adjust it using a screwdriver. Just be sure you mark where it is now so you can put it back there when winter comes. Below is a link that'll give you more info about it.
http://www.garagedoorsofwilkes.com/basics.htm