I'm in Costa Rica right now (I know, I know, I should be on the water not on the computer). June is still supposed to be dry around here. December was also "supposed" to be dry around here - it was far from dry. Maybe 70-80% of days were rainy and pretty large floods and landslides made it impossible to get on the water somedays.
Global climate change is definitely affecting when (and how much) it rains around here. A decision to come down here boils down to how willing you are to leave things up to chance. You might not see rain the whole time you are here. Or, you might just land on a random week of high rain. With that in mind, there are some rivers that are only runnable at lower flows and some that are damn released and should have runnable flows all year (so you aren't SOL if it doesn't rain at all).
Chasing Jaguars has pretty good information in it about when rivers are runnable.
Hope that helped a little