We leave for Costa Rica tomorrow and I just saw the extended weather is for thunderstorms everyday. I know it is the rainy season, but does it usually rain all day this time of the year?

an afternoon or evening torrent is what is normal, but it can rain all day, not much different than living in southern US. Usually only lasts for about an hour. Plan most of your outdoor activities for early in the day and have a great time. Also, northern parts of CR see a lot less rain than south, so if you are going to the Guanacaste area, you may not see any rain at all.

If you further research the weather patterns, you may find that it rains an average of 21 days during September. Guanacaste, Liberia areas are a little drier and ~10 degrees warmer now when compared to San Jose, Manuel Antonio areas.

Also, those weather reports you see online don't mean that it will rain all day every day (although that is certainly possible)! But you can probably expect afternoon showers most days.

http://costa-rica-guide.com/Weather/...rMap.html#rain

Check out this site. It will give you some idea of how much rain in a given month at different locations around the country. But still, it's luck of the draw. We've seen full days of rain during the dry season in Guanacaste and we've seen 4-5 days of absolutely NO rain during the rainy season.

It is SO hard to predict weather. Yes, northern areas will be dryer, but you will probably still have SOME ran. And as other have said, mostly afternoon showers.
Now that being said, we were in Nosara 8/21-8/27 and it rained a LOT. It was mostly cloudy and rainy everyday. We had maybe 5 hours of sun the entire week! We were told that they had over 45 inches of rain in July, and flooded a lot of the hotels. I don't think this is the norm, just a wet year this year.

We were in CR August 16 to 26. We had only one day of rain when we were in MA. No rain during the day at Arenal, Monteverde, Alajuela or Heredia. Some showers and downpours in the evenings and during the nights.

what a switch! Usually MA gets more rain than Nosara! As I said, you can't really predict the weather! LOL!