We went to CN Aug. 14th -21st, and out of the seven days, it did not rain just one day. It clouded over in the afternoon with a black sky, and rained or threatened to rain and thunder most of the afternoon. Some days it was just for an hour, but more often it was most of the afternoon. We will not go back in the summer again for this reason. Others will disagree with me, but this was our experience so since you asked, I feel like I have to be honest. Threr were a few mosquiotes out at night.
We've been to CN twice. First trip was around Sept. the 12th in 2009 and June 9th 2011. I thought it seemed a little more hot and humid in September than June, but it was relatively the same. In our week in September it rained for 30 to 45 minutes each day in the afternoon. It gave us a break to go back to the room take an afernoon nap and enjoy the rest of the day. Other then that we had not other weather to speak of. Our week in June started with several straight days of rain then nothing but sunshine the rest of the trip. We had a lot more people in our June trip then what we had in the September trip. This time even while it rained hard we stayed out in the ocean or pool and had a great time. Next year we are doing a mid April trip.
My wife and I have gone to CSA for the last 2 years on the last week of August and into the first week of September absolutely no weather related or bug problems. You cannot see CN from CSA but it is only about 5-8 miles as the crow flies, so the weather should be the same. We did a private dinner on the beach 2 years ago and a 40th vow renewal last year. There were no bugs to speak of on the private dinner in fact the bug spray that we brought with us that year was never opened. It did rain very lightly for the 4PM renewal on the beach, but the skies cleared about 15 minutes before the ceremony and did not start again until we were cutting the cake at Lemongrass resturant. In the 14 total days that we spent during the last 2 visits it only rained about 2-3 afternoons and it was very light and did not last long. I would not worry about going at that time, if the weather starts getting bad I would highly recommend going across the street to the spa and getting a 1 hour deep tissue full body massage. We upgraded the vow renewal and my wife and I got that massage for 1 hour. It thundered the whole time that we were getting this done and it has to be the most relaxing thing that we did. When we walked outside afterwards it had not rained. This is Jamaica mon, so go with the flow and no worries. We are booked again this year from 8/24 thru 9/2/2012. If we had worries we would not have chosen those dates again.