There are a gazillion threads on the weather and they all say the same thing - don't worry about it!
On one of my trips, the forecast was 80% rain every day (including the scary thunderbolts) - what we actually got was a 30 second sprinkle one day and about 5 minutes of fairly heavy rain the next. Other than that, it was sunny and HOT the whole time.
The only forecast that I do watch prior to travelling to Mexico is this one - http://www.nhc.noaa.gov/gtwo.php?basin=atlc&fdays=2 - I want to know if anything significant is headed to the region, especially a hurricane - as that usually results in delayed/cancelled flights.
FWIW, over the years, I've been through two tropical storms in the region. The first one (several years ago) moved in during the evening but was gone by the morning and the rest of our vacation was sunny and hot. The second one came in during the day and we were able to sit at a bar and watch it move in over the ocean. It rained hard for the rest of the day and a portion of the evening but again, by next morning there wasn't a cloud in the sky.