Punta Cana is blessed with a tropical sun year-round, so expect an annual warm climate with the hottest temperatures occurring in summer June-September. The temperature averages 87 F/31 C with seasonal fluctuations of 4-6 degrees depending on the month. Humidity can be bad at that time so it's good to stay poolside or in an air-conditioned environment.
The Dominican Republic is located in the Caribbean hurricane zone. While the hurricane season officially lasts June-November, peak season in the Punta Cana area usually occurs in September and October.