Except in the central mountains, temperatures don't vary much in the Dominican Republic, averaging a summery 28°C-31°C (81°F-87°F) in most places for most of the year. In the mountains, sunny days climb to 24°C (75°F) but can just as easily fall to single digits at night or on cloudy days. Tropical humidity can make the temperatures feel higher, though sea breezes help mitigate the effect.
There are two rainy seasons, October to May along the northern coast and May to October in the south; bring an umbrella if you plan to travel the entire country. Dominican rain isn't your garden-variety 'just-enough-to-cool-things-off' precipitation native to Hawaii or Central America - this stuff drenches waterfall-style and could easily last half a day.